Lead-free low temperature solder paste usually refers to solder paste containing "bismuth (Bi)" metal.

The melting point of our general lead-free solder paste SAC305 is 217°C, but the melting point of Sn64Bi35Ag1 is only 178°C, while the melting point of Sn42Bi58 is as low as 138°C, which means it contains "bismuth (Bi)" tin The melting temperature of the paste is about 39°C to 79°C lower than that of the SAC305 lead-free solder paste.

 

 

Advantages of low temperature solder paste: low temperature matching material characteristics

The biggest advantage of lowering the melting point of solder paste is that it can solve the technical problem of insufficient temperature resistance of soldering materials. In addition, it can also achieve cost-saving benefits, because the reflow furnace temperature can be soldered without adjusting too high, and the most commonly used object should be FPC Or LED light bar welding!

 

Just as we encountered before, RD requires a 3-layer FPC to make HotBar soldering, and the FPC can only have a pad on one side without a via hole, so the HotBar hot press The heat of the head (Thermodes) cannot directly contact the solder pads of the FPC and PCB to effectively conduct heat to melt the solder. The heat of the thermal head must pass through the three-layer FPC before it can be conducted to the solder surface. You can imagine the degree of heat accumulation. Because I was worried that the heat of the HotBar was too high or heated for too long to cause the problem of FPC burnt, in fact, I was more worried about the problem of quality reliability in the future, so I later chose to use "lead-free low temperature solder paste."

 

Disadvantages of low temperature solder paste: poor solder strength

However, the disadvantage of low-temperature solder paste is that its solder strength will be worse, that is, the solder joints are more likely to be broken due to external forces, so do not break the de-panel after SMT, and stamp holes are not recommended. The design is done to remove the board edge; in addition, the cooling rate after reflow is not recommended to be too fast, otherwise the problem of solder joint embrittlement is prone to occur.

Secondly, because low-temperature solder paste is not the mainstream in the market, delivery time, inventory management, and avoiding mixing must all be overcome.

Ag is the only difference between SnBi and SnBiAg

As for the difference between SnBi and SnBiAg, it lies in Ag (silver). Generally speaking, "silver" is added to solder paste to improve wettability, strengthen solder joint strength, and improve fatigue resistance, which is beneficial for products to pass through cold and heat cycles. Tested, but if the silver content is too much, it will increase the brittleness, so the most commonly used solder paste contains no more than 3% of silver.

 

Application case: Feasibility evaluation of low-temperature solder paste for HotBar soldering

I’ve asked you about the company’s special requirement for HotBar recently. This problem has been plagued by the work bears, that is, RD requires the use of a three-layer FPC to make the HotBar, and the FPC can only have a pad on one side. That is, the heat of the thermal head cannot directly contact the solder pads of the FPC and PCB to conduct heat and melt the solder. The heat of the thermal head needs to pass through the three-layer FPC to be conducted to the solder surface.

In fact, such a HotBar process is not impossible. The main concern is that too high heat or heating for too long may scorch the FPC, which will cause subsequent quality reliability problems.

After thinking about it, in addition to using the HotBar machine to complete the process in a traditional way, we came up with two ways to achieve this requirement:

Use low temperature solder paste and use HotBar machine to complete FPC soldering. The disadvantage is that the reliability of the general low-temperature solder paste is relatively poor, and it is easy to be brittle and cannot withstand too much pulling force. Therefore, this process cannot print low-temperature solder paste on the PCB side. If there is only HotBar and other small resistance and small capacitors on the PCB, you can consider printing low-temperature solder paste directly. Otherwise, it is recommended to print the low-temperature solder paste on the FPC through the reflow oven. Take it for the HotBar process.

 The FPC is directly welded through the SMT furnace. The disadvantage is that the FPC may need hand ornaments, and a furnace fixture must be made to fix and press the FPC. I haven't actually done such a process, but it should be feasible, because I have seen other people's products made in this way.

In addition, some people suggest whether ACF can be used to replace the HotBar process? In fact, ACF is mostly used in the COG process. Even if most LCM FPCs now use ACF as the welding medium, the bonding force of ACF is too small. Under the ACF area of ​​10mmx3mm, the X-direction is resistant to peeling. The force is about 500 grams, and the anti-peeling force of Y-direction is about 200g. It can be pulled up by pulling it at will. Therefore, it is necessary to add additional protective materials to increase its anti-peeling force. At present, the use of silica gel is more common. (silicone) Covered on COG and FPC. Moreover, ACF has two fatal injuries. The first is its poor reliability. After a long time of use, it is easy to peel off, especially in a high temperature and high humidity environment. The second is that the storage environment of ACF raw materials is very important. It is prone to qualitative change under high temperature and high humidity environment, resulting in poor bonding.

 

Low temperature solder paste

For this purpose, this time we chose Indium 5.7LT 58Bi/42Sn (bismuth tin) low temperature solder paste, its melting point is only 138°C, and the recommended peak value is 175°C. After the HotBar hot pressing is completed, the peeling force of the HotBar tested is 1.5Kgf, which is lower than I expected. In addition, we are also pushing LED parts that also use low-temperature solder paste, and the thrust is 4.0Kgf.

Basically, this result is barely acceptable. If a better process is not found, this process condition will be selected first.

(We have also tried using SAC305 solder paste. The tensile force is very good, reaching about 4.5Kgf, but the FPC soft board will appear burnt marks.)