Does the low-temperature solder paste (LTS) process have a chance to become mainstream in the future?

In the past, in order to meet the requirements of the Restriction of Hazardous Substances Directive 2002/95/EC of the European Union, the solder in the PCBA process was changed from SnPb to SAC alloy, but the melting temperature of the solder was relatively increased. In response to the general trend of energy saving and carbon reduction, it seems that more and more companies are trying to develop the SAC high-temperature process in the direction of Low-Temperature Soldering.

In fact, after the soldering process was transferred to SAC alloy, the reflow peak temperature of the SMT production line also increased from the original 220˚C to about 250˚C, and the increase in soldering temperature also means the increase in parts materials and production costs, for example, It is necessary to use more high-temperature-resistant materials. The biggest change is the change in 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 the SnBi and SnBiAg alloys with Bi added on the basis of Sn. Therefore, today we will roughly discuss the advantages and disadvantages, feasibility, and possible future trends of LTS.

Advantages of the LTS process:

 》Energy saving and carbon reduction, reducing energy consumption. Because the solder alloy with a lower melting point is used, the temperature and time required to form the solder will be reduced and reduced, and the energy consumption will be relatively reduced, achieving the purpose of energy saving and carbon reduction.

 》Reduce the demand for high-temperature materials and reduce the cost of materials. The use of materials with lower temperature resistance above room temperature usually means lower material costs, at least the processing costs of materials are lower.

》Reduce the process threshold and improve the 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 temperatures will also be reduced by about 50%. Deformation and warping of the board is one of the main causes of HIP/HoP soldering of large leadless parts such as BGA and LGA and is also one of the important killers of MLCC rupture.

Disadvantages of LTS process:

 》Long-term reliability of solder joints is poor.

 》The biggest disadvantage of low-temperature solder is that the mechanical strength of its solder joints is not good, that is to say, 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 drop impact.

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

 The disadvantage of hot-tearing is likely to appear on the surface of PCB pads in the mixed soldering process of SAC solder balls and SnBi solder paste. In fact, hot-tearing is also prone to appear in the mixed process of lead-free and tin-lead, especially in BGA, which has already existed. Pre-soldered component 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 temperature profile should be used when SAC alloy BGA solder balls are mixed with LTS?

In fact, when using LTS, it is best to use 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, due to the helplessness of reality, there are almost no BGAs with low-temperature solder balls on the market at present, so we have to settle for the next best thing and use BGA solder balls of LTS and SAC alloys.

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 temperature 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, thereby changing the alloy ratio of the SnBi formula, so that there is an opportunity to slightly increase the solidification temperature of the SnBi area, and it is recommended to accelerate the cooling rate after the peak temperature, especially Accelerate the cooling 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. So, the above is simply a bunch of nonsense!

I believe that most 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. Personally, I 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 BGA 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 of course 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. As for how to evaluate the solderability of LTS, there are several laboratories in Taiwan that provide similar services. You can consult these laboratories first. 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 LTS? Strengthen solder strength?

At present, the more feasible LTS solder joint reinforcement solution is to use underfill. This solution actually existed when Chip Scale Package and flip-chip first appeared and was later applied to BGA. Generally, epoxy material glue is used to spot BGA. Or the edge of similar parts, through the principle of capillary action, the glue penetrates and fills the bottom of the part, and then heats and solidifies to achieve the purpose of filling gaps and strengthening solder joints. Later, some people use glue with relatively high viscosity to selectively spot on BGA Coner bond or BGA edge bond to strengthen the fixation.

Later, the so-called underfill sheet also appeared. After the board was printed with solder paste, it was placed on the BGA position of the PCB through the SMT placement machine (avoiding the solder joints), and then the BGA was placed on it, and finally, it was reflowed together with the BGA. The high temperature of the furnace melts the film to fill the gaps, and it solidifies after cooling. However, it should be noted that the underfill will only work after the board assembly is completed and the function test is correct, 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 of LTS applications, 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 much, and the strengthening of 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. 【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. It is recommended to consider the use of products with low yield rates before use.

According to the author’s own application experience, 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. Problematic solder joints will still cause problems. Therefore, to untie the bell, the person who tied the bell must find a way to minimize the source of stress affecting the solder joints.

Which products have the opportunity to adopt the LTS process?

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

》The product will not move after installation. Handheld devices are not recommended. Handheld devices are prone to mechanical shock stress caused by falling due to carry-on operation.

 》The service life of the product design is preferably within 5 years or less. It is recommended to perform Mean Time Between Failures assessment.

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

 》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 product use environment should be lower than 40˚C, and the maximum operating temperature should not exceed 85˚C.

 》Used in a general indoor environment, without severe high and low temperature fluctuations. Not recommended for use in vehicles or outdoor environments.

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

The future development trend of low-temperature process?

From the perspective of energy saving and carbon reduction, the LTS 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 LTS solder paste has a fatal shortcoming, which is the general 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 the action of vibration or thermal stress are not suitable for LTS process. It can only be said that although LTS solder paste can meet the demands of energy saving and carbon reduction, there is still a long way to go, and maybe LTS will not be able to completely replace it in the end. SAC, more likely to be LTS in parallel with SAC.