Why the low temperature solder has been used

Why the low temperature solder has been used
In the past, in response to the European Union’s Restriction of Hazardous Substances Directive 2002/95/EC, RoHS, the solder in the PCBA process was changed from tin-lead (SnPb) to tin-silver-copper (SAC) alloys, however, relatively increase the welding temperature of the solder. In response to the general trend of energy saving and carbon reduction, it seems that increasing number of companies are trying to transform the SAC high temperature process to the low temperature process.
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    Which kind of low temperature solder is most popular

    Which kind of low temperature solder is most popular

    In fact, after the soldering process was transferred to SAC alloy, the peak reflow temperature of the SMT production line also rose from the original 220˚C to about 250˚C, and the increase in soldering temperature also means that part of materials and production costs are reduced. The need to use more high temperature resistant materials, the biggest change is that the engineering plastic materials, in addition, high temperature also deteriorates the quality of production, for example, materials are more likely to deform at high temperatures and cause poor welding.

    At present, the most well known low temperature solder is an alloy of tin-bismuth (SnBi) and tin-bismuth-silver (SnBiAg) based on tin (Sn) with bismuth (Bi) added.

    Advantages of low temperature solder process

    Energy saving and carbon reduction

    In low temperature solder process, because the solder alloy with a lower melting point is used, thus leading to the reduction of temperature, time and the energy consumption.

    Reduce the demand for high-temperature materials

    The use of materials with lower temperature resistance above room temperature usually means lower material costs during low temperature solder process

    Lower the process threshold and improve production yield

    Changing the solder alloy from SAC to SnBi will reduce the maximum temperature in the reflow furnace from 250˚C to about 175˚C, and correspondingly, the deformation rate of the circuit board at high temperature will also be reduced by about 50% which is one of the main causes of HIP/HoP soldering of large leadless parts such as BGA and LGA, and MLCC rupture.

     

    leadless parts such as BGA and LGA

    Disadvantages of low temperature solder process

    The long-term reliability of solder joints is poor

    The biggest disadvantage of low temperature solder is that the solder joints are relatively brittle and prone to tin cracking due to stress. Compared with SnPb and SAC alloy solder, the solder strength of SnBi alloy is very weak against thermal shock and impact drop.

    Hot-tearing defects are prone to occur in the reflow process

    Hot-tearing tends to appear on the surface of PCB pads in the hybrid soldering process of SAC solder balls, SnBi solder paste lead-free and tin-lead, especially in BGA with pre-soldered part solder joints. This is because during the soldering process, the SAC solder ball has a high melting point and is not easy to melt. 

    Even after melting, it will solidify earlier during the cooling process, while the SnBi solder paste will definitely melt during the reflow process and cool down. It also cures more slowly than SAC. Imagine that during the cooling process of the reflow furnace, the BGA solder balls have solidified or are not melted at all, leaving only a small portion of SnBi solder in a slurry state. 

    At this time, the PCB and BGA carrier board also gradually recover from the high temperature deformation. Once the gap between the BGA carrier board and the PCB is small deformation at high temperature and the gap becomes larger after returning to temperature (deformation recovery), it will pull the slurry SnBi solder that has not yet fully cured, thus forming a torn hot -tearing cracks.

    What kind of temperature profile should be used when BGA solder balls of SAC alloy are mixed with low temperature solder paste

    In fact, it is helpful to combine low temperature solder paste, low temperature solder balls and low temperature profiles at the same time, so as to obtain all the benefits of low temperature solder paste and the best soldering effect and quality. However, for the lack of BGAs with low temperature solder balls on the market, so PCB Manufacture havs to resort to low-temperature solder paste and SAC alloy BGA solder balls.

    If you want to achieve the best quality effect of SAC mixed with low temperature solder paste, you have to find a way to reduce the impact of hot-tearing, and the best reflow profile is to follow the temperature profile of SAC, because the high-temperature profile can be melted, at the same time SAC and SnBi alloy allow SAC to diffuse into the SnBi alloy area.

    SAC-alloy

    Thereby changing the alloy ratio of the SnBi formula, which can slightly increase the solidification temperature of the SnBi area, and it is recommended to accelerate the cooling rate after the peak temperature, especially rate between 217°C (SAC305) and 138°C (Sn42Bi58), the purpose is to let the SnBi solder area solidify immediately after the SAC solder area solidifies in the shortest time. But in this way, all the advantages of using LTS will be lost, and the solder strength is not as good as that of SAC alloy, so it is better to use SAC solder paste directly.

    Most of the cases where low-temperature solder paste is used are because the parts cannot withstand the high-temperature profile of SAC. In this case, only the low-temperature profile of low-temperature solder paste can be used. Experts suggest that the peak temperature of reflow should be reduced as much as possible without affecting the soldering quality. The purpose is to reduce the heat of PCB and reflow carrier board during reflow.

    At the same time, it is necessary to accelerate the cooling rate after the peak temperature of reflow. The purpose is to solidify the low-temperature solder before the deformation of the board recovers. However, if the cooling rate is excessively accelerated, there may be a risk of worsening the cracking of the BGA solder. For evaluation, a better temperature and cooling rate is selected after reliability tests and comparisons.It is not recommended to increase the peak reflow temperature, because the higher the temperature, the greater the deformation of the PCB and BGA carrier

    How to strengthen the mechanical strength of low temperature paste

    epoxy resin glue

    At present, the more feasible low temperature solder joint reinforcement solution is to use underfill. This solution actually existed when CSP and flip-chip appeared, and was later applied to BGA. Use epoxy resin glue to point on the edge of BGA or similar parts, and use the principle of capillary action to allow the glue to penetrate and fill the bottom of the part, and then heat and solidify to achieve the purpose of filling gaps and strengthening solder joints. Some use glue with a relatively high viscosity to selectively point on the four corners of the BGA (coner bond) or the four edges of the BGA (edge bond) to strengthen the fixation.

    Here comes to underfilm. After the board was printed with solder paste, it was placed on the BGA position of the PCB through an SMT placement machine (avoiding solder joints), and then the BGA was placed on it. The high temperature of the reflow furnace is used to melt the film to fill the gap, and then solidify after cooling. However, it should be noted that the underfill will only work after the board assembly and the function test, while the underfilm is added during the SMT process. If the yield rate of the product is not high, rework will be very troublesome.

    In addition, with the increase application of low temperature solder, there are also so-called epoxy paste and epoxy flux made as the times require. Epoxy paste is to add epoxy to the solder paste, directly print the solder paste and heat it after reflow, but since it is added to the solder paste, its dosage cannot be too much, and the solder strength of BGA parts may be limited. But if it is only for chip components or LED light boards, it should still have some effect.

    LED light boards 

    Epoxy flux uses solder paste printing and dispensing before mounting, which is a bit similar to underfilm. The effects of the above two processes of adding epoxy have yet to be further verified, and both of them have been completed before the test. Adding underfill can indeed strengthen the BGA’s ability to resist stress, but it can only delay the cracking of the solder due to stress, but cannot completely cure it. That is to say, after a period of use, the problematic solder joints will still cause problems. 

    Therefore, to untie the bell,it is necessary to find a way to minimize the source of stress affecting the solder joints.

    Which kind of products are able to adopt the low temperature solder process

    Now that we have learned that the solder joints of low temperature solder process products are relatively brittle and not resistant to stress, as long as the employ situation of electronic products is not under severe thermal stress (high and low temperature cycle) changes or mechanical stress (drop impact). If there is no need for long-term life design guarantee, it should be considered to use low temperature paste process. After all, save energy and cost. Here are some industry guideline references for citing low temperature solder:

    The product design service life is preferably within 5 years or less. It is recommended to perform MTBF (Mean Time Between Failures) evaluation.

    It is better if the main parts have an additional protection mechanism for solder joints, such as dispensing or caulking.

    solder-joints

    It is better if the IO parts have additional anti-insertion stress mechanism design, such as anti-over-insertion, anti-shaking and other mechanism designs.

    The operating condition of product is best below 40˚C, and the maximum operating temperature should not exceed 85˚C.

    It is generally used in indoor environment without severe high and low temperature fluctuations. Not recommended for using in vehicles or outdoor environments.

    At present, it is seen that low temperature solder is mostly used in LED lights, and mini-LED is also used in a small part, and some PC industries are also under evaluation.

    Conclusion

    From the perspective of energy saving and carbon reduction, the low temperature solder paste process is indeed more energy-saving, and it can also reduce the requirements for parts on high-temperature plastic materials and save costs. However, the current low temperature solder paste has a fatal shortcoming, which is the poor reliability. 

    Solder joints are relatively brittle, and may not have much impact on some small parts, but for some parts that have stress-bearing requirements, such as I/O parts, or products that may bend the circuit board after being subjected to external forces, or often Products under vibration or thermal stress are not suitable for low temperature solder process. 

    It can only be said that although low temperature solder paste can meet the demands of energy saving and carbon reduction, there is still a long way to go, and maybe low temperature solder will not be able to completely replace it in the end. SAC, more likely to be low temperature solder in parallel with SAC.

    FAQ

    At present, the most well-known low-temperature solder is an alloy of tin-bismuth (SnBi) and tin-bismuth-silver (SnBiAg) based on tin (Sn) with bismuth (Bi) added
    Energy saving and carbon reduction Reduce the demand for high-temperature materials Lower the process threshold and improve production yield
    The long-term reliability of solder joints is poor. Hot-tearing defects are prone to occur in the reflow process.

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