Lead-free technology has caused the greatest change in the welding process and is the most difficult part of the entire process technology. This change is due to changes in melting point and surface tension of the weld metal after removal of the lead metal. Changes in the characteristics of these two aspects, so that the original use of tin in the flux formula must be redesigned or adjusted. The change in melting temperature and the composition of the flux also cause changes in the process parameters of the welding process. From the current research results, the melting temperature of all the alternative alloys is higher than that of the existing tin-lead alloys. For example, from the point of view of the 'tin-silver-copper' alloy that is currently more widely accepted by the industry, the melting point is 217°C. Taking this as an example, the adoption of lead-free technology will greatly reduce the process window in the welding process. Theoretically, the shrinkage of the process window has dropped from 37oC of tin-lead solder to only 23oC, with a shrinkage of about 38% (see Figure 1). In fact, the shrinkage of the process window is still greater than the above theoretical value. The reason is that in actual work, our profiling practices contain certain uncertainties, plus DFM restrictions, and we must take good care of the solder joint 'appearance' (many factories still regard the appearance as the main According to the quality inspection, etc., etc., the window of this reflow soldering process is only about 14oC (approximately 53% shrinkage). This is only 14oC process window, in fact, in the process modulation is very challenging. The requirements for equipment (reflow ovens) and DFM are also much higher than those for tin-lead technologies.
Theoretically, in the welding process, the temperature of the solder joints only needs to reach the melting temperature of the solder alloy. However, under actual conditions, the solder that has just reached the melting point temperature has poor wettability. So we must increase the temperature of the actual solder joints to increase the wetting ability. Since the wettability of lead-free alloys is worse than that of tin-lead alloys, this approach is even more necessary in lead-free technologies. The devices and boards on PCBA are all subject to temperature extremes, and the current requirement for this withstand temperature in lead-free technology is 260oC. Although this temperature has been improved compared with the 240°C technology of lead-containing technology, the solder joint temperature is greatly increased due to the influence of the melting point temperature and the wettability considerations, which results in allowable process windows (upper and lower limits of temperature). It is much smaller in lead-free technology.
In fact, if the device supplier only meets the 260oC upper limit of the international proposal in the device design, the user will face even greater problems. The welding temperature process window possessed may not even reach the 14oC mentioned above. This is because of the packaging design of some devices such as BGA, in convection heating applications, the temperature of the package body is often higher than the solder joint temperature at the bottom. This is not originally a big problem, and the problem is exacerbated by the fact that these devices are generally devices with large heat capacity and their package thermal conductivity is not very good. Since there are always some devices with a much smaller thermal capacity on the same PCBA (Note 3), it is very difficult to reduce the actual temperature difference through process adjustment and ensure that they are within the process window.
Not only is the shrinking of the process window a huge challenge for craftsmen, but the increase in welding temperature also makes the welding work more difficult. One of these is oxidation during high temperature welding. As we all know, the oxide layer can make welding difficult, poor wetting, and causing welds that affect the life of solder joints. The degree of oxidation, in addition to adequate control of the incoming material itself, the user's inventory conditions and time, processing prior to processing (eg, dehumidifying baking), and the thermal energy experienced during the preheat (or isothermal) phase of the welding ( Temperature and time etc. are all decisive factors. The increased temperature of the lead-free technology is causing the soldering tip to cause more oxidation in the preheating section. If the paste does not have enough flux capacity, or if the reflow profile is improperly set in the 'clean/de-oxidize' section, solder back problems may occur during reflow.
The 'popcorn' phenomenon is another issue that will increase in lead-free technology. Some studies in the industry have pointed out that due to the increase in temperature, the sensitivity of many ICs in lead-free soldering will increase by one to two levels. In other words, the user's moisture control or treatment must also be strengthened. This will have a more serious impact on those users who are producing in small quantities. Because many users of small batch production have a longer time of incoming inventory. If the moisture-proof facilities in inventory are not ideal, the 'popcorn' problem must be prevented by pre-assembly bake dehumidification. This practice has been more frequent since it entered the lead-free era due to its greater sensitivity to moisture absorption. Although baking can solve the 'popcorn' problem, but the baking process will exacerbate the oxidation of the device's soldering end, bringing about the difficulty of welding. One possible practice is to use inert 'tombstones' as another serious problem in lead-free technology than in lead-containing technology. This is because of the strong surface tension of lead-free alloys. The principle of the solution is the same as that of the lead-containing technology in which the size of the solder terminal and the pad, and the heat capacity at both ends are most effective through the DFM control. Secondly, the temperature difference between the two ends of the device can be reduced through process adjustment. It should be noted that although the principle remains the same, the lead-free process window will be smaller, so users must first ensure that the furnace they are using has sufficient capacity. That is, good heating efficiency and stable air flow.
'Stomata' is a problem that cannot be completely solved in the tin-lead technology. After entering the lead-free technology, this problem will appear even worse with the increase of surface tension of lead-free alloys. To eliminate the 'stomatal' problem, there are three factors that must be closely matched and given care. It is the solder paste characteristics (selection of solder pastes), DFM (device solder end structures, pad and stencil opening designs), and reflow process (setting of temperature profile). The control principle is not different from the lead-containing technology, but the window is smaller.
Since the lead-free soldering process window is significantly reduced from the leaded solder process window, some people in the industry believe that the use of a nitrogen soldering environment may be necessary. Nitrogen welding can reduce the surface tension of molten tin and increase its wettability. It also prevents oxidation during warm-up. However, nitrogen is not universal, it can not solve all the problems brought about by lead-free. In particular, it is impossible to solve the problems that have already been caused before the welding process. Such as solder paste, reflow capability, DFM and other issues. Moreover, the use of nitrogen increases costs, so it should not be a primary consideration. Should be positioned as a 'remedy'. In other words, the correct treatment attitude should be to consider whether to implement the nitrogen welding process after implementing the 'technical integration' to confirm that other effective factors cannot be improved or properly controlled. There are not many users of nitrogen in the country, but in the two companies that I contact, in fact, there is no need to use nitrogen. Its process problems should be solved from other more cost-effective methods. So here's a reminder to users that although nitrogen will help, you don't have to resort to it. It is not advisable not to hurry to use the decision before you have any knowledge of other aspects.
The small process window not only has a high requirement for the accuracy of the process modulation, but also requires that the process stability must also be very high. Otherwise, even if the process is set to the optimization point, the process shift will quickly shift the quality out of the controlled area. To stabilize the process, equipment is a key factor. In the current reflow soldering equipment, the design of the furnace using the forced hot air convection principle is the mainstream. The hot-air convection technology can replace the early gas phase and later infrared radiation technology, because of its controllability of the heating rate and the constant temperature capability. Unfortunately, hot air convection is its weakness in terms of heating efficiency, heating uniformity, and repeatability. These weaknesses are not serious in the lead-containing technology and can be accepted in many cases. With the shrinking of the lead-free technology in the process window and the higher repetitiveness requirements, hot air convection technology will be challenged. Some low-to-medium-sized devices that are not well-designed in hot-air convection technology will not be able to effectively support users who understand the process and care about quality.
The principle of hot air reflow furnaces is through hot air as a medium for heat transfer. Air itself is not a good heat conductor. However, the purpose of heat transfer must be achieved through sufficient 'convection'. So how the furnace controls the design of the internal air flow is a key. The air flow is very difficult to control accurately. Even a well-designed furnace, the heat transfer benefit will be due to changes in the gas pressure in the furnace (changes from the exhaust system, fan aging, gradual blockage of the air outlet, etc.), load changes (into the furnace time or interval time), Paste volatiles, equipment aging and other variables change. Therefore, if you want to best control the welding process, you must have a continuous supervision approach.
At present, many users in China are not very concerned about the process control of welding. Many users actually do not have any monitoring methods throughout the welding process. Some of the better, will use the test board regularly measured changes in furnace temperature through the furnace. Although this is also a feasible solution, there are two weaknesses:
1. High cost - The measurement board must be a product to make sense. Empty board does not really measure all possible changes. The effect of the analog board can be satisfactory, but the design has a certain degree of difficulty. If you use a real product, the cost may be high. Moreover, the test board is not infinitely long and must be replaced after a certain number of times. In addition to sheet metal costs, the personnel and time required for testing and certification are also elements of cost.
2. Missed inspections - The above approach cannot be continuous. General users will perform at the beginning of shifts or shifts, as well as in the middle of a shift. So it belongs to a kind of sampling inspection technology. And the sample size is not high. Therefore, the missed false positive rate is also high.
Theoretically, in the welding process, the temperature of the solder joints only needs to reach the melting temperature of the solder alloy. However, under actual conditions, the solder that has just reached the melting point temperature has poor wettability. So we must increase the temperature of the actual solder joints to increase the wetting ability. Since the wettability of lead-free alloys is worse than that of tin-lead alloys, this approach is even more necessary in lead-free technologies. The devices and boards on PCBA are all subject to temperature extremes, and the current requirement for this withstand temperature in lead-free technology is 260oC. Although this temperature has been improved compared with the 240°C technology of lead-containing technology, the solder joint temperature is greatly increased due to the influence of the melting point temperature and the wettability considerations, which results in allowable process windows (upper and lower limits of temperature). It is much smaller in lead-free technology.
In fact, if the device supplier only meets the 260oC upper limit of the international proposal in the device design, the user will face even greater problems. The welding temperature process window possessed may not even reach the 14oC mentioned above. This is because of the packaging design of some devices such as BGA, in convection heating applications, the temperature of the package body is often higher than the solder joint temperature at the bottom. This is not originally a big problem, and the problem is exacerbated by the fact that these devices are generally devices with large heat capacity and their package thermal conductivity is not very good. Since there are always some devices with a much smaller thermal capacity on the same PCBA (Note 3), it is very difficult to reduce the actual temperature difference through process adjustment and ensure that they are within the process window.
Not only is the shrinking of the process window a huge challenge for craftsmen, but the increase in welding temperature also makes the welding work more difficult. One of these is oxidation during high temperature welding. As we all know, the oxide layer can make welding difficult, poor wetting, and causing welds that affect the life of solder joints. The degree of oxidation, in addition to adequate control of the incoming material itself, the user's inventory conditions and time, processing prior to processing (eg, dehumidifying baking), and the thermal energy experienced during the preheat (or isothermal) phase of the welding ( Temperature and time etc. are all decisive factors. The increased temperature of the lead-free technology is causing the soldering tip to cause more oxidation in the preheating section. If the paste does not have enough flux capacity, or if the reflow profile is improperly set in the 'clean/de-oxidize' section, solder back problems may occur during reflow.
The 'popcorn' phenomenon is another issue that will increase in lead-free technology. Some studies in the industry have pointed out that due to the increase in temperature, the sensitivity of many ICs in lead-free soldering will increase by one to two levels. In other words, the user's moisture control or treatment must also be strengthened. This will have a more serious impact on those users who are producing in small quantities. Because many users of small batch production have a longer time of incoming inventory. If the moisture-proof facilities in inventory are not ideal, the 'popcorn' problem must be prevented by pre-assembly bake dehumidification. This practice has been more frequent since it entered the lead-free era due to its greater sensitivity to moisture absorption. Although baking can solve the 'popcorn' problem, but the baking process will exacerbate the oxidation of the device's soldering end, bringing about the difficulty of welding. One possible practice is to use inert 'tombstones' as another serious problem in lead-free technology than in lead-containing technology. This is because of the strong surface tension of lead-free alloys. The principle of the solution is the same as that of the lead-containing technology in which the size of the solder terminal and the pad, and the heat capacity at both ends are most effective through the DFM control. Secondly, the temperature difference between the two ends of the device can be reduced through process adjustment. It should be noted that although the principle remains the same, the lead-free process window will be smaller, so users must first ensure that the furnace they are using has sufficient capacity. That is, good heating efficiency and stable air flow.
'Stomata' is a problem that cannot be completely solved in the tin-lead technology. After entering the lead-free technology, this problem will appear even worse with the increase of surface tension of lead-free alloys. To eliminate the 'stomatal' problem, there are three factors that must be closely matched and given care. It is the solder paste characteristics (selection of solder pastes), DFM (device solder end structures, pad and stencil opening designs), and reflow process (setting of temperature profile). The control principle is not different from the lead-containing technology, but the window is smaller.
Since the lead-free soldering process window is significantly reduced from the leaded solder process window, some people in the industry believe that the use of a nitrogen soldering environment may be necessary. Nitrogen welding can reduce the surface tension of molten tin and increase its wettability. It also prevents oxidation during warm-up. However, nitrogen is not universal, it can not solve all the problems brought about by lead-free. In particular, it is impossible to solve the problems that have already been caused before the welding process. Such as solder paste, reflow capability, DFM and other issues. Moreover, the use of nitrogen increases costs, so it should not be a primary consideration. Should be positioned as a 'remedy'. In other words, the correct treatment attitude should be to consider whether to implement the nitrogen welding process after implementing the 'technical integration' to confirm that other effective factors cannot be improved or properly controlled. There are not many users of nitrogen in the country, but in the two companies that I contact, in fact, there is no need to use nitrogen. Its process problems should be solved from other more cost-effective methods. So here's a reminder to users that although nitrogen will help, you don't have to resort to it. It is not advisable not to hurry to use the decision before you have any knowledge of other aspects.
The small process window not only has a high requirement for the accuracy of the process modulation, but also requires that the process stability must also be very high. Otherwise, even if the process is set to the optimization point, the process shift will quickly shift the quality out of the controlled area. To stabilize the process, equipment is a key factor. In the current reflow soldering equipment, the design of the furnace using the forced hot air convection principle is the mainstream. The hot-air convection technology can replace the early gas phase and later infrared radiation technology, because of its controllability of the heating rate and the constant temperature capability. Unfortunately, hot air convection is its weakness in terms of heating efficiency, heating uniformity, and repeatability. These weaknesses are not serious in the lead-containing technology and can be accepted in many cases. With the shrinking of the lead-free technology in the process window and the higher repetitiveness requirements, hot air convection technology will be challenged. Some low-to-medium-sized devices that are not well-designed in hot-air convection technology will not be able to effectively support users who understand the process and care about quality.
The principle of hot air reflow furnaces is through hot air as a medium for heat transfer. Air itself is not a good heat conductor. However, the purpose of heat transfer must be achieved through sufficient 'convection'. So how the furnace controls the design of the internal air flow is a key. The air flow is very difficult to control accurately. Even a well-designed furnace, the heat transfer benefit will be due to changes in the gas pressure in the furnace (changes from the exhaust system, fan aging, gradual blockage of the air outlet, etc.), load changes (into the furnace time or interval time), Paste volatiles, equipment aging and other variables change. Therefore, if you want to best control the welding process, you must have a continuous supervision approach.
At present, many users in China are not very concerned about the process control of welding. Many users actually do not have any monitoring methods throughout the welding process. Some of the better, will use the test board regularly measured changes in furnace temperature through the furnace. Although this is also a feasible solution, there are two weaknesses:
1. High cost - The measurement board must be a product to make sense. Empty board does not really measure all possible changes. The effect of the analog board can be satisfactory, but the design has a certain degree of difficulty. If you use a real product, the cost may be high. Moreover, the test board is not infinitely long and must be replaced after a certain number of times. In addition to sheet metal costs, the personnel and time required for testing and certification are also elements of cost.
2. Missed inspections - The above approach cannot be continuous. General users will perform at the beginning of shifts or shifts, as well as in the middle of a shift. So it belongs to a kind of sampling inspection technology. And the sample size is not high. Therefore, the missed false positive rate is also high.