PCB Technical - Low temperature solder paste production HotBar soldering

Regarding the special needs of HotBar, this problem has always plagued PCB companies, 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, the heat of the hot press head cannot directly contact. To the solder pads of the FPC and PCB to conduct heat conduction 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 the FPC may be burnt if the heat is too high or heated for too long, 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:

1. Use low temperature solder paste and use HotBar machine to complete FPC welding.

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 are 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 a reflow oven. Take it for the HotBar process.

2. The FPC is directly welded through the furnace with SMT.

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. You can pull it up by just pulling it, so most of them need to add additional protective materials to increase its anti-peeling force. At present, it is more common to see the use of silica gel. (silicone) Covered on COG and FPC. Moreover, ACF has two fatal injuries. The first is its poor reliability. After a long period 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 PB process is not found, this PCB process condition will be selected first.