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

 

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

 

 

 

 

 

 

 

 

Advantages of low-temperature solder paste: low-temperature mating material properties

 

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 the benefit of cost saving, because the reflow oven temperature can be soldered without raising the temperature too high, and the most commonly used object should be FPC. Or welding LED light strips!

 

 

 

Just like when we met RD before, we asked to use a three-layer FPC to make HotBar welding, and the FPC can only have pads on one side and no via holes. In this way, the hot pressing of HotBar The heat of the thermodes cannot directly contact the FPC and PCB pads to effectively conduct heat to melt the solder. The heat of the thermodes must pass through the three layers of FPC before it can be conducted to the solder surface. You can imagine the degree of heat energy accumulation. Because I was worried that the HotBar's heat would be too high or heated for too long, causing the FPC to burn. In fact, I was even more worried that it would cause quality and reliability problems 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 deteriorate, that is, the solder joints are more likely to break due to external force, so the de-panel after SMT must not be broken by hand, and stamp holes are not recommended. It is designed to remove the edge of the board; in addition, it is not recommended to cool down too fast after reflow, otherwise the problem of solder joint embrittlement will easily occur.

 

Secondly, because low-temperature solder paste is not mainstream in the market, delivery time, inventory management, and avoiding mixing of materials must 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 to products through hot and cold cycles. However, if the silver content is too much, it will increase the brittleness, so the silver content in the commonly used solder paste will not exceed 3%.

 

 

 

Application case: Feasibility assessment of HotBar soldering using low-temperature solder paste

I have asked everyone about the company's recent special needs regarding HotBar. This problem has been bothering the work bear. That is, RD requires the use of a three-layer FPC to make HotBar, and the FPC can only have a pad on one side. That is to say, the heat of the thermal head cannot directly contact the soldering pads of the FPC and PCB to conduct heat and melt the solder. The heat of the thermal head needs to pass through three layers of FPC to be directed to the solder surface.

 

In fact, such a HotBar process is not impossible. The main concern is that the heat may be too high or heated for too long, which may burn the FPC, causing subsequent quality and reliability issues.

 

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

 

 

Use low temperature solder paste and use HotBar machine to complete FPC soldering. The disadvantage is that the reliability of low-temperature solder paste is generally poor, and it is easily brittle and cannot withstand too much pulling force. Therefore, this process cannot print low-temperature solder paste on the PCB board end. If there are only HotBar and other small resistors and small capacitors on the PCB, you can consider printing low-temperature solder paste directly. Otherwise, it is recommended to print low-temperature solder paste on the FPC and pass it through the reflow oven. Use it for the HotBar process.

 

  Directly put the FPC into SMT and pass through the furnace for welding. The disadvantage is that the FPC may require hand ornaments, and a jig needs to be made to fix and hold down the FPC. I have never 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 have suggested whether ACF can be used to replace the HotBar process? In fact, ACF is mostly used in the COG process. Even most LCM FPCs now use ACF as the welding medium. However, because the bonding force of ACF is too small, under the ACF area of 10mmx3mm, the X-direction's anti-peeling The force is about 500g, and the anti-peeling force of Y-direction is about 200g. It can be pulled up by just pulling it, so most of them need to add additional protective materials to increase its anti-peeling force. Currently, it is more common to use silicone. (silicone) covering COG and FPC. Moreover, ACF has two fatal flaws. The first is its poor reliability. After long-term use, it is prone to peeling off, especially in high temperature and high humidity environments. Secondly, the storage environment of ACF raw materials is very important. Under high temperature and high humidity environment, it is easy to deteriorate, 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 peak value is recommended to be 175°C. After completing the HotBar hot pressing, the peeling force of the HotBar was tested to be 1.5Kgf, which was lower than I expected. In addition, we also pushed LED parts that also used low-temperature solder paste, and the pushing force was 4.0Kgf.

 

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

 

(We have also tried using SAC305 solder paste, and the pulling force results are very good, reaching about 4.5Kgf, but the FPC soft board will have scorch marks.)