Considering the environmental and health factors, the European Union has passed legislation to stop using lead-containing solder in 2008, and the United States and Japan are also actively considering adopting legislation to reduce and prohibit the use of harmful elements such as lead. The Poison of Lead At present, the solder used in the global electronic industry consumes about 20,000 tons of lead every year, accounting for about 5% of the world's annual lead production. Lead and lead compounds have been listed as one of the 65,438+07 chemicals that are most harmful to human body and environment by the US Environmental Protection Agency (EPA). At present, lead-containing alloy solder powders commonly used for lead-free solder include Sn-Pb, Sn-Pb-Ag, Sn-Pb-Bi, etc. Commonly used alloy compositions are 63%Sn/37%Pb and 62%Sn/36%Pb/2%Ag. Different alloy proportions have different melting temperatures. For standard Sn63 and Sn62 solder alloys, the peak temperature of the reflow temperature curve is between 203 and 230 degrees. However, the melting point of most lead-free solder pastes is 30 to 45 degrees higher than that of Sn63 alloy. Therefore, the basic requirements of lead-free solder are internationally recognized as: Sn-based solder alloy with other alloying elements such as Ag, Cu, Sb and In, and the mass fraction of Pb is below 0.2%. Lead-free solder is not a new technology, but today's lead-free solder research is to find a substitute for Sn-Pb solder with annual consumption of 50 ~ 60 thousand tons. Therefore, the replacement alloy should meet the following requirements:
(1) Its global reserves are sufficient to meet the market demand. Some elements, such as indium and bismuth, have small reserves and can only be used as trace additives in lead-free solder;
(2) non-toxic. Some substitute elements, such as cadmium and tellurium, are toxic. Some elements, such as antimony, can also be considered toxic if the toxicity standard is changed;
(3) It can be processed into all required shapes, including welding wires for manual welding and repair; Solder powder for solder paste; Welding rods used for wave soldering, etc. Not all alloys can be processed into all shapes, for example, the increase of bismuth content will cause the alloy to become brittle and cannot be drawn into filaments;
(4) The phase transition temperature (solid/liquidus temperature) is similar to that of Sn-Pb solder;
(5) Appropriate physical properties, especially electrical conductivity, thermal conductivity and thermal expansion coefficient;
(6) Compatibility with existing component substrates/leads and PCB materials in metallurgical properties;
(7) Sufficient mechanical properties: shear strength, creep resistance, isothermal fatigue resistance, thermal engine fatigue resistance and stability of metallographic structure;
(8) Good wettability;
(9) acceptable cost price.
The cost of 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 content of Bi should be less than 2.0%. The early R&D plan focused on the determination of new alloy composition, the study of multiphase diagram and the investigation of basic properties such as wettability and strength. The later research and development plan mainly focuses on five alloy series: SnCu, SnAg, SnAgCu, SnAgCuSb and SnAgBi. The fatigue performance, production behavior and process optimization are discussed in depth. Table 2.3 IPC, the performance evaluation standard of lead-free solder proposed by NCMS National Manufacturing Science Center, also released the research report of "Lead-free Electronic Product Assembly Guide" in June 2000.
At present, the main conclusions about lead-free solder in the world are as follows: There are many kinds of existing lead-free solder, and none of them can provide a comprehensive solution for the direct replacement of SnPb solder.
(1) For some special processes, some specific lead-free solders can be directly replaced;
(2) At present, the most attractive lead-free solder is Sn-Ag-Cu series. Other possible combinations include Sn-0.7Cu, Sn-3.5Ag and sn-ag-bi;
(3) There is no suitable lead-free substitute for high lead and high melting point solder at present;
(4) At present, the chemical system of flux does not need to be greatly changed;
(5) The reliability of solder joint formed by lead-free solder is better than that of SnPb alloy.
Comparison of several lead-free solders
(1)SnCu: The price is the cheapest; Maximum melting point; The mechanical properties are the worst.
(2)SnAg: good mechanical properties, good weldability and good thermal fatigue reliability. * * * The melting point of the crystal is 2265438 0℃. The difference between SnAg and SnAgCu combination is very small, and its choice mainly depends on other factors such as price and supply.
(3)SnAgCu(Sb): In recent years, it has been known that there is ternary * * * crystal between Sn-Ag-Cu, and its melting point is lower than that of Sn-Ag*** crystal. Of course, the exact composition of this ternary crystal is still controversial. Compared with tin silver and tin copper, this combination has better reliability and weldability. In addition, the high temperature reliability can be further improved by adding 0.5%Sb.
(4)SnAgBi(Cu)(Ge): low melting point, 200 ~ 210℃; Good reliability; Solderability is the best among all lead-free solders, which has been confirmed by Panasonic. Adding Cu or Ge can further improve the strength; The disadvantage is that it contains the problem of rising wetting angle caused by Bi.
(5)SnZnBi: Sn-Pb*** crystal with the closest melting point; However, containing Zn brings many problems, such as the shelf life of solder paste, a large number of active solder residues, oxidation problems and potential corrosion problems. Not recommended at present. 2.2 alloy selection: Sn/Ag/Cu alloy (Sn/Ag3.0/Cu0.5) is adopted in this reflow soldering process design, because this alloy is considered as the first choice of international industry and has been recommended by members of industrial and research associations. Because although some guilds have also proposed and studied another alloy Sn/0.7Cu (mass percentage), some enterprises have adopted this alloy in production. However, compared with the reliability and wettability of Sn/Cu alloy, considering that reflow soldering and wave soldering use the same alloy, Sn/Ag/Cu alloy has become the best choice for process development test. Properties of Sn/Ag3.0/Cu0.5 alloy: Dissolution temperature: solidus 2 17℃/ liquidus 220℃; Cost: 0. 10 USD /cm3 and Sn/Cu solder price ratio: 2.7 mechanical strength: 48kg/mm2 elongation: 75% wettability: according to the properties of Sn/Ag/Cu alloy, the melting temperature of solder alloy is 36℃ higher than that of the original Sn/Pb alloy, and the price after commercialization is also higher than that of the original. The process welding temperature adopts the process curve recommended by Japan for this alloy solder, as shown in Figure 2. 1.
The typical process curve of lead-free reflow soldering recommended by Japan shows that there are three main points in the recommended process curve:
(1) The heating rate in the preheating zone should be as slow as possible (choose the value of 2 ~ 3℃/s) to control the bridge between solder joint and solder ball caused by solder paste edge collapse.
(2) The preheating requirement must be within the range of (45 ~ 90 seconds, 120 ~ 160℃) to control the defects in reflow soldering caused by the temperature difference of PCB substrate and the change of flux performance.
(3) The highest welding temperature is above 230℃ and kept for 20 ~ 30 seconds to ensure the wettability of welding. Cooling rate selection -4℃/s 6 Defects in reflow soldering and their solutions Welding defects can be divided into main defects, secondary defects and surface defects. Any defect that invalidates the function of shape memory alloy is called main defect; Secondary defect refers to the defect that the wettability between solder joints is good, which will not cause the loss of SMA function, but may affect the product life; Surface defect means that it does not affect the function and life of the product. It is influenced by many parameters such as solder paste, substrate, solderability of components, printing, mounting accuracy, welding process and so on. In the research and production of SMT technology, we know that reasonable surface assembly technology plays a vital role in controlling and improving the quality of SMT production.
Tin Beads in Reflow Welding
Formation mechanism of solder balls in (1) reflow soldering. Solder balls (or solder balls) appearing in reflow soldering are often hidden between the edges of two welding ends of rectangular chip components or between fine-pitch pins, as shown in Figure 6. 1 and 6.2. In the process of component installation, solder paste is placed between the pins and pads of chip components. When the printed circuit board passes through the reflow oven, the solder paste melts and becomes liquid. If it does not wet well with the pad and the device pin, the liquid solder will shrink, the weld will not be completely filled, and all solder particles will not aggregate into solder joints. Part of the liquid solder will flow out of the weld to form weld beads. Therefore, the poor wettability of solder with pads and device pins is the fundamental reason for the formation of solder beads. Figure 6. 1 An example of a slightly larger chip component. Figure 6.2 is not just solder balls and solder paste scattered around the pins. In the printing process, due to the deviation between the template and the pad, if the deviation is too large, the pot paste will overflow the pad, and tin balls will easily appear after heating. The pressure of Z-axis in the process of mounting is an important cause of tin beads, which is often ignored by people. Because the Z uranium head is located according to the thickness of components, some mounters will cause tin buds to be squeezed out of the pads at the moment when components are attached to PCB. This grouping obviously leads to tin beads. In this case, the size of the produced tin beads is slightly larger. Usually, tin beads can be prevented by readjusting the height of Z uranium.
(2) Cause analysis and control methods There are many reasons for the poor wettability of solder. The following mainly analyzes the reasons and solutions related to related processes:
(1) The reflux temperature curve is set incorrectly. The reflow of solder paste is related to temperature and time. If the temperature or time is not enough, the solder paste will not reflow. The temperature in the preheating zone rises too fast and the time is too short, which makes the moisture and solvent in the solder paste not completely volatilize. When it reaches the reflow area, it will cause water and solvent to boil and splash solder beads. Practice has proved that it is ideal to control the heating rate of preheating zone at1~ 4℃/s.
(2) If the solder balls always appear in the same position, it is necessary to check the design structure of the metal template. The corrosion accuracy of the template opening size can not meet the requirements, the pad size is too large, and the surface material is soft (such as copper template), which will cause the outline of printed solder paste to be unclear and bridge each other. This situation often occurs in pad printing of fine pitch devices, and a large number of solder balls will inevitably be produced between pins after reflow soldering. Therefore, according to the different shapes and center distances of pad patterns, appropriate template materials and template manufacturing processes should be selected to ensure the printing quality of solder paste.
(3) If the time from mounting to reflow soldering is too long, the solder particles in the solder paste will be oxidized, the flux will deteriorate, and the activity will be reduced, resulting in solder paste not reflow soldering and tin beads. Choosing a solder paste with a longer working life (we think it is at least 4 hours) will alleviate this influence.
(4) In addition, insufficient cleaning of printed boards printed with solder paste will leave solder paste on the surface of printed boards and through holes. Before reflow soldering, the printed solder paste is deformed when the component is attached. These are also the causes of tin beads. Therefore, in the production process, it is necessary to strengthen the sense of responsibility of operators and technicians, carry out production in strict accordance with process requirements and operating procedures, and strengthen the quality control of the process. 6.2 Vertical Chip Problem (Manhattan Phenomenon) One end of the chip assembly is soldered on the pad, and the other end is inclined. This phenomenon is called Manhattan phenomenon, as shown in Figure 6.5. The main reason for this phenomenon is that the two ends of the component are heated unevenly and the solder paste melts one after another. Under the following circumstances, the heating at both ends of the member will be uneven: Figure 6.5 Vertical phenomenon Figure 6.6 The member deviates from the pad, so the stress on both sides is unbalanced, resulting in vertical phenomenon.
(1) The arrangement direction of components is not designed correctly. We assume that there is a reflow limit line on the width of the reflow oven, and the solder paste will melt immediately once it passes, as shown in Figure 6.7. One end of the chip-type rectangular component first passes through the reflow solder limit line, and the solder paste melts first, completely infiltrating the metal surface of the component end and having liquid surface tension; While that other end doe not reach the liquid phase temperature of 183℃, so the solder paste doe not melt, and only the adhesive force of the flux is far less than the surface tension of the reflowed solder paste, so that the component end at the unmelted end stands upright. Therefore, both ends of the component should be kept within the reflow limit line at the same time, so that the solder paste on the pads at both ends can be melted at the same time, forming a balanced liquid surface tension and keeping the component position unchanged. Figure 6.7 The tin green powder on one side of the pad melts. When the tension between the two pads is unbalanced, a monument will appear.
(2) In the process of gas phase welding, the preheating of printed circuit components is insufficient. Gas phase welding melts solder paste by releasing heat when inert liquid vapor condenses on component pins and PCB pads. Gas phase welding is divided into equilibrium zone and saturated steam zone. In the saturated steam zone, the welding temperature is as high as 265438 07℃. In the production process, we found that if the components to be soldered are not fully preheated and are subjected to temperature difference changes above 65,438+000℃, the vaporization force of gas phase welding can easily make the chip components with package size less than 65,438+0206 float, resulting in the phenomenon of vertical chip. We preheat the welded components in the high and low temperature box at the temperature of 145 ~ 150℃ for 1 ~ 2 min, then preheat them in the equilibrium zone of gas phase welding for about 1min, and finally slowly enter the saturated steam zone for welding, thus eliminating the vertical phenomenon.
(3) The influence of pad design quality. If the size of a pair of pads of chip components is different or asymmetrical, the amount of printed solder paste will also be inconsistent. Small pads react quickly to temperature, and the solder paste on them is easy to melt, while large pads are the opposite. Therefore, after the solder paste on the small pad melts, the component will straighten and stand up under the surface tension of the solder paste. The width or gap of the pad is too large, and the vertical piece phenomenon may also occur. The premise of solving this defect is to design the pad strictly according to the standard specification. 6.3 Bridging is also one of the common defects in SMT production, which will cause short circuit between components and must be repaired when bridging occurs. The process of connection.
(1) Quality problems of solder paste The content of metal in solder paste is high, especially when the printing time is long, it is easy to increase. The solder paste has low viscosity and overflows the pad after preheating; The slump of solder paste is poor, and Han overflows after preheating, which will lead to IC pin bridging. The solution is to adjust the solder paste.
(2) The printing system has poor repeatability and uneven alignment, and the solder paste is printed outside the silver bars, which is common in the production of fine pitch QFP; Poor alignment between steel plate and PCB, improper design of steel plate window size/thickness and uneven alloy coating on PCB pad design will all lead to excessive solder paste and bridge. The solution is to adjust the printing machine and improve the coating of PCB pads.
(3) The bonding pressure is too high, and the leaching of solder paste after pressing is a common reason in production, so the height of Z axis should be adjusted. If the accuracy of the patch is not enough, it will cause the components to shift and the IC pins to deform, so it is necessary to improve according to the reasons.
(4) The preheating temperature is too fast, and the solvent in the solder paste cannot be volatilized. 6.4 Suction/Wicking Phenomenon 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 gas phase reflow welding. Capillary phenomenon is that solder leaves the pad and travels between the pin and the chip body along the pin, which will form a serious virtual soldering phenomenon. Fig. 6.8 The reason for the wicking phenomenon is generally believed to be that the thermal conductivity of the component pins is high and the temperature rises rapidly, so that the solder wets the pins first. The wetting force between the solder and the pins is much greater than that between the solder and the pads, and the upward inclination of the pins will aggravate the wicking phenomenon. In infrared reflow soldering, organic flux in PCB substrate and solder is an excellent absorbing medium for infrared rays, but lead can partially reflect infrared rays. Comparatively speaking, the solder melts first, and its wetting force with the pad is greater than that with the lead, so the solder will not rise along the lead, and the probability of capillary phenomenon is much smaller. The solutions are as follows: when the SMA is reflowed in gas phase, it should be fully preheated before being put into the gas phase furnace; Carefully check and ensure the solderability of PCB pads, and PCB with poor solderability shall not be used for production; The flatness of components can not be ignored, and devices with poor flatness are not suitable for production. 6.5 After welding, light green bubbles will appear around individual solder joints. In severe cases, bubbles with the size of fingernails will appear, which will not only affect the appearance quality, but also affect the performance in severe cases. This is one of the common problems in the welding process. The fundamental reason for the foaming of solder mask is that there is gas/water vapor between solder mask and anode substrate. A small amount of gas/water vapor will be entrained in different technological processes. When encountering high temperature, the gas will expand, resulting in delamination between the solder mask and the anode substrate. Welding, the temperature of the pad is relatively high, so bubbles appear around the pad first. At present, it is often necessary to clean and dry before the next working procedure. For example, after etching, the solder mask should be pasted after drying. At this time, if the drying temperature is not enough, water vapor will be carried into the next process. The storage environment of PCB before processing is not good, the humidity is too high, and it is not dried in time when welding; In the process of wave soldering, aqueous flux is often used. If the preheating temperature of PCB is not enough, the water vapor in the flux will enter the PCB substrate along the hole wall of the through hole, and the water vapor will first enter around the pad, and bubbles will be generated after encountering the high welding temperature. The solution is; (1) All links should be strictly controlled, and the purchased PCB should be inspected and put into storage. Generally, under standard conditions, there should be no foaming phenomenon; (2) PCBs should be stored in a ventilated and dry environment for no more than 6 months; (3) Before soldering, the PCB shall be pre-baked in an oven at105℃ for 4h ~ 6h; 6.6 PCB distortion PCB distortion is a common problem in SMT mass production, which will have a considerable impact on assembly and testing. Therefore, we should try to avoid this problem in production. The reasons for PCB deformation are: (1) improper selection of raw materials for PCB itself, low Tg of PCB, especially for paper-based PCB, which will bend due to its high processing temperature. (2) Unreasonable PCB design and uneven distribution of components will cause excessive thermal stress of PCB, and connectors and sockets with larger shapes will also affect the expansion and contraction of PCB, even cause permanent deformation. (3) Double-sided PCB, if one copper foil is too big (such as ground wire) and the other copper foil is too small, it will cause uneven shrinkage and deformation on both sides. (4) Too high reflow temperature will also cause PCB deformation. In view of the above reasons, the solutions are as follows: if the price and space allow, choose PCB with high Tg or increase the thickness of PCB to obtain the best aspect ratio; The PCB board should be designed reasonably, the areas of steel foil on both sides should be balanced, the places without circuits should be covered with steel layers, which appear in the form of a net to increase the rigidity of the PCB board, and the PCB board should be preheated before installation, provided that105℃/4h; Adjust the fixture or clamping distance to ensure the space for PCB to expand when heated; Welding process temperature should be as low as possible; When there is slight deformation, it can be put into the positioning fixture, and the temperature is raised to release the stress, which will generally achieve satisfactory results. 6.7 Open circuit after pin welding/virtual welding After IC pin welding, some pins appear virtual welding, which is a common welding defect. There are many reasons. The main reason is that the flatness of * * * is poor, especially for QFP devices. Due to improper storage, the pins are deformed and sometimes difficult to find (some mounters do not have the function of checking the flatness of * * *), and the process is shown in Figure 6.9. Figure 6.9 *** Components with poor flatness need to be welded after welding. Therefore, attention should be paid to the storage of components, and components should not be taken away or unpacked. Second, the solderability of the pin is not good. If the IC is stored for a long time, the lead will turn yellow, and poor solderability will also lead to virtual soldering. Check the weldability of components during production, and pay special attention to the fact that the storage period should not be too long (within one year from the date of production), and the packaging bag should not be opened casually during storage. Third, the quality of solder paste is poor, the metal content is low and the solderability is poor. Usually, the solder paste used for soldering QFP devices should have a metal content of not less than 90%. Fourthly, the preheating temperature is too high, which is easy to cause IC pin oxidation and make solderability worse. Fifth, the size of the template window is small, which leads to insufficient solder paste. Usually, after the template is made, the size of the template window should be carefully checked, which should not be too large or too small, and should match the size of the PCB pad. 6.8 Cracking of Chip Components In SMC production, cracking of chip components is common in multilayer chip capacitors (MLCC), which is mainly caused by acting force and mechanical stress. (1) has great structural fragility for MLCC capacitors. In general, MLCC is composed of multilayer ceramic capacitors, which have low strength and are extremely intolerant of thermal and mechanical shocks. (2) In the process of mounting, the Z-axis suction height of the mounter, especially some mounters without Z-axis soft landing function, is determined by the thickness of the mounting element rather than the pressure sensor, so the tolerance of the element thickness will cause cracks. (3) 3) The warpage stress of PCB, especially the warpage stress after welding, is easy to cause component cracking. (4) When some PCB boards are divided, components will be damaged. The preventive measures are: carefully adjust the welding process curve, especially the temperature in the preheating zone should not be too low; During the mounting process, the Z-axis adsorption height of the mounter should be carefully adjusted. Pay attention to the shape of the scraper of the puzzle; The warpage of PCB, especially after welding, should be corrected in a targeted way. If it is the quality problem of PCB, another key consideration is needed. 6.9 Other common welding defects (1) have poor wettability, which is characterized by poor tin eating on PCB pads or components pins. Cause: PCB pad of component pin is oxidized/polluted; The reflow soldering temperature is too high; The quality of solder paste is poor. It will lead to poor wettability, and in severe cases, virtual welding will occur. (2) The amount of tin is very small, which is manifested in insufficient solder joints and small meniscus at the root of IC pins. Reason: the print template window is small; Wick phenomenon (temperature curve difference); The metal content of solder paste is very low. All these will lead to low tin content and insufficient solder joint strength. (3) Pin damage Pin damage is characterized by poor flatness or bending of device pins, which directly affects the welding quality. Cause: Damage during transportation/pick-and-place. Therefore, components, especially FQFP, should be carefully preserved. (4) Contaminants cover the liner. Contaminants will cover the liner, which happens from time to time in production. Cause: paper from the scene; Foreign objects on the tape; Touch the PCB board or component by hand; The position of the character mapping table is wrong. Therefore, we should pay attention to the cleanliness of the production site and standardize the process. (5) The amount of solder paste is insufficient, which often appears in production. Reason: printing after the first PCB/ machine stops printing; Printing process parameters change; The steel window is blocked; The quality of solder paste becomes worse. One of the above reasons will lead to insufficient tin content, and the problem should be solved accordingly. (6) The solder paste is angular. Solder paste is angular, which often appears in production and is not easy to find. If it is serious, it will be welded. Reason: The screen lifting speed of the printing machine is too fast; The wall of the template hole is not smooth, which makes the solder paste appear as an ingot. 7 Summarize a lot of research on lead-free soldering technology at home and abroad at present, and deeply study a variety of lead-free solders including Sn-Cu system, Sn-Ag-Cu system, Sn-Ag-Bi-Cu system, Sn-Bi system and Sn-Sb system. The international industrial research association and other electronic industry associations have also recommended the technological parameters of several alloy ratios of typical alloy materials such as Sn-Ag-Cu series. Some powerful enterprises have carried out repeated experimental research on the basis of this research result, constantly optimizing the process parameters and obtaining the maximum benefit as much as possible. This topic refers to domestic and foreign literature and related journals, and selects appropriate parameters; Select the reflow soldering equipment that meets the technical requirements on SMT related websites to form a lead-free reflow soldering process. Finally, the possible welding defects in the welding process are analyzed theoretically and the corresponding solutions are put forward. This topic is a theoretical study of technology. Due to the lack of equipment and my superficial and incomplete understanding of SMT, fallacies are inevitable. Thank you for your criticism and correction.