Lead-free technology causes the greatest change in the welding process and is the most difficult part of the overall process technology. This change is due to changes in the melting point and surface tension of the weld metal after the lead metal is removed. These two characteristics change, so that the flux formulation originally used in tin-lead must be redesigned or adjusted. The change in melting point temperature and the difference in flux composition also cause changes in the process parameters of the welding process. From the current research results, the melting point temperature of all the alternative alloys is higher than the existing tin-lead alloy. For example, from the 'tin-silver-copper' alloy, which is currently more widely accepted by the industry, its melting point is 217oC. Taking this as an example, the adoption of lead-free technology will result in a significant reduction in the process window in the soldering process. Theoretically, the shrinkage of the process window has dropped from 37oC for 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 theoretical value above. The reason is that in actual work, our temperature measurement (Profiling) method has certain uncertainty, plus DFM restrictions, and to take good care of the solder joint 'appearance' (many factories still use appearance as the main The quality inspection basis, etc., the window of this reflow soldering process is only about 14oC (about 53% shrinkage). This is only a 14oC process window, which is actually very challenging in terms of process modulation. The requirements for equipment (reflow oven) and DFM are also much higher than the requirements for tin-lead technology.
Theoretically, in the soldering process, the temperature of the solder joints is as long as the melting temperature of the solder alloy is reached. However, in actual cases, the solder which has just reached the melting point temperature has a particularly poor wettability. So we have to increase the temperature of the actual solder joint to increase the wetting ability. Since the wettability of lead-free alloys is worse than that of tin-lead alloys, this practice is even more necessary in lead-free technology. Devices and boards on PCBA have temperature extremes, and the current temperature requirements for lead-free technology are 260oC. Although this temperature is improved compared to 240oC for lead-containing technology, the solder joint temperature is increased by the influence of melting point temperature and wettability, which results in an allowable process window (upper and lower limits of temperature). It is much smaller in lead-free technology.
In fact, if the device supplier only meets the internationally recommended 260oC limit for device design, the problem faced by users is even greater. The welding temperature process window that is owned may not even reach the above mentioned 14oC. This is because some devices, such as BGA package designs, in convection heating applications, the package body temperature is often higher than the bottom solder joint temperature. This is not a big problem. What makes the problem worse is that these devices are generally also devices with large heat capacity, and the thermal conductivity of the package is not very good. Since there are always some devices with much smaller heat capacity on the same PCBA (Note 3), it is very difficult to reduce the actual temperature difference by process adjustment and ensure that they are all within the process window.
Not only is the shrinking of the process window a huge challenge for the craftsman, but the increase in the welding temperature also makes the welding work more difficult. One of them is the oxidation phenomenon during high temperature welding. We all know that oxide layers can make soldering difficult, poorly wetted, and cause solder joints that affect solder joint life. The degree of oxidation, in addition to sufficient control of the incoming material itself, the user's inventory conditions and time, pre-processing treatment (such as dehumidification baking), and the heat energy in the preheating (or constant temperature) stage of welding ( Temperature and time) and so on are the determining factors. The increase in temperature of the lead-free technology is causing the weld end to cause more oxidation in the preheating section. If the flux of the solder paste is insufficient, or if the reflow profile is improperly set in the 'cleaning/deoxidizing' section, poor soldering may occur during reflow.
The 'popcorn' phenomenon is another problem that will be exacerbated in lead-free technology. There are some research reports in the industry that many ICs have improved moisture resistance by one or two levels in lead-free soldering due to temperature rise. In other words, the user's moisture control or handling must also be enhanced. This will have a more serious impact on users who are in small batch production. Because many users of small batch production have a long time of incoming inventory time. If the moisture-proof facilities in stock are not ideal, it is necessary to prevent the 'popcorn' problem by baking and dehumidifying before assembly. This practice is more frequent after entering the lead-free era because it is more sensitive to moisture absorption. Although baking can solve the problem of 'popcorn', the baking process will aggravate the oxidation of the soldered end of the device and bring about the difficulty of soldering. A viable option is to use inertia.
'Tianbei' is 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 solution is the same as the lead-containing technology, where the DFM controls the solder tip and pad size, as well as the thermal capacity at both ends. Secondly, the temperature difference between the two ends of the device can be reduced by process adjustment. It should be noted that although the principle is unchanged, the lead-free process window will be smaller, so the user must first ensure that the furnace used by itself has sufficient capacity. That is, there is good heating efficiency and stable airflow.
'Pneumatic holes' in the tin-lead technology is already a problem that is not easy to solve completely. After entering the lead-free technology, this problem will become more serious with the increase of the surface tension of lead-free alloys. To eliminate the 'stomach' problem, there are three factors that must be closely coordinated and cared for. It is the solder paste characteristics (certified selection of solder paste), DFM (device solder joint structure, pad and stencil opening design), and reflow process (temperature curve setting). The control principle is no different from the lead-containing technology, except that the window is smaller.
Since the lead-free soldering process window is significantly smaller than the lead-containing soldering process window, some 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 preheating. But nitrogen is not omnipotent, it can't solve all the problems caused by lead. In particular, it is impossible to solve the problems that have been caused before the welding process. For example, solder paste, reflow oven capability, DFM and other issues. And the use of nitrogen increases costs, so it should not be a primary consideration. Should be positioned as a 'remedy'. That is to say, the correct handling attitude should be considered after the implementation of 'technical integration' to confirm that other effective factors cannot be improved or properly controlled before considering whether to implement the nitrogen welding process. There are not many users of nitrogen in China, but in the two companies I contact, virtually no nitrogen is needed. Its process problems should be solved by other more cost-effective practices. So here to remind users, although nitrogen will help, you don't have to resort to it. It is not advisable to rush to make a decision before you have other knowledge.
The small process window not only requires high process modulation accuracy, but also requires that the process stability must be very high. Otherwise, even if the process is set to the optimum point, the offset of the process will cause the mass to be quickly shifted out of the controlled zone. Equipment is a key factor in order to be stable. In current reflow soldering equipment, furnace design using the principle of forced hot air convection is a mainstream. Hot air convection technology can replace the early gas phase and later infrared radiation technology, because of its controllable heating rate and constant temperature. But unfortunately, hot air convection is a weakness in heating efficiency and heating uniformity and repeatability. These weaknesses are not serious in lead-based technology and can be accepted in many cases. Hot air convection technology will be challenged as lead-free technology shrinks over the process window and demands for repeatability. Some low- and medium-grade equipment 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 a hot air reflow oven is through hot air as a medium for heat transfer. Air itself is not a good thermal conductor. And enough 'convection' must be used to achieve heat transfer. So how the furnace controls the design of the internal airflow is a key. The flow of air is very difficult to control accurately. Even for a well-designed furnace, the heat transfer efficiency will be due to changes in the air pressure in the furnace (from changes in the exhaust system, aging of the fan, gradual blockage of the air outlet, etc.), load changes (inlet time or interval), The solder paste volatiles, the aging of the equipment, and the like vary. Therefore, if you want to best control the welding process, you must have a constant supervision.
At present, many users in China are not very concerned about the process control of welding. Many users do not have any monitoring means throughout the welding process. Some of the better ones will use the test board to periodically measure the change in furnace temperature through the furnace. Although this is also a viable option, there are two weaknesses:
1. High cost – the measurement board must be a product to make sense. Empty boards do not really detect all possible changes. The effect of the analog board is satisfactory, but the design is somewhat difficult. If you use an actual product, the cost can be high. Moreover, the test board is not infinite life and must be replaced after a certain number of times. In addition to the cost of the board, the personnel, time, etc. required to test the certification are also elements of cost.
2. Missed inspections – the above practices cannot be continuous. The average user will be at the beginning of the shift or at the shift, as well as at some time in the middle of the shift. Therefore, it belongs to a sampling inspection technique. Moreover, the number of samples sampled is not high. Therefore, the false positive rate of missed detection is also high.
Theoretically, in the soldering process, the temperature of the solder joints is as long as the melting temperature of the solder alloy is reached. However, in actual cases, the solder which has just reached the melting point temperature has a particularly poor wettability. So we have to increase the temperature of the actual solder joint to increase the wetting ability. Since the wettability of lead-free alloys is worse than that of tin-lead alloys, this practice is even more necessary in lead-free technology. Devices and boards on PCBA have temperature extremes, and the current temperature requirements for lead-free technology are 260oC. Although this temperature is improved compared to 240oC for lead-containing technology, the solder joint temperature is increased by the influence of melting point temperature and wettability, which results in an allowable process window (upper and lower limits of temperature). It is much smaller in lead-free technology.
In fact, if the device supplier only meets the internationally recommended 260oC limit for device design, the problem faced by users is even greater. The welding temperature process window that is owned may not even reach the above mentioned 14oC. This is because some devices, such as BGA package designs, in convection heating applications, the package body temperature is often higher than the bottom solder joint temperature. This is not a big problem. What makes the problem worse is that these devices are generally also devices with large heat capacity, and the thermal conductivity of the package is not very good. Since there are always some devices with much smaller heat capacity on the same PCBA (Note 3), it is very difficult to reduce the actual temperature difference by process adjustment and ensure that they are all within the process window.
Not only is the shrinking of the process window a huge challenge for the craftsman, but the increase in the welding temperature also makes the welding work more difficult. One of them is the oxidation phenomenon during high temperature welding. We all know that oxide layers can make soldering difficult, poorly wetted, and cause solder joints that affect solder joint life. The degree of oxidation, in addition to sufficient control of the incoming material itself, the user's inventory conditions and time, pre-processing treatment (such as dehumidification baking), and the heat energy in the preheating (or constant temperature) stage of welding ( Temperature and time) and so on are the determining factors. The increase in temperature of the lead-free technology is causing the weld end to cause more oxidation in the preheating section. If the flux of the solder paste is insufficient, or if the reflow profile is improperly set in the 'cleaning/deoxidizing' section, poor soldering may occur during reflow.
The 'popcorn' phenomenon is another problem that will be exacerbated in lead-free technology. There are some research reports in the industry that many ICs have improved moisture resistance by one or two levels in lead-free soldering due to temperature rise. In other words, the user's moisture control or handling must also be enhanced. This will have a more serious impact on users who are in small batch production. Because many users of small batch production have a long time of incoming inventory time. If the moisture-proof facilities in stock are not ideal, it is necessary to prevent the 'popcorn' problem by baking and dehumidifying before assembly. This practice is more frequent after entering the lead-free era because it is more sensitive to moisture absorption. Although baking can solve the problem of 'popcorn', the baking process will aggravate the oxidation of the soldered end of the device and bring about the difficulty of soldering. A viable option is to use inertia.
'Tianbei' is 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 solution is the same as the lead-containing technology, where the DFM controls the solder tip and pad size, as well as the thermal capacity at both ends. Secondly, the temperature difference between the two ends of the device can be reduced by process adjustment. It should be noted that although the principle is unchanged, the lead-free process window will be smaller, so the user must first ensure that the furnace used by itself has sufficient capacity. That is, there is good heating efficiency and stable airflow.
'Pneumatic holes' in the tin-lead technology is already a problem that is not easy to solve completely. After entering the lead-free technology, this problem will become more serious with the increase of the surface tension of lead-free alloys. To eliminate the 'stomach' problem, there are three factors that must be closely coordinated and cared for. It is the solder paste characteristics (certified selection of solder paste), DFM (device solder joint structure, pad and stencil opening design), and reflow process (temperature curve setting). The control principle is no different from the lead-containing technology, except that the window is smaller.
Since the lead-free soldering process window is significantly smaller than the lead-containing soldering process window, some 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 preheating. But nitrogen is not omnipotent, it can't solve all the problems caused by lead. In particular, it is impossible to solve the problems that have been caused before the welding process. For example, solder paste, reflow oven capability, DFM and other issues. And the use of nitrogen increases costs, so it should not be a primary consideration. Should be positioned as a 'remedy'. That is to say, the correct handling attitude should be considered after the implementation of 'technical integration' to confirm that other effective factors cannot be improved or properly controlled before considering whether to implement the nitrogen welding process. There are not many users of nitrogen in China, but in the two companies I contact, virtually no nitrogen is needed. Its process problems should be solved by other more cost-effective practices. So here to remind users, although nitrogen will help, you don't have to resort to it. It is not advisable to rush to make a decision before you have other knowledge.
The small process window not only requires high process modulation accuracy, but also requires that the process stability must be very high. Otherwise, even if the process is set to the optimum point, the offset of the process will cause the mass to be quickly shifted out of the controlled zone. Equipment is a key factor in order to be stable. In current reflow soldering equipment, furnace design using the principle of forced hot air convection is a mainstream. Hot air convection technology can replace the early gas phase and later infrared radiation technology, because of its controllable heating rate and constant temperature. But unfortunately, hot air convection is a weakness in heating efficiency and heating uniformity and repeatability. These weaknesses are not serious in lead-based technology and can be accepted in many cases. Hot air convection technology will be challenged as lead-free technology shrinks over the process window and demands for repeatability. Some low- and medium-grade equipment 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 a hot air reflow oven is through hot air as a medium for heat transfer. Air itself is not a good thermal conductor. And enough 'convection' must be used to achieve heat transfer. So how the furnace controls the design of the internal airflow is a key. The flow of air is very difficult to control accurately. Even for a well-designed furnace, the heat transfer efficiency will be due to changes in the air pressure in the furnace (from changes in the exhaust system, aging of the fan, gradual blockage of the air outlet, etc.), load changes (inlet time or interval), The solder paste volatiles, the aging of the equipment, and the like vary. Therefore, if you want to best control the welding process, you must have a constant supervision.
At present, many users in China are not very concerned about the process control of welding. Many users do not have any monitoring means throughout the welding process. Some of the better ones will use the test board to periodically measure the change in furnace temperature through the furnace. Although this is also a viable option, there are two weaknesses:
1. High cost – the measurement board must be a product to make sense. Empty boards do not really detect all possible changes. The effect of the analog board is satisfactory, but the design is somewhat difficult. If you use an actual product, the cost can be high. Moreover, the test board is not infinite life and must be replaced after a certain number of times. In addition to the cost of the board, the personnel, time, etc. required to test the certification are also elements of cost.
2. Missed inspections – the above practices cannot be continuous. The average user will be at the beginning of the shift or at the shift, as well as at some time in the middle of the shift. Therefore, it belongs to a sampling inspection technique. Moreover, the number of samples sampled is not high. Therefore, the false positive rate of missed detection is also high.
Cangnan Younglite Electrical Co., Ltd. is located in Ningbo City, the most important seaport city in China. We are only 40 Kilometers far from Ningbo Airport. Near the express way, the transportation is very convenient.We are specialized in manufacturing and exporting LED Flashlight, headlamp, camping lantern and so on.We have outstanding management, strong production capability and advanced technology ability to design and develop many new items which have good quality control and testing at every stage of the production process. We can ensure that each product is eco-friendly and will provide long life and high performance service. Currently, our products are very popular in the USA, Europe, South Asia, the Middle East and Hong Kong."Quality first, delivery on time, reputation as life" is our premier principle. Let us make better quality products to you-----our valued customers.
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