Considering the environment and health factors, the European Union has passed legislation to stop the use of lead-containing brazing materials in 2008, and the United States and Japan are also actively considering the adoption of legislation to reduce and prohibit the use of lead and other harmful elements. The toxicity of lead currently consumes about 20,000t of lead per year in the global electronic industry brazing materials, accounting for about 5% of the world's total annual lead production. Lead and lead compounds have been included by the Environmental Protection Agency (EPA) in the first 17 kinds of chemical substances that are most harmful to human beings and the environment. Lead-free brazing materials Currently commonly used lead alloy solder powders are tin and lead (Sn-Pb), tin and lead and silver (Sn-Pb-Ag), tin and lead and bismuth (Sn-Pb-Bi), etc., commonly used alloy composition of 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 temperature profile 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 alloys. Therefore, the basic requirements of lead-free brazing materials The internationally recognized definition of lead-free brazing materials is: soft brazing alloys mainly used for electronic assemblies with Sn as the base, with other alloying elements added such as Ag, Cu, Sb, and In, and with a mass fraction of Pb of 0.2% or less. Lead-free brazing is not a new technology, but today's lead-free brazing research is to seek the annual use of 5 to 60,000 tons of Sn-Pb brazing material alternative products. Therefore, alternative alloys should meet the following requirements:
(1) Their global reserves are sufficient to meet market demand. Certain elements, such as indium and bismuth, have small reserves and can therefore only be used as trace additions to lead-free brazing materials;
(2) Non-toxic. Certain alternative elements under consideration, such as cadmium and tellurium are toxic. Certain elements, such as antimony, can also be considered toxic if the toxicity criteria are changed;
(3) Can be processed into all forms needed, including wire for hand soldering and repair; solder powder for brazing paste; and solder rods for wave soldering. Not all alloys can be processed into all forms, e.g. increased levels of bismuth will cause the alloy to become brittle and cannot be drawn into filaments;
(4) Phase transition temperature (solid/liquid phase line temperature) similar to Sn-Pb brazing materials;
(5) Suitable physical properties, in particular electrical and thermal conductivity, thermal expansion coefficients;
(6) Metallographic properties similar to those of existing component substrate/lead and PCB materials in terms of metallurgical properties;
(7) Adequate mechanical properties: shear strength, creep resistance, isothermal fatigue resistance, thermo-mechanical fatigue resistance, stability of the metallurgical organization;
(8) Good wettability;
(9) Acceptable cost price.
The cost of a new lead-free brazing material should be less than 22.2/kg, so the mass fraction of In in it should be less than 1.5% and the Bi content should be less than 2.0%. Early R&D programs focused on determining new alloy compositions, multivariate phase diagram studies, and basic performance investigations such as wettability and strength. Later R&D programs focused on five alloy series:SnCu, SnAg, SnAgCu, SnAgCuSb, and SnAgBi. and in-depth discussion of their fatigue properties, production behavior, and process optimization. Table 2.3 NCMS U.S. National Center for Manufacturing Sciences proposed by the lead-free brazing performance evaluation standards IPC also in June 2000 released a study report "A guide line for assembly of lead-free electronics".
The main international conclusions on lead-free brazing materials are as follows: there are now a wide range of lead-free brazing materials available, none of which provides a comprehensive solution for the direct replacement of SnPb brazing materials.
(1) For some specific processes, direct substitution can be achieved with certain specific Pb-free brazing compounds;
(2) For the time being, the most attractive Pb-free brazing compounds are the Sn-Ag-Cu series. Other promising combinations include Sn-0.7Cu, Sn-3.5Ag, and Sn-Ag-Bi;
(3) There are currently no suitable lead-free alternatives to high-lead, high-melting-point brazing compounds;
(4) At this time, it appears that the brazing flux chemistry system does not require major changes;
(5) The reliability of lead-free brazing compounds for forming solder joints is superior to that of SnPb alloys.
Comparison of several types of lead-free brazing materials
(1) SnCu: the cheapest; highest melting point; the worst mechanical properties.
(2) SnAg: good mechanical properties, good solderability, good thermal fatigue reliability, *** crystal composition when the melting point of 221 ℃. SnAg and SnAgCu combination of the differences between the very small, and its choice depends mainly on price, availability and other factors.
(3) SnAgCu (Sb): until the last few years it was not known that there is a ternary **** crystal between Sn-Ag-Cu, and its melting point is lower than that of the Sn-Ag **** crystal, of course, the exact composition of the ternary **** crystal is still controversial. The combination has better reliability and solderability than Sn-Ag and Sn-Cu. And its high temperature reliability can be further improved by adding 0.5% Sb.
(4) SnAgBi (Cu) (Ge): low melting point, 200 ~ 210 ℃; good reliability; in all the lead-free brazing material in the solderability is the best, has been confirmed by Matsushita; adding Cu or Ge can further improve the strength; drawbacks are the problem of rising defects in the angle of wetting brought about by the inclusion of Bi.
(5) SnZnBi: melting point closest to the Sn-Pb **** crystal; but contains Zn brings many problems, such as brazing paste retention period, a large number of active brazing flux residue, oxidation problems, potential corrosive problems. It is not recommended at present. 2.2 Selection of alloy From the above, the reflow process design solder alloy Sn/Ag/Cu alloy (Sn/Ag3.0/Cu0.5), because the alloy is considered to be the first choice in the international industry and recommended by members of industry and research guilds. This is because another alloy, Sn/0.7Cu (mass percentage), has been proposed and studied by some guilds and is used by some companies in their production. But relative to the reliability and wettability of Sn / Cu alloy, in addition to considering the use of the same alloy in the reflow soldering and wave soldering, Sn / Ag / Cu alloy has become the best choice for process development trials. Sn/Ag3.0/Cu0.5 alloy properties: Dissolution temperature: solid-phase line 217 ℃/liquid-phase line 220 ℃; Cost: $ 0.10/cm3 and Sn / Cu solder price ratio: 2.7 Mechanical Strength: 48kg/mm2 Elongation: 75% Wettability: good From the performance of the Sn/Ag/Cu alloy can be seen: solder alloy melting temperature is 36 ℃ higher than the original Sn/Pb alloy. ℃, the formation of commercialization after the price is also higher than the original. Process soldering temperature using Japan's recommended process curve for this alloy solder, see Figure 2.1.
Japan's recommended lead-free reflow soldering typical process curve Description: the recommended process curve has three important points:
(1) preheating zone to try to slow down the rate of warming (select the value of 2 ~ 3 ℃ / s), in order to control the solder paste by the collapse of the edge caused by the solder joints of the bridge, the solder ball and so on.
(2)
(2) Preheating requirements must be (45 ~ 90sec, 120 ~ 160 ℃) within the range, in order to control the temperature difference by the PCB substrate and flux performance changes and other factors such as the occurrence of reflow soldering bad.
(3) The maximum temperature of the soldering above 230 ℃, keep 20 ~ 30sec, to ensure that the soldering wet. Cooling speed selection -4 ℃ / s 6 Reflow soldering defects and their solutions Welding defects can be divided into major defects, minor defects and surface defects. Any defect that makes the SMA function fail is called major defects; minor defects are defects that the wetting between the weld joints is still good and will not cause the loss of SMA function, but there is the possibility of affecting the product's life; surface defects are defects that do not affect the product's function and life. It is affected by many parameters, such as solder paste, substrate, component solderability, printing, mounting accuracy and soldering process. We carry out 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.
Reflow solder beads
(1) Reflow solder beads in the formation of the mechanism of reflow soldering beads (or solder ball), often hidden in the rectangular chip components between the two soldering ends of the side or fine-pitch between the pins, such as Figure 6.1, 6.2. In the process of component placement, the solder paste is placed on the chip components between the pins and the pads, along with the printed circuit board through the reflow soldering furnace, the paste melted into a liquid, if with the pads and solder paste, the paste will melt into a liquid. As the printed board passes through the reflow oven, the solder paste melts and becomes liquid. If it is not wetted properly with the pads and device pins, etc., the liquid solder will not fill the solder joints adequately due to shrinkage and all the solder particles will not be able to polymerize into a single solder joint. Part of the liquid solder will flow out of the solder seam to form a bead. Therefore, poor wetting of the solder with the pads and device pins is the root cause of the formation of tin beads. Figure 6.1 An example of a chip component has a slightly larger particle size of tin balls Figure 6.2 Than the pins around the dispersed tin balls Solder paste in the printing process, due to the template and pad alignment offset, if the offset is too large will lead to the potting paste diffuse to the outside of the pads, heated and prone to the emergence of tin beads. Patch the process of Z-axis pressure is caused by an important reason for the tin bead, often not people calendar attention, part of the mounter due to the Z uranium head is based on the thickness of the components to locate. Therefore, it will cause the components pasted to the PCB will be a moment of tin lei extrusion to the pad outside the phenomenon, this part of the group will obviously cause tin beads. This situation produces a slightly larger size of tin beads, usually as long as the re-adjustment of Z uranium height, you can prevent the production of tin beads.
(2) Cause analysis and control methods Caused by poor solder wettability of many reasons, the following main analysis and related processes related to the causes and solutions:
(1) reflow temperature profile is not set properly. The reflow of solder paste is related to the temperature and time, if not reached a sufficient temperature or time, the solder paste will not reflow. Preheating zone temperature rise too fast, too short a time, so that the solder paste inside the moisture and solvent is not completely evaporated out, to reach the reflow temperature zone, caused by the moisture, solvent boiling, spattering out of tin beads. Practice has proved that the preheating zone temperature rise rate control in 1 ~ 4 ℃ / s is more ideal.
(2) If always in the same position on the tin beads, it is necessary to check the metal template design structure. Template opening size corrosion accuracy fails to meet the requirements, pad size is large, as well as the surface material is softer (such as copper templates), will cause the shape of the printed solder paste outline is not clear, bridging each other, this situation occurs more often than not in the pad printing on fine-pitch devices, reflow soldering is bound to result in the production of a large number of tin beads between the pins. Therefore, the different shapes of pad graphics and center distance, select the appropriate template materials and template production process to ensure the quality of solder paste printing.
(3) If the time from the patch to reflow soldering is too long, the solder paste due to the oxidation of solder particles, flux deterioration, reduced activity, will lead to the solder paste does not reflow, resulting in tin beads. Select a longer working life of the solder paste (we believe that at least 4h), it will mitigate this effect.
(4) In addition, the solder paste misprinted board cleaning is not sufficient, the solder paste will be left on the surface of the board and through-hole. Before reflow soldering, paste the components, so that the printed solder paste deformation. These are also causes of tin beads. Therefore, we should strengthen the responsibility of operators and craftsmen in the production process, strictly follow the process requirements and operating procedures for production, and strengthen the quality control of the process. 6.2 Standing piece of the problem (Manhattan phenomenon) shape piece of components of one end of the welded on the pad, while the other end of the warping, this phenomenon is known as the Manhattan phenomenon, see Figure 6.5. caused by this phenomenon is the main reason for the two ends of the components are not uniformly heated, the melting of the solder paste has been caused by the successive. In the following cases will cause uneven heating of the two ends of the component: Figure 6.5 standing piece of the phenomenon Figure 6.6 component deviates from the pad so the two sides of the force imbalance generated by the standing piece of the phenomenon.
(1) Incorrect design of component arrangement. We envision a reflow limit line across the width of the furnace in the reflow oven, once the solder paste through it will melt immediately, as shown in Figure 6.7. Sheet rectangular components of an end first through the reflow limit line, the solder paste first melted, fully infiltrated component end of the metal surface, with liquid surface tension; and the other end did not reach 183 ℃ liquid phase temperature, the solder paste is not melted, only the adhesive force of the flux, which is much smaller than the surface tension of the reflow solder paste, and, therefore, so that the component end of the unmelted end of the upwardly upright. Therefore, should keep both ends of the component at the same time into the reflow limit line, so that both ends of the solder paste on the pad at the same time to melt, the formation of a balanced liquid surface tension, to maintain the position of the component is unchanged. Figure 6.7 soldering pad side of the end of the melting of tin green. Unbalanced tension between the two pads will appear standing monument.
(2) in the vapor phase welding printed circuit assembly preheating is not sufficient. Vapor phase soldering is the use of inert liquid vapor condensation in the component pins and PCB pads, the release of heat and melt the solder paste. Vapor phase soldering equilibrium zone and saturated steam zone, in the saturated steam zone soldering temperature up to 217 ℃, in the production process, we found that, if the soldered component preheating is not sufficient, subjected to temperature changes of more than 100 ℃, the vaporization of vapor phase soldering is easy to less than the size of the 1206 package of chip components floating, resulting in the phenomenon of the stand-up piece. We through the welded components in the high and low temperature box 145 ~ 150 ℃ temperature preheating 1 ~ 2min, and then in the equilibrium zone of vapor phase welding and then preheating 1min or so, and finally slowly into the saturated vapor zone welding, eliminating the phenomenon of standing piece.
(3) the impact of pad design quality. If the sheet components of a pair of pads of different sizes or asymmetry, will also cause inconsistencies in the amount of printed solder paste, small pads on the temperature response is fast, the solder paste on it is easy to melt, large pads on the contrary, so when the small pads on the melting of the solder paste in the role of the surface tension of the solder paste, the components will be pulled straight and vertical. The width or gap of the pad is too large, may also appear standing piece phenomenon. Strictly following the standard specification for pad design is a prerequisite for solving this defect. 6.3 Bridging Bridging is also one of the common defects in SMT production, it will cause a short circuit between the components, encounter bridging must be reworked. Bridging this occurs in the process.
(1) solder paste quality problems The metal content in the paste is high, especially after a long time of printing. Easy to increase the metal content; solder paste viscosity is low, after preheating diffuse flow to the outside of the pad; solder paste collapse degree is poor, after preheating Han diffuse to the outside of the pad, will lead to IC pin bridging. The solution is to adjust the solder paste.
(2) Printing system Repeat accuracy of the printing machine is poor, alignment is not aligned, solder paste printing to the outside of the silver bar, this situation is mostly seen in the production of fine-pitch QFP; steel plate alignment is not good and PCB alignment is not good as well as steel window size / thickness design is not right with the design of the alloy of PCB pads plating is not uniform, resulting in the amount of solder paste on the side of the amount of more, will cause bridging. The solution is to adjust the printing machine to improve the PCB pad coating layer.
(3) placement placement pressure is too large, the paste is pressurized by the dip is common in the production of the reason, should be adjusted to the height of the Z-axis. If there is insufficient placement accuracy, component displacement and IC pin deformation, it should be improved for the reasons.
(4) preheating The warming speed is too fast, the solvent in the solder paste can not evaporate in time. 6.4 Suction / core suction phenomenon Core suction phenomenon, also known as core phenomenon is one of the common welding defects such as Figure 6.8, mostly seen in the vapor phase reflow soldering. Core suction phenomenon is the solder out of the pad along the pin up to the pin and the chip body between the formation of serious soldering phenomenon. Figure 6.8 core suction phenomenon The reason is usually considered to be the thermal conductivity of the component pins. Warming rapidly, so that the solder priority wetting pins, solder and pins between the wetting force is much greater than the wetting force between the solder and the pad, the pins of the warping will intensify the core suck phenomenon occurs. In the infrared reflow soldering, PCB substrate and solder in the organic flux is infrared excellent absorption medium, while the pin can be part of the infrared reflection, compared to the solder priority melting, it is greater than the wetting force with the pad and the wetting force between it and the pin, the solder will not be rising along the pin, the probability of core sucking phenomenon is much smaller. The solution is: in the vapor phase reflow soldering should first be fully preheated SMA and then into the vapor phase furnace; should be carefully checked and ensure that the PCB board pad weldability, weldability is not good PCB should not be used in the production; components of the * * * * surface can not be ignored, on the * * * * surface of the poor device should not be used in the production. 6.5 After welding printed board solder resist film blistering Printed board components after welding, will appear around the individual solder joints light green bubbles, and in serious cases, there will be a fingernail-sized bubbles, not only affecting the quality of appearance, but also affect the performance in serious cases, is one of the problems often occurring in the welding process. The root cause of soldermask blistering lies in the presence of gas/water vapor between the soldermask and the positive substrate. Trace amounts of gas/water vapor are entrained into the different processes, and when high temperatures are encountered, the gas expands, leading to delamination of the soldermask and the male substrate. With soldering, the pad temperature is relatively high, so gas bubbles first appear around the pad. Now the process often need to clean, dry and then do the next process, such as corrosion carving, should be dry before applying soldermask, at this time, if the drying temperature is not enough, it will entrain water vapor into the next process. pcb processing before the storage of the environment is not good, the humidity is too high, and welding is not timely drying process; in the wave soldering process, the use of fluxes containing water, if the pcb preheating temperature is not enough, the flux in the water vapor will be Along the hole wall of the through-hole into the interior of the PCB substrate, the pad around the first to enter the water vapor, meet the welding high temperature after these conditions will produce bubbles. The solution is; (1) should strictly control all aspects of the purchase of PCB should be inspected after storage. Normally standard circumstances, there should be no bubbling phenomenon; (2) PCB should be stored in a ventilated dry environment, the storage period of not more than 6 months; (3) PCB in the welding should be placed in the oven before pre-drying 105 ℃ / 4h ~ 6h; 6.6 PCB distortion PCB distortion is a frequent problem in the production of SMT, it will be on the assembly and testing to bring a considerable impact, and therefore, in the production should be Try to avoid the emergence of this problem, PCB distortion is due to the following reasons: (1) PCB itself is not the right choice of raw materials, PCB Tg is low, especially paper-based PCB, its processing temperature is too high, will make the PCB become bent. (2) PCB design is not reasonable, uneven distribution of components will cause PCB thermal stress is too large, the shape of the larger connectors and sockets will also affect the expansion and contraction of the PCB, and even a permanent distortion. (3) Double-sided PCB, if one side of the copper foil retention is too large (such as ground), while the other side of the copper foil is too little, will cause uneven contraction and deformation of both sides. (4) Reflow soldering in the temperature is too high will also cause PCB distortion. For the above reasons, the solution is as follows: in the case of price and space allowances, the choice of Tg high PCB or increase the thickness of the PCB, in order to obtain the best aspect ratio; reasonable design of the PCB, both sides of the steel foil area should be balanced, in the absence of circuitry covered with steel layer, and in the form of a network to increase the rigidity of the PCB, in the placement of the PCB before preheating, the conditions of 105 ℃ / 4h; adjust the fixture or clamping distance to ensure that the PCB is subject to thermal expansion of the space; welding process temperature is adjusted as low as possible; there has been a mild distortion, it can be placed in the positioning fixture, temperature reset to release the stress, and generally will achieve satisfactory results. 6.7 IC pin welding pin open circuit / virtual welding IC pin welding after the emergence of some of the pin virtual welding, is a common welding defects, resulting in a number of reasons, the main reason, one is the **** surface of the poor, especially the QFP device. Due to improper storage, resulting in pin deformation, sometimes not easy to be found (part of the mounter does not check the *** face of the function), resulting in the process shown in Figure 6.9. Figure 6.9 *** surface poor component welding appeared after the need to weld Therefore, attention should be paid to the custody of the device, do not just take the components or open the package. Second, the pin weldability is not good. IC stored for a long time, the pin yellowing, weldability is not good will also cause false soldering, production should check the weldability of components, pay special attention to than the storage period should not be too long (one year from the date of manufacture), storage should not be subjected to high temperature, high humidity, do not casually open the bag. Third, the poor quality of solder paste, low metal content, poor weldability, usually used for QFP device solder paste, metal content should not be less than 90%. Fourth, the preheating temperature is too high, easy to cause IC pin oxidation, so that the weldability becomes poor. Fifth, the template window size is small, so that the amount of solder paste is not enough. Usually after the template manufacturing, should carefully check the template window size, should not be too large or too small, and pay attention to the size of the pad with the PCB matching. 6.8 Chip component cracking In SMC production, chip component cracking is common in multilayer chip capacitors (MLCC), the cause of which is mainly due to effect force and mechanical stress. (1) For MLCC capacitors, there is a great deal of vulnerability in its structure, usually MLCC is made of multilayer ceramic capacitors stacked together, low strength, extremely intolerant of thermal and mechanical impact. (2) the placement process, the placement machine z-axis of the suction and discharge height, especially some do not have the z-axis soft landing function of the placement machine, suction and discharge height by the thickness of the chip components rather than by the pressure sensor to decide, so the component thickness tolerance will cause cracking. (3) PCB warpage stress, especially after welding, warpage stress will easily cause cracking of components. (4) Some of the PCB of the collocation board in the division, it will damage the components. Preventive measures are: carefully adjust the welding process curve, especially the preheating zone temperature can not be too low; patch should be carefully adjusted patch machine z-axis suction and discharge height; should pay attention to the shape of the scraper of the patchwork board; PCB warpage. Especially after welding warpage, there should be targeted correction, such as PCB board quality issues, need to focus on another consideration. 6.9 Other common welding defects (1) poor wettability poor wettability, manifested in the PCB pad to eat tin is not good or component pins to eat tin is not good. Causes: component pins PCB pad has been oxidized / polluted; too high reflow temperature; poor quality solder paste. All will lead to poor wettability, and in serious cases, there will be soldering. (2) The amount of tin is very little The amount of tin is very little, the performance of the solder joints are not full, IC pin root crescent surface is small. Causes: printing template window is small; wick phenomenon (temperature profile); paste metal content is low. These will lead to a small amount of tin, solder joint strength is not enough. (3) damaged pins damaged pins, manifested in the device pin **** surface is not good or bent, directly affecting the quality of welding. Causes: transportation / pick and place when the touch. For this reason, components should be carefully stored, especially FQFP. (4) contaminants covered pads contaminants covered pads, which occur from time to time in production. Causes: Paper from the site; foreign objects from the tape; human touch PCB pads or components; character map is not in the right position. Thus, the production should pay attention to the production site clean, the process should be standardized. (5) insufficient amount of solder paste Insufficient amount of solder paste, the production of the phenomenon often occurs. Causes: the first PCB printing / printing after the machine stops; printing process parameters change; steel plate window blocking; paste quality deterioration. One of the above reasons will cause insufficient tin volume, should be targeted to solve the problem. (6) paste is angular paste is angular, often occurring in the production, and not easy to find, when the seriousness of continuous welding. Causes: the printing machine to lift the network too fast; template hole wall is not smooth, easy to make the solder paste in the shape of a treasure. 7 Summary At present, domestic and foreign lead-free soldering technology has been carried out a lot of research on the proposed variety of lead-free solder, including Sn-Cu series, Sn-Ag-Cu series, Sn-Ag-Bi-Cu series, Sn-Bi series, Sn-Sb series and so on have a more in-depth study. International Industrial Research Association and other electronic industry associations of typical alloy materials such as Sn-Ag-Cu series of several alloy ratio also have recommended process parameters; some powerful enterprises are based on the results of this research repeated experiments and research on the process parameters continue to optimize, as far as possible to achieve the maximum degree of efficiency. This topic with reference to domestic and foreign literature and relevant journals, select the appropriate parameters; and selected SMT-related websites on the market to meet the process requirements of reflow soldering equipment composed of lead-free reflow soldering process. Finally, the welding process may appear in the welding defects to make theoretical analysis, and put forward relative solutions. This topic is the theoretical study of the process, due to lack of equipment, but also because of my shallow knowledge of SMT is not comprehensive, fallacies are inevitable. I hope you criticize and correct, thank you.