Electronic – What solder for low temp removal and resoldering to pcb


Have an issue with some legacy system – need to replace throughhole parts on PCB in situ while PCB is sandwiched between two sheets of acrylic with only small holes to access the leads on one side (i.e the back access is through a 3mm wide hole through a 10mm sheet of acrylic) and the front access is also tight (but just able to get a fine soldering tip in).

What we've been doing so far is prewarm the PCB to about 80C while trying to shield as much acrylic as possible. Then warm the leads (these have been soldered w leaded solder, (Sn60/Pb40) and remove but it is extremely difficult to get enough heat in due to the tight access and large ground planes and is causing issues with pad removal etc.

So I was considering using a low temp solder alloy but I am not experienced with them and from what I have seen, most are used mostly for when you want to reuse the IC not the PCB.

I'm sure it will work well for removal but its when we add the replacement part I am concerned that leftover low temp solder will produce a weak joint? I have read that many low temp solders alloy with leaded solder to produce brittle alloys – are there any that don't? Cost is not a major issue.


Best Answer

For chips that are hard to get off, I use low temp solder, then use a wick to clean up the remaining solder. It cleans off just like regular solder, if you clean it off good and put new solder on it.

The alloy contents of low temperature solders such as chip quick low temp solder (which is what I use) have a high bismuth content in them (specifically for one solder Sn42/Bi57.6/Ag0.4) this lowers the melting point, however it does make it more brittle.

"All combinations involving bismuth were brittle, caused by the stiffening effect of solder due to the homogenized presence of bismuth in the joint, thus the brittle IMC interface became the weakest link upon shearing," said Ning-Cheng Lee, vice president of technology at Indium.

Source: http://www.ipc.org/feature-article.aspx?aid=Improving-Reliability-with-Low-Temperature-Solders

Use the low temp solder and clean it off, then put better solder on. If regular solder cannot be applied after cleaning off then a different method of heating will need to be found or live with the risk of being brittle and use a solder with a high bismuth content.