What solder for low temp removal and resoldering to pcb

Question

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.

Thanks

Answer 1

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.

Answer 2

ChipQuik is the product that I use. It is available from many on-line sources including direct from the company and from Amazon.

To use it, flood the leads with flux. ChipQuik sells a sticky rosin-based flux which does work well but is messy and (by its very nature, sticky). Instead, I use a no-clean flux that I get from Amazon: Amtech NC-559 flux. I have purchased both 30cc and 100 gram containers - both are identical but the 30cc offering is stupid expensive at Can $50 (but immediate shipment). The 100 gram bottle is Can $36 and contains about triple the amount in the 30cc syringe. But the 100 gram bottle took many weeks to arrive at my shop.

You may want to watch YouTube videos of Louis Rossman showing how he uses the stuff. Be warned: I think that Louis uses FAR too much of the flux - you simply need only a tiny amount dispensed as a 1.5mm or 2mm line down the length of the chip leads.

After the component has been removed, use a desoldering station or solder wick to remove all of the ChipQuik solder from the pads. Again - a small amount of the Amtech flux helps. Clean the board with IPA before proceeding any further and inspect.

Install the new chip(s) and solder with 63/37 solder. I prefer Kester 44 flux but that does leave a non-conductive rosin residue. Clean with isopropyl alcohol (IPA) and toothbrush or Q-tips. The alcohol also cleans the Amtech flux perfectly.

So: flux the leads of the original chip. Apply ChipQuik solder to all of the leads. You will notice that the solder stays molten for much longer than normal. When the solder on all of the leads is molten, just lift the chip out of the board.

Apply more flux and remove all traces of the ChiQuik solder from the pads and holes. Add flux as needed. Clean both sides of the board with alcohol.

Install the new chip. Apply yet more flux and solder the chip. Use an easily-cleaned no-clean flux solder if you wish.

One final cleaning with the IPA and you should be good to go.

Although I prefer to use my desoldering station for most of these jobs, there are cases where only something like ChipQuik will do the job. I've used a LOT of ChipQuik over the years and can attest to how well it works.