Lead-free soldering reliability
2022-09-15 03:04:01
Introduction: Considering environmental and health factors, the EU has passed legislation to stop using leaded solder in 2008. The United States and Japan are also actively considering legislation to reduce and prohibit the use of harmful elements such as lead. Lead poisoning At present, the annual consumption of solder in the global electronics industry is about 20,000 tons, which is about...
Taking into account environmental and health factors, the EU has passed legislation to stop using lead-based solders in 2008, and the United States and Japan are actively considering legislation to reduce and prohibit the use of harmful elements such as lead. Lead poisoning At present, the annual consumption of solder in the global electronics industry is about 20,000 tons, which is about 5% of the world total lead production. Lead and lead compounds have been listed by the Environmental Protection Agency (EPA) as one of the top 17 chemicals that pose the greatest harm to humans and the environment. Lead-free solders commonly used lead-containing alloy solder powders are tin-lead (Sn-Pb), tin-lead-silver (Sn-Pb-Ag), tin-lead-bismuth (Sn-Pb-Bi), etc. The alloy composition was 63% Sn/37% Pb and 62% Sn/36% Pb/2% Ag. Different alloy ratios have different melting temperatures. For standard Sn63 and Sn62 solder alloys, the peak temperature of the reflow profile is between 203 and 230 degrees. However, most lead-free solder pastes have a melting point 30 to 45 degrees higher than that of Sn63 alloy. Therefore, the basic requirements for lead-free solders are currently internationally recognized as lead-free solders: Sn is the base and Ag is added. , Cu, Sb, In other alloying elements, and Pb mass fraction of 0.2% or less, mainly used for electronic assembly of solder alloys. Lead-free solder is not a new technology, but today's lead-free solder research is looking for alternatives to Sn-Pb solders that use between 50,000 and 60,000 tons per year. Therefore, alternative alloys should meet the following requirements:
(1) Its global reserves are sufficient to meet market demand. Certain elements, such as indium and antimony, have small reserves and can therefore only be used as trace additions in lead-free solders;
(2) Non-toxic. Some of the alternative elements under consideration, such as cadmium and strontium, are toxic. And some elements, such as cockroaches, can also be considered toxic if they change the toxicity criteria;
(3) It can be processed into all forms required, including wire for manual welding and repair; solder powder for solder paste; solder bar for wave soldering. Not all alloys can be processed into all forms, such as an increase in the content of niobium, which will cause the alloy to become brittle and not to be drawn into a filament;
(4) The phase transition temperature (solid/liquidus temperature) is similar to that of the Sn-Pb solder;
(5) suitable physical properties, in particular electrical conductivity, thermal conductivity, thermal expansion coefficient;
(6) Metallographically compatible with existing component substrates/leads and PCB materials;
(7) Sufficient mechanical properties: shear strength, creep resistance, isothermal fatigue resistance, thermal fatigue resistance, stability of metallographic structure;
(8) good wettability;
(9) Acceptable cost price.
The cost of the new lead-free solder should be less than 22.2/kg, so the mass fraction of In should be less than 1.5% and the Bi content should be less than 2.0%. Early R&D programs focused on identifying new alloy compositions, multiphase phase diagram studies, and basic properties such as wettability and strength. The later research and development program focused on five alloy series: SnCu, SnAg, SnAgCu, SnAgCuSb and SnAgBi. And in-depth discussion of its fatigue performance, production behavior and process optimization. Table 2.3 NCMS National Manufacturing Science Center's lead-free solder performance evaluation standard IPC also released the research report "A guide line for assembly of lead-free electronics" in June 2000.
At present, the main conclusions about lead-free solders in the world are as follows: There are many kinds of lead-free solders available, and none of them can provide a comprehensive solution for the direct replacement of SnPb solders.
(1) For certain special processes, certain specific lead-free solders can be directly replaced;
(2) At present, the most attractive lead-free solder is the Sn-Ag-Cu series. Other potential combinations include Sn-0.7Cu, Sn-3.5Ag, and Sn-Ag-Bi;
(3) There is currently no suitable lead-free substitute for high-lead and high-melting solder;
(4) It appears that the chemical system of the flux does not require major changes;
(5) The reliability of solder joints formed by lead-free solder is superior to that of SnPb alloy.
Comparison of several lead-free solders
(1) SnCu: the cheapest price; the highest melting point; the worst mechanical properties.
(2) SnAg: good mechanical properties, good weldability, good thermal fatigue reliability, and a melting point of 221 ° C for the eutectic composition. The difference between the SnAg and SnAgCu combinations is small, and the choice depends mainly on other factors such as price and supply.
(3) SnAgCu(Sb): It is only until recent years that there is a ternary eutectic between Sn-Ag-Cu, and its melting point is lower than that of Sn-Ag eutectic. Of course, the exact composition of the ternary eutectic is still controversial. . This combination is more reliable and solderable than Sn-Ag and Sn-Cu. Moreover, after adding 0.5% Sb, the high temperature reliability can be further improved.
(4) SnAgBi (Cu) (Ge): low melting point, 200 ~ 210 ° C; good reliability; the best solderability in all lead-free solders, has been confirmed by Matsushita; adding Cu or Ge can further improve the strength The disadvantage is that Bi contains a problem that the wetting angle rises.
(5) SnZnBi: The melting point is closest to the Sn-Pb eutectic; however, Zn contains many problems, such as solder paste shelf life, a large amount of active flux residue, oxidation problems, and potential corrosive problems. Currently not recommended. 2.2 Selecting the alloy from above, this reflow process design uses Sn/Ag/Cu alloy (Sn/Ag3.0/Cu0.5) as the alloy is considered to be the first choice in the international industry and has been awarded the Industrial and Research Association. Member's recommendation. Because some squads have proposed and studied another alloy, Sn/0.7Cu (mass percent), some companies also use this alloy in production. However, compared with the reliability and wettability of Sn/Cu alloys, and considering the use of the same alloys in reflow and wave soldering, Sn/Ag/Cu alloys have become the best choice for process development experiments. Sn/Ag3.0/Cu0.5 alloy properties: Dissolution temperature: solid phase line 217 ° C / liquidus 220 ° C; cost: 0.10 US dollars / cm 3 and Sn / Cu solder price ratio: 2.7 mechanical strength: 48 kg / mm 2 elongation : 75% Wettability: It is known from the properties of Sn/Ag/Cu alloy that the melting temperature of the solder alloy is 36 °C higher than that of the original Sn/Pb alloy, and the price after commercialization is also higher than the original. The process soldering temperature uses the recommended process curve for this alloy solder in Japan, as shown in Figure 2.1.
The typical process curve for lead-free reflow soldering recommended by Japan: There are three important points on the recommended process curve:
(1) The heating rate in the preheating zone should be as slow as possible (select the value 2~3°C/s) to control the bridging of solder joints, solder balls, etc. caused by the collapse of the solder paste.
(2) The preheating requirement must be within the range of (45 to 90 sec, 120 to 160 °C) to control the defects during reflow soldering due to factors such as temperature difference of the PCB substrate and changes in flux properties.
(3) The maximum temperature of soldering is above 230 °C for 20 to 30 sec to ensure the wettability of the solder. Cooling speed selection -4 °C / s 6 Defects in reflow soldering and their solutions Welding defects can be divided into major defects, minor defects and surface defects. The defects that invalidate the SMA function are called main defects; the secondary defects are that the wetting between the solder joints is good, and the SMA function is not lost, but there are possible defects that affect the product life; the surface defects are those that do not affect the product. Function and life. It is affected by many parameters such as solder paste, substrate, component solderability, printing, placement accuracy, and soldering process. In the SMT process research and production, we know that reasonable surface assembly process technology plays a vital role in controlling and improving the quality of SMT production.
Tin beads in reflow soldering
(1) Mechanism of formation of tin beads in reflow soldering Tin beads (or solder balls) appearing in reflow soldering are often hidden between the side of the soldered end of a rectangular chip component or between fine pitch pins, as shown in Figure 6.1. 6.2. During the component placement process, the solder paste is placed between the pins of the chip component and the pad. As the printed board passes through the reflow oven, the solder paste melts into a liquid, if with the pad and device pins. If the wetting is poor, the liquid solder will not fill the weld seam due to shrinkage, and all the solder particles cannot be aggregated into one solder joint. Part of the liquid solder will flow out of the weld to form tin beads. Therefore, poor solder wettability with pads and device leads is the root cause of solder ball formation. Figure 6.1 One example of a chip component has a slightly larger particle size. Figure 6.2 A solder ball solder paste is scattered around the pin. In the printing process, if the template is offset from the pad, if the offset is too large, the pan will be caused. The paste flows over the pad and is prone to solder beads after heating. The pressure of the Z-axis during the patching process is an important cause of tin beads, which is often not noticed by people. Some of the placement machines are positioned according to the thickness of the components. Therefore, the phenomenon that the components are attached to the PCB and the tin buds are squeezed out of the pad for a moment, this part of the group will obviously cause the tin beads. The size of the tin beads produced in this case is slightly larger, and it is usually possible to prevent the generation of tin beads by simply adjusting the Z uranium height.
(2) There are many reasons for the poor solder wettability caused by the cause analysis and control methods. The following mainly analyze the causes and solutions related to the related processes:
(1) The reflow temperature curve is not set properly. The reflow of solder paste is related to temperature and time. If sufficient temperature or time is not reached, the solder paste will not reflow. The temperature rise in the preheating zone is too fast, and the time is too short, so that the moisture and solvent inside the solder paste are not completely volatilized. When reaching the reflow soldering temperature zone, the water and the solvent boil and the tin beads are splashed. Practice has proved that it is desirable to control the rising speed of the preheating zone temperature to 1 to 4 ° C / s.
(2) If tin beads are always present at the same location, it is necessary to check the metal template design structure. The stencil opening size corrosion accuracy is not up to standard, the pad size is too large, and the surface material is soft (such as copper stencil), which will cause the outline of the printed solder paste to be unclear and bridged with each other. When the pad of the device is printed, reflow soldering will inevitably result in a large amount of solder balls between the pins. Therefore, appropriate template materials and template fabrication processes should be selected for the different shapes and center distances of the land patterns to ensure the quality of the solder paste.
(3) If the time from patch to reflow is too long, the solder particles will deteriorate and the activity will decrease due to oxidation of solder particles in the solder paste, which will cause the solder paste to not reflow and produce solder beads. Selecting a solder paste with a longer working life (we think at least 4h) will alleviate this effect.
(4) In addition, the solder paste misprinted printed board is not sufficiently cleaned, and the solder paste remains on the surface of the printed board and through holes. Before the reflow soldering, the printed solder paste is deformed when the components are placed. These are also the reasons for the tin beads. Therefore, it is necessary to strengthen the responsibility of operators and process personnel in the production process, strictly follow the process requirements and operating procedures for production, and strengthen the quality control of the process. 6.2 Problem of the slab (Manhattan phenomenon) One end of the chip component is soldered to the pad and the other end is erected. This phenomenon is called Manhattan phenomenon, see Figure 6.5. The main cause of this phenomenon is that the ends of the element are unevenly heated, and the solder paste is melted one after another. In the following cases, the two ends of the component will be unevenly heated: Figure 6.5 Phenomena Phenomena Figure 6.6 The component is offset from the pad, so the force imbalance on both sides produces a slab phenomenon.
(1) The component arrangement direction is not designed correctly. We envision a reflow line across the width of the furnace in the reflow oven that melts as soon as the solder paste passes through it, as shown in Figure 6.7. One end of the chip-shaped rectangular component is first passed through a reflow soldering limit line, the solder paste is first melted, completely immersing the metal surface of the component end, and has a liquid surface tension; and the other end does not reach the liquid phase temperature of 183 ° C, the solder paste is not melted, Only the adhesion of the flux, which is much less than the surface tension of the reflow solder paste, thus erecting the ends of the unmelted end up. Therefore, both ends of the component should be kept into the reflow limit line at the same time, so that the solder paste on both ends of the pad is simultaneously melted to form a balanced liquid surface tension, keeping the position of the component unchanged. Figure 6.7. The solder on one side of the pad is melted. If the tension of the two pads is unbalanced, a monument will appear.
(2) The preheating of the printed circuit board is insufficient when performing vapor phase welding. Vapor phase welding uses an inert liquid vapor to condense heat on the component leads and PCB pads to release the solder paste. In the vapor phase welding equilibrium zone and the saturated steam zone, the welding temperature in the saturated steam zone is as high as 217 °C. During the production process, we found that if the preheated component of the welded component is insufficient, it will undergo a temperature difference of more than 100 °C, and vaporization of the vapor phase welding. It is easy to float a chip component of less than 1206 package size, resulting in a slab phenomenon. We preheated the welded component in the high and low temperature chamber at 145 ~ 150 °C for 1-2 minutes, then preheated for about 1 min in the equilibrium zone of the vapor phase welding, and finally slowly entered the saturated steam zone for welding, eliminating the stand. Piece phenomenon.
(3) The impact of pad design quality. If the pair of pads of the chip component are different in size or asymmetry, the amount of solder paste printed may be inconsistent, the small pad has a fast response to temperature, the solder paste on the solder paste is easy to melt, and the large pad is opposite, so when After the solder paste on the small pad is melted, the component is straightened and erected under the surface tension of the solder paste. If the width or gap of the pad is too large, a slab phenomenon may occur. Pad design in strict accordance with standard specifications is a prerequisite for addressing this defect. 6.3 Bridge bridging is also one of the common defects in SMT production. It can cause short circuits between components and must be repaired when bridging occurs. Bridge the process of this happening.
(1) Solder paste quality problem The metal content in solder paste is high, especially after printing time is too long. It is prone to increase the metal content; the solder paste has low viscosity, and it flows to the outside of the pad after preheating; the solder paste has a poor slump, and the Han solder will be outside the pad after preheating, which will cause the IC pin to be bridged. The solution is to adjust the solder paste.
(2) Printing system printing machine has poor repeatability, misalignment, and solder paste printed outside the silver strip. This situation is more common in fine pitch QFP production; poor alignment of the steel plate and poor PCB alignment and steel window size/ The thickness design is not uniform with the PCB pad design, and the amount of solder paste is too large, which will cause bridging. The solution is to adjust the press to improve the PCB pad coating.
(3) The pressure of the placing and placing is too large, and the sinking of the solder paste is the most common cause in production. The Z-axis height should be adjusted. If the placement accuracy is not enough, the component is shifted and the IC pin is deformed, it should be improved for the reason.
(4) The preheating heating rate is too fast, and the solvent in the solder paste is not as volatile. 6.4 Suction/wicking phenomenon The wicking phenomenon, also known as core pulling phenomenon, is one of the common welding defects as shown in Figure 6.8, which is more common in vapor phase reflow soldering. The wicking phenomenon is that the solder detaches from the pad along the pin to the pin and the chip body, which causes a serious solder joint phenomenon. Figure 6.8 The cause of the wicking phenomenon is generally considered to be the thermal conductivity of the component leads. The temperature rises rapidly, so that the solder preferentially wets the pin, the wetting force between the solder and the pin is much greater than the wetting force between the solder and the pad, and the upturn of the pin further exacerbates the wicking phenomenon. In infrared reflow soldering, the organic flux in the PCB substrate and solder is an excellent absorption medium for infrared rays, while the pins can partially reflect infrared rays. In contrast, the solder preferentially melts, and its wetting force with the pad is greater than It has a wetting force with the pins, so the solder does not rise along the pins, and the probability of wicking is much smaller. The solution is: in the vapor phase reflow soldering, the SMA should be fully preheated before being placed in the vapor phase furnace; the solderability of the PCB board pads should be carefully checked and ensured, and the solderability of the PCB is not applied. Production; the coplanarity of components cannot be ignored, and devices with poor coplanarity are not used for production. 6.5 After soldering, the solder mask of the solder mask of the printed circuit board will show light green vesicles around the individual solder joints after soldering. In case of serious, the foam of the size of the fingernail will appear, which not only affects the appearance quality. In severe cases, it also affects performance and is one of the problems that often occur in the welding process. The root cause of solder mask blistering is the presence of gas/water vapor between the solder mask and the anode substrate. Traces of gas/water vapor are entrained into different processes, and when exposed to high temperatures, the gas expands, causing delamination of the solder mask and the anode substrate. Soldering, the pad temperature is relatively high, so the bubble first appears around the pad. Now the processing often needs to be cleaned, and then do the next process after drying. If it is etched, it should be dried and then adhered to the solder mask. At this time, if the drying temperature is not enough, the water vapor will be entrained into the next process. The storage environment before PCB processing is not good, the humidity is too high, and the welding is not dried in time. In the wave soldering process, the water-containing flux is often used. If the PCB preheating temperature is not enough, the water vapor in the flux will follow the through hole. The hole wall enters the inside of the PCB substrate, and the water vapor is first introduced around the pad, and these conditions generate bubbles after encountering the high temperature of the solder. The solution is: (1) All links should be strictly controlled, and the purchased PCB should be inspected and stored. Under normal conditions, foaming should not occur; (2) PCB should be stored in a ventilated and dry environment, and the storage period should not exceed 6 months; (3) PCB should be pre-baked in an oven at 105 °C/4h before welding. ~6h; 6.6 PCB distortion PCB distortion problem is a common problem in SMT production, it will have a considerable impact on assembly and testing, so this problem should be avoided in production, the reasons for PCB distortion are as follows Several kinds: (1) improper selection of raw materials for PCB itself, low Tg of PCB, especially paper-based PCB, the processing temperature is too high, which will make the PCB bend. (2) Unreasonable PCB design, uneven component distribution will cause excessive thermal stress on the PCB. Connectors and sockets with larger shapes will also affect the expansion and contraction of the PCB, and even permanent distortion. (3) Double-sided PCB, if one side of the copper foil remains too large (such as the ground wire), and the other side of the copper foil is too small, it will cause uneven deformation of both sides and deformation. (4) Excessive temperature in reflow soldering can also cause distortion of the PCB. For the above reasons, the solution is as follows: in the case of price and space, choose Tg high PCB or increase PCB thickness to obtain the best aspect ratio; rationally design PCB, double-sided steel foil area should be balanced, In the place where there is no circuit, it is covered with steel layer and appears in the form of network to increase the rigidity of the PCB. Preheat the PCB before the patch, the condition is 105 °C / 4h; adjust the fixture or clamping distance to ensure the PCB is protected. The space of thermal expansion; the temperature of the welding process is reduced as much as possible; when slight distortion has occurred, it can be placed in the positioning fixture, and the temperature is reset to release the stress, and generally satisfactory results are obtained. 6.7 After the IC pin is soldered, the open lead/virtual solder IC pin is soldered after soldering. It is a common soldering defect. There are many reasons for this. The main reason is that the coplanarity is poor, especially for QFP devices. Due to improper storage, the pins are deformed and sometimes difficult to be found (some of the placement machines do not check the coplanarity function), and the resulting process is shown in Figure 6.9. Figure 6.9 Components with poor coplanarity need to be soldered after soldering. Therefore, pay attention to the storage of the device. Do not take the components or open the package. Second, the pin solderability is not good. IC storage time is long, the pins are yellow, solderability is not good, it will cause solder joints. The solderability of components should be checked during production. Pay special attention to the storage period should not be too long (one year from the date of manufacture), custody It should be free from high temperature and high humidity, and do not open the bag casually. Third, the quality of the solder paste is poor, the metal content is low, and the solderability is poor. It is usually used for the solder paste for soldering of QFP devices, and the metal content should be no less than 90%. Fourth, the preheating temperature is too high, which may cause oxidation of the IC pin, which deteriorates the solderability. Fifth, the size of the template window is small, so that the amount of solder paste is not enough. Usually after the template is manufactured, the template window size should be carefully checked, not too large or too small, and pay attention to the PCB pad size. 6.8 Chip Component Cracking In SMC production, cracking of chip components is common in multilayer chip capacitors (MLCC), which are mainly caused by effect forces and mechanical stresses. (1) For MLCC type capacitors, there is a great vulnerability in its structure. Generally, MLCC is made up of multilayer ceramic capacitors, which has low strength and is extremely resistant to thermal and mechanical forces. (2) During the patching process, the z-axis suction and discharge height of the placement machine, especially some placement machines that do not have the z-axis soft landing function, the suction height is determined by the thickness of the chip component rather than by the pressure sensor. Therefore, the tolerance of the thickness of the component causes cracking. (3) The warp stress of the PCB, especially after soldering, the stress of the warp is likely to cause cracking of the component. (4) Some of the PCBs of the board will damage the components when they are divided. The prevention method is: carefully adjust the welding process curve, especially the temperature of the preheating zone should not be too low; the patch should be carefully adjusted to the suction and discharge height of the z-axis of the placement machine; the shape of the blade of the panel should be noted; the curvature of the PCB. In particular, the warpage after welding should be corrected. If it is a PCB quality problem, it should be considered. 6.9 Other common soldering defects (1) Poor wettability with poor wettability, which is manifested in poor soldering of PCB pads or poor soldering of component leads. Cause: The component lead PCB pad has been oxidized/contaminated; the reflow temperature is too high; the quality of the solder paste is poor. Both will result in poor wettability and severe soldering in severe cases. (2) The amount of tin is very small, and the amount of tin is small. The solder joint is not full, and the IC pin has a small meniscus. Cause: The printing template window is small; the wick phenomenon (poor temperature curve); the solder paste metal content is low. These will result in a small amount of tin and insufficient solder joint strength. (3) The damaged pin of the pin is damaged, which is characterized by poor coplanarity or bending of the device pin, which directly affects the soldering quality. Cause: It is damaged when transporting/picking. Care should be taken to store components, especially FQFP. (4) Contaminants cover the pad. Contaminants cover the pad and occur during production. Cause: Paper from the scene; foreign matter from the tape; hand touch PCB pads or components; the character map is not in the right position. Therefore, attention should be paid to the cleaning of the production site during production, and the process should be standardized. (5) The amount of solder paste is insufficient, and the amount of solder paste is insufficient, which often occurs in production. Cause: The first PCB printing / printing after the machine stops; the printing process parameters change; the steel plate window is blocked; the solder paste quality deteriorates. One of the above reasons will cause the tin volume to be insufficient, and the problem should be solved in a targeted manner. (6) The solder paste has an angular solder paste which is horny and often occurs in production, and is not easy to find and will be welded when it is severe. Cause: The lifting speed of the printing press is too fast; the wall of the template hole is not smooth, and the solder paste is easy to make the ingot shape. 7 Summary At present, a lot of researches have been done on lead-free soldering technology at home and abroad. The various lead-free solders proposed include Sn-Cu series, Sn-Ag-Cu series, Sn-Ag-Bi-Cu series, Sn-Bi. The series, Sn-Sb series, etc. have more in-depth research. The International Industrial Research Association and other electronic industry associations also have recommended process parameters for typical alloy materials such as the Sn-Ag-Cu series of alloys; some powerful companies are conducting trial and error research based on the results of this research. Process parameters are continually optimized to maximize the benefits. This topic refers to the domestic and foreign literature and related journals, selects the appropriate parameters; and selects the SMT related website to log out the market reflow soldering equipment that meets the process requirements to form a lead-free reflow process. Finally, theoretical analysis of the welding defects that may occur during the welding process is made, and a relative solution is proposed. This topic is the theoretical study of the process. Due to the lack of equipment, and because of the shallow and incomplete knowledge of my SMT, it is inevitable that delays will occur. I hope everyone will criticize and correct me. I am grateful.
Taking into account environmental and health factors, the EU has passed legislation to stop using lead-based solders in 2008, and the United States and Japan are actively considering legislation to reduce and prohibit the use of harmful elements such as lead. Lead poisoning At present, the annual consumption of solder in the global electronics industry is about 20,000 tons, which is about 5% of the world total lead production. Lead and lead compounds have been listed by the Environmental Protection Agency (EPA) as one of the top 17 chemicals that pose the greatest harm to humans and the environment. Lead-free solders commonly used lead-containing alloy solder powders are tin-lead (Sn-Pb), tin-lead-silver (Sn-Pb-Ag), tin-lead-bismuth (Sn-Pb-Bi), etc. The alloy composition was 63% Sn/37% Pb and 62% Sn/36% Pb/2% Ag. Different alloy ratios have different melting temperatures. For standard Sn63 and Sn62 solder alloys, the peak temperature of the reflow profile is between 203 and 230 degrees. However, most lead-free solder pastes have a melting point 30 to 45 degrees higher than that of Sn63 alloy. Therefore, the basic requirements for lead-free solders are currently internationally recognized as lead-free solders: Sn is the base and Ag is added. , Cu, Sb, In other alloying elements, and Pb mass fraction of 0.2% or less, mainly used for electronic assembly of solder alloys. Lead-free solder is not a new technology, but today's lead-free solder research is looking for alternatives to Sn-Pb solders that use between 50,000 and 60,000 tons per year. Therefore, alternative alloys should meet the following requirements:
(1) Its global reserves are sufficient to meet market demand. Certain elements, such as indium and antimony, have small reserves and can therefore only be used as trace additions in lead-free solders;
(2) Non-toxic. Some of the alternative elements under consideration, such as cadmium and strontium, are toxic. And some elements, such as cockroaches, can also be considered toxic if they change the toxicity criteria;
(3) It can be processed into all forms required, including wire for manual welding and repair; solder powder for solder paste; solder bar for wave soldering. Not all alloys can be processed into all forms, such as an increase in the content of niobium, which will cause the alloy to become brittle and not to be drawn into a filament;
(4) The phase transition temperature (solid/liquidus temperature) is similar to that of the Sn-Pb solder;
(5) suitable physical properties, in particular electrical conductivity, thermal conductivity, thermal expansion coefficient;
(6) Metallographically compatible with existing component substrates/leads and PCB materials;
(7) Sufficient mechanical properties: shear strength, creep resistance, isothermal fatigue resistance, thermal fatigue resistance, stability of metallographic structure;
(8) good wettability;
(9) Acceptable cost price.
The cost of the new lead-free solder should be less than 22.2/kg, so the mass fraction of In should be less than 1.5% and the Bi content should be less than 2.0%. Early R&D programs focused on identifying new alloy compositions, multiphase phase diagram studies, and basic properties such as wettability and strength. The later research and development program focused on five alloy series: SnCu, SnAg, SnAgCu, SnAgCuSb and SnAgBi. And in-depth discussion of its fatigue performance, production behavior and process optimization. Table 2.3 NCMS National Manufacturing Science Center's lead-free solder performance evaluation standard IPC also released the research report "A guide line for assembly of lead-free electronics" in June 2000.
At present, the main conclusions about lead-free solders in the world are as follows: There are many kinds of lead-free solders available, and none of them can provide a comprehensive solution for the direct replacement of SnPb solders.
(1) For certain special processes, certain specific lead-free solders can be directly replaced;
(2) At present, the most attractive lead-free solder is the Sn-Ag-Cu series. Other potential combinations include Sn-0.7Cu, Sn-3.5Ag, and Sn-Ag-Bi;
(3) There is currently no suitable lead-free substitute for high-lead and high-melting solder;
(4) It appears that the chemical system of the flux does not require major changes;
(5) The reliability of solder joints formed by lead-free solder is superior to that of SnPb alloy.
Comparison of several lead-free solders
(1) SnCu: the cheapest price; the highest melting point; the worst mechanical properties.
(2) SnAg: good mechanical properties, good weldability, good thermal fatigue reliability, and a melting point of 221 ° C for the eutectic composition. The difference between the SnAg and SnAgCu combinations is small, and the choice depends mainly on other factors such as price and supply.
(3) SnAgCu(Sb): It is only until recent years that there is a ternary eutectic between Sn-Ag-Cu, and its melting point is lower than that of Sn-Ag eutectic. Of course, the exact composition of the ternary eutectic is still controversial. . This combination is more reliable and solderable than Sn-Ag and Sn-Cu. Moreover, after adding 0.5% Sb, the high temperature reliability can be further improved.
(4) SnAgBi (Cu) (Ge): low melting point, 200 ~ 210 ° C; good reliability; the best solderability in all lead-free solders, has been confirmed by Matsushita; adding Cu or Ge can further improve the strength The disadvantage is that Bi contains a problem that the wetting angle rises.
(5) SnZnBi: The melting point is closest to the Sn-Pb eutectic; however, Zn contains many problems, such as solder paste shelf life, a large amount of active flux residue, oxidation problems, and potential corrosive problems. Currently not recommended. 2.2 Selecting the alloy from above, this reflow process design uses Sn/Ag/Cu alloy (Sn/Ag3.0/Cu0.5) as the alloy is considered to be the first choice in the international industry and has been awarded the Industrial and Research Association. Member's recommendation. Because some squads have proposed and studied another alloy, Sn/0.7Cu (mass percent), some companies also use this alloy in production. However, compared with the reliability and wettability of Sn/Cu alloys, and considering the use of the same alloys in reflow and wave soldering, Sn/Ag/Cu alloys have become the best choice for process development experiments. Sn/Ag3.0/Cu0.5 alloy properties: Dissolution temperature: solid phase line 217 ° C / liquidus 220 ° C; cost: 0.10 US dollars / cm 3 and Sn / Cu solder price ratio: 2.7 mechanical strength: 48 kg / mm 2 elongation : 75% Wettability: It is known from the properties of Sn/Ag/Cu alloy that the melting temperature of the solder alloy is 36 °C higher than that of the original Sn/Pb alloy, and the price after commercialization is also higher than the original. The process soldering temperature uses the recommended process curve for this alloy solder in Japan, as shown in Figure 2.1.
The typical process curve for lead-free reflow soldering recommended by Japan: There are three important points on the recommended process curve:
(1) The heating rate in the preheating zone should be as slow as possible (select the value 2~3°C/s) to control the bridging of solder joints, solder balls, etc. caused by the collapse of the solder paste.
(2) The preheating requirement must be within the range of (45 to 90 sec, 120 to 160 °C) to control the defects during reflow soldering due to factors such as temperature difference of the PCB substrate and changes in flux properties.
(3) The maximum temperature of soldering is above 230 °C for 20 to 30 sec to ensure the wettability of the solder. Cooling speed selection -4 °C / s 6 Defects in reflow soldering and their solutions Welding defects can be divided into major defects, minor defects and surface defects. The defects that invalidate the SMA function are called main defects; the secondary defects are that the wetting between the solder joints is good, and the SMA function is not lost, but there are possible defects that affect the product life; the surface defects are those that do not affect the product. Function and life. It is affected by many parameters such as solder paste, substrate, component solderability, printing, placement accuracy, and soldering process. In the SMT process research and production, we know that reasonable surface assembly process technology plays a vital role in controlling and improving the quality of SMT production.
Tin beads in reflow soldering
(1) Mechanism of formation of tin beads in reflow soldering Tin beads (or solder balls) appearing in reflow soldering are often hidden between the side of the soldered end of a rectangular chip component or between fine pitch pins, as shown in Figure 6.1. 6.2. During the component placement process, the solder paste is placed between the pins of the chip component and the pad. As the printed board passes through the reflow oven, the solder paste melts into a liquid, if with the pad and device pins. If the wetting is poor, the liquid solder will not fill the weld seam due to shrinkage, and all the solder particles cannot be aggregated into one solder joint. Part of the liquid solder will flow out of the weld to form tin beads. Therefore, poor solder wettability with pads and device leads is the root cause of solder ball formation. Figure 6.1 One example of a chip component has a slightly larger particle size. Figure 6.2 A solder ball solder paste is scattered around the pin. In the printing process, if the template is offset from the pad, if the offset is too large, the pan will be caused. The paste flows over the pad and is prone to solder beads after heating. The pressure of the Z-axis during the patching process is an important cause of tin beads, which is often not noticed by people. Some of the placement machines are positioned according to the thickness of the components. Therefore, the phenomenon that the components are attached to the PCB and the tin buds are squeezed out of the pad for a moment, this part of the group will obviously cause the tin beads. The size of the tin beads produced in this case is slightly larger, and it is usually possible to prevent the generation of tin beads by simply adjusting the Z uranium height.
(2) There are many reasons for the poor solder wettability caused by the cause analysis and control methods. The following mainly analyze the causes and solutions related to the related processes:
(1) The reflow temperature curve is not set properly. The reflow of solder paste is related to temperature and time. If sufficient temperature or time is not reached, the solder paste will not reflow. The temperature rise in the preheating zone is too fast, and the time is too short, so that the moisture and solvent inside the solder paste are not completely volatilized. When reaching the reflow soldering temperature zone, the water and the solvent boil and the tin beads are splashed. Practice has proved that it is desirable to control the rising speed of the preheating zone temperature to 1 to 4 ° C / s.
(2) If tin beads are always present at the same location, it is necessary to check the metal template design structure. The stencil opening size corrosion accuracy is not up to standard, the pad size is too large, and the surface material is soft (such as copper stencil), which will cause the outline of the printed solder paste to be unclear and bridged with each other. When the pad of the device is printed, reflow soldering will inevitably result in a large amount of solder balls between the pins. Therefore, appropriate template materials and template fabrication processes should be selected for the different shapes and center distances of the land patterns to ensure the quality of the solder paste.
(3) If the time from patch to reflow is too long, the solder particles will deteriorate and the activity will decrease due to oxidation of solder particles in the solder paste, which will cause the solder paste to not reflow and produce solder beads. Selecting a solder paste with a longer working life (we think at least 4h) will alleviate this effect.
(4) In addition, the solder paste misprinted printed board is not sufficiently cleaned, and the solder paste remains on the surface of the printed board and through holes. Before the reflow soldering, the printed solder paste is deformed when the components are placed. These are also the reasons for the tin beads. Therefore, it is necessary to strengthen the responsibility of operators and process personnel in the production process, strictly follow the process requirements and operating procedures for production, and strengthen the quality control of the process. 6.2 Problem of the slab (Manhattan phenomenon) One end of the chip component is soldered to the pad and the other end is erected. This phenomenon is called Manhattan phenomenon, see Figure 6.5. The main cause of this phenomenon is that the ends of the element are unevenly heated, and the solder paste is melted one after another. In the following cases, the two ends of the component will be unevenly heated: Figure 6.5 Phenomena Phenomena Figure 6.6 The component is offset from the pad, so the force imbalance on both sides produces a slab phenomenon.
(1) The component arrangement direction is not designed correctly. We envision a reflow line across the width of the furnace in the reflow oven that melts as soon as the solder paste passes through it, as shown in Figure 6.7. One end of the chip-shaped rectangular component is first passed through a reflow soldering limit line, the solder paste is first melted, completely immersing the metal surface of the component end, and has a liquid surface tension; and the other end does not reach the liquid phase temperature of 183 ° C, the solder paste is not melted, Only the adhesion of the flux, which is much less than the surface tension of the reflow solder paste, thus erecting the ends of the unmelted end up. Therefore, both ends of the component should be kept into the reflow limit line at the same time, so that the solder paste on both ends of the pad is simultaneously melted to form a balanced liquid surface tension, keeping the position of the component unchanged. Figure 6.7. The solder on one side of the pad is melted. If the tension of the two pads is unbalanced, a monument will appear.
(2) The preheating of the printed circuit board is insufficient when performing vapor phase welding. Vapor phase welding uses an inert liquid vapor to condense heat on the component leads and PCB pads to release the solder paste. In the vapor phase welding equilibrium zone and the saturated steam zone, the welding temperature in the saturated steam zone is as high as 217 °C. During the production process, we found that if the preheated component of the welded component is insufficient, it will undergo a temperature difference of more than 100 °C, and vaporization of the vapor phase welding. It is easy to float a chip component of less than 1206 package size, resulting in a slab phenomenon. We preheated the welded component in the high and low temperature chamber at 145 ~ 150 °C for 1-2 minutes, then preheated for about 1 min in the equilibrium zone of the vapor phase welding, and finally slowly entered the saturated steam zone for welding, eliminating the stand. Piece phenomenon.
(3) The impact of pad design quality. If the pair of pads of the chip component are different in size or asymmetry, the amount of solder paste printed may be inconsistent, the small pad has a fast response to temperature, the solder paste on the solder paste is easy to melt, and the large pad is opposite, so when After the solder paste on the small pad is melted, the component is straightened and erected under the surface tension of the solder paste. If the width or gap of the pad is too large, a slab phenomenon may occur. Pad design in strict accordance with standard specifications is a prerequisite for addressing this defect. 6.3 Bridge bridging is also one of the common defects in SMT production. It can cause short circuits between components and must be repaired when bridging occurs. Bridge the process of this happening.
(1) Solder paste quality problem The metal content in solder paste is high, especially after printing time is too long. It is prone to increase the metal content; the solder paste has low viscosity, and it flows to the outside of the pad after preheating; the solder paste has a poor slump, and the Han solder will be outside the pad after preheating, which will cause the IC pin to be bridged. The solution is to adjust the solder paste.
(2) Printing system printing machine has poor repeatability, misalignment, and solder paste printed outside the silver strip. This situation is more common in fine pitch QFP production; poor alignment of the steel plate and poor PCB alignment and steel window size/ The thickness design is not uniform with the PCB pad design, and the amount of solder paste is too large, which will cause bridging. The solution is to adjust the press to improve the PCB pad coating.
(3) The pressure of the placing and placing is too large, and the sinking of the solder paste is the most common cause in production. The Z-axis height should be adjusted. If the placement accuracy is not enough, the component is shifted and the IC pin is deformed, it should be improved for the reason.
(4) The preheating heating rate is too fast, and the solvent in the solder paste is not as volatile. 6.4 Suction/wicking phenomenon The wicking phenomenon, also known as core pulling phenomenon, is one of the common welding defects as shown in Figure 6.8, which is more common in vapor phase reflow soldering. The wicking phenomenon is that the solder detaches from the pad along the pin to the pin and the chip body, which causes a serious solder joint phenomenon. Figure 6.8 The cause of the wicking phenomenon is generally considered to be the thermal conductivity of the component leads. The temperature rises rapidly, so that the solder preferentially wets the pin, the wetting force between the solder and the pin is much greater than the wetting force between the solder and the pad, and the upturn of the pin further exacerbates the wicking phenomenon. In infrared reflow soldering, the organic flux in the PCB substrate and solder is an excellent absorption medium for infrared rays, while the pins can partially reflect infrared rays. In contrast, the solder preferentially melts, and its wetting force with the pad is greater than It has a wetting force with the pins, so the solder does not rise along the pins, and the probability of wicking is much smaller. The solution is: in the vapor phase reflow soldering, the SMA should be fully preheated before being placed in the vapor phase furnace; the solderability of the PCB board pads should be carefully checked and ensured, and the solderability of the PCB is not applied. Production; the coplanarity of components cannot be ignored, and devices with poor coplanarity are not used for production. 6.5 After soldering, the solder mask of the solder mask of the printed circuit board will show light green vesicles around the individual solder joints after soldering. In case of serious, the foam of the size of the fingernail will appear, which not only affects the appearance quality. In severe cases, it also affects performance and is one of the problems that often occur in the welding process. The root cause of solder mask blistering is the presence of gas/water vapor between the solder mask and the anode substrate. Traces of gas/water vapor are entrained into different processes, and when exposed to high temperatures, the gas expands, causing delamination of the solder mask and the anode substrate. Soldering, the pad temperature is relatively high, so the bubble first appears around the pad. Now the processing often needs to be cleaned, and then do the next process after drying. If it is etched, it should be dried and then adhered to the solder mask. At this time, if the drying temperature is not enough, the water vapor will be entrained into the next process. The storage environment before PCB processing is not good, the humidity is too high, and the welding is not dried in time. In the wave soldering process, the water-containing flux is often used. If the PCB preheating temperature is not enough, the water vapor in the flux will follow the through hole. The hole wall enters the inside of the PCB substrate, and the water vapor is first introduced around the pad, and these conditions generate bubbles after encountering the high temperature of the solder. The solution is: (1) All links should be strictly controlled, and the purchased PCB should be inspected and stored. Under normal conditions, foaming should not occur; (2) PCB should be stored in a ventilated and dry environment, and the storage period should not exceed 6 months; (3) PCB should be pre-baked in an oven at 105 °C/4h before welding. ~6h; 6.6 PCB distortion PCB distortion problem is a common problem in SMT production, it will have a considerable impact on assembly and testing, so this problem should be avoided in production, the reasons for PCB distortion are as follows Several kinds: (1) improper selection of raw materials for PCB itself, low Tg of PCB, especially paper-based PCB, the processing temperature is too high, which will make the PCB bend. (2) Unreasonable PCB design, uneven component distribution will cause excessive thermal stress on the PCB. Connectors and sockets with larger shapes will also affect the expansion and contraction of the PCB, and even permanent distortion. (3) Double-sided PCB, if one side of the copper foil remains too large (such as the ground wire), and the other side of the copper foil is too small, it will cause uneven deformation of both sides and deformation. (4) Excessive temperature in reflow soldering can also cause distortion of the PCB. For the above reasons, the solution is as follows: in the case of price and space, choose Tg high PCB or increase PCB thickness to obtain the best aspect ratio; rationally design PCB, double-sided steel foil area should be balanced, In the place where there is no circuit, it is covered with steel layer and appears in the form of network to increase the rigidity of the PCB. Preheat the PCB before the patch, the condition is 105 °C / 4h; adjust the fixture or clamping distance to ensure the PCB is protected. The space of thermal expansion; the temperature of the welding process is reduced as much as possible; when slight distortion has occurred, it can be placed in the positioning fixture, and the temperature is reset to release the stress, and generally satisfactory results are obtained. 6.7 After the IC pin is soldered, the open lead/virtual solder IC pin is soldered after soldering. It is a common soldering defect. There are many reasons for this. The main reason is that the coplanarity is poor, especially for QFP devices. Due to improper storage, the pins are deformed and sometimes difficult to be found (some of the placement machines do not check the coplanarity function), and the resulting process is shown in Figure 6.9. Figure 6.9 Components with poor coplanarity need to be soldered after soldering. Therefore, pay attention to the storage of the device. Do not take the components or open the package. Second, the pin solderability is not good. IC storage time is long, the pins are yellow, solderability is not good, it will cause solder joints. The solderability of components should be checked during production. Pay special attention to the storage period should not be too long (one year from the date of manufacture), custody It should be free from high temperature and high humidity, and do not open the bag casually. Third, the quality of the solder paste is poor, the metal content is low, and the solderability is poor. It is usually used for the solder paste for soldering of QFP devices, and the metal content should be no less than 90%. Fourth, the preheating temperature is too high, which may cause oxidation of the IC pin, which deteriorates the solderability. Fifth, the size of the template window is small, so that the amount of solder paste is not enough. Usually after the template is manufactured, the template window size should be carefully checked, not too large or too small, and pay attention to the PCB pad size. 6.8 Chip Component Cracking In SMC production, cracking of chip components is common in multilayer chip capacitors (MLCC), which are mainly caused by effect forces and mechanical stresses. (1) For MLCC type capacitors, there is a great vulnerability in its structure. Generally, MLCC is made up of multilayer ceramic capacitors, which has low strength and is extremely resistant to thermal and mechanical forces. (2) During the patching process, the z-axis suction and discharge height of the placement machine, especially some placement machines that do not have the z-axis soft landing function, the suction height is determined by the thickness of the chip component rather than by the pressure sensor. Therefore, the tolerance of the thickness of the component causes cracking. (3) The warp stress of the PCB, especially after soldering, the stress of the warp is likely to cause cracking of the component. (4) Some of the PCBs of the board will damage the components when they are divided. The prevention method is: carefully adjust the welding process curve, especially the temperature of the preheating zone should not be too low; the patch should be carefully adjusted to the suction and discharge height of the z-axis of the placement machine; the shape of the blade of the panel should be noted; the curvature of the PCB. In particular, the warpage after welding should be corrected. If it is a PCB quality problem, it should be considered. 6.9 Other common soldering defects (1) Poor wettability with poor wettability, which is manifested in poor soldering of PCB pads or poor soldering of component leads. Cause: The component lead PCB pad has been oxidized/contaminated; the reflow temperature is too high; the quality of the solder paste is poor. Both will result in poor wettability and severe soldering in severe cases. (2) The amount of tin is very small, and the amount of tin is small. The solder joint is not full, and the IC pin has a small meniscus. Cause: The printing template window is small; the wick phenomenon (poor temperature curve); the solder paste metal content is low. These will result in a small amount of tin and insufficient solder joint strength. (3) The damaged pin of the pin is damaged, which is characterized by poor coplanarity or bending of the device pin, which directly affects the soldering quality. Cause: It is damaged when transporting/picking. Care should be taken to store components, especially FQFP. (4) Contaminants cover the pad. Contaminants cover the pad and occur during production. Cause: Paper from the scene; foreign matter from the tape; hand touch PCB pads or components; the character map is not in the right position. Therefore, attention should be paid to the cleaning of the production site during production, and the process should be standardized. (5) The amount of solder paste is insufficient, and the amount of solder paste is insufficient, which often occurs in production. Cause: The first PCB printing / printing after the machine stops; the printing process parameters change; the steel plate window is blocked; the solder paste quality deteriorates. One of the above reasons will cause the tin volume to be insufficient, and the problem should be solved in a targeted manner. (6) The solder paste has an angular solder paste which is horny and often occurs in production, and is not easy to find and will be welded when it is severe. Cause: The lifting speed of the printing press is too fast; the wall of the template hole is not smooth, and the solder paste is easy to make the ingot shape. 7 Summary At present, a lot of researches have been done on lead-free soldering technology at home and abroad. The various lead-free solders proposed include Sn-Cu series, Sn-Ag-Cu series, Sn-Ag-Bi-Cu series, Sn-Bi. The series, Sn-Sb series, etc. have more in-depth research. The International Industrial Research Association and other electronic industry associations also have recommended process parameters for typical alloy materials such as the Sn-Ag-Cu series of alloys; some powerful companies are conducting trial and error research based on the results of this research. Process parameters are continually optimized to maximize the benefits. This topic refers to the domestic and foreign literature and related journals, selects the appropriate parameters; and selects the SMT related website to log out the market reflow soldering equipment that meets the process requirements to form a lead-free reflow process. Finally, theoretical analysis of the welding defects that may occur during the welding process is made, and a relative solution is proposed. This topic is the theoretical study of the process. Due to the lack of equipment, and because of the shallow and incomplete knowledge of my SMT, it is inevitable that delays will occur. I hope everyone will criticize and correct me. I am grateful.
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| Brand Name | HP |
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| Life test | CSA B125-98 ASSE 1016(<=100,000 cycles) |
| Outflow temperature Auto-adjusting sensitivity |
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| EN1111 AS-4032.1,cUPC | |
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