Electronics > Manufacturing & Assembly

Lead free solder joint rework

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IconicPCB:
A client has a problem with lead free solder joints.
The joints are cracking open. The assemblies are failing.
I have been asked to reflow the joints with the view to repairing the assemblies.Now that is not a problem. I can certainly reflow the assembly.

My question is .... will reflowing the joints provide a reasonable resolution to this problem or will they fail in a similar fashion in not too distant a future.

Stray Electron:
  Lead free solder is absolute shit in my opinion.  A friend of mine has a job at KSC resoldering circuit boards for satellites with leaded solder after they had countless failures with lead free solder.  He's been at it for about four years now and there is no end in sight. I worked for a large aerospace company that built systems for the .MIL and we never stopped using leaded solder. That should tell you something.

Kasper:

--- Quote from: Stray Electron on August 09, 2022, 03:26:20 am ---[...] I worked for a large aerospace company that built systems for the .MIL and we never stopped using leaded solder. That should tell you something.

--- End quote ---

All that says is they don't like change.  Changes require documentation and testing.  In aerospace for .mil, that's a huge hurdle.

I used to work in avionics and we used all sorts of old components and did lots of mods.  New compoments and new design would have been much better but it would have required years of work to re-certify it.

Whales:
There are lots of variables that control solder joint strength and resilience to cracking. 

* Solder alloy (initial used)
* Solder alloy & gradients (final product, affected by working time, pad plating, tip material, etc)
* Forces being applied (large/heavy cantilevered parts or human-handled parts, vibration, multiple mounting surfaces, etc)
* Geometry of solder joint: gap between pad and leg
* Wetting of solder (unclean joints only partly attach)
* Heating & cooling of solder (cold joints, grain structure, etc)
* Probably more
If the client says they are seeing this particular problem often: I would not rely on just reflowing the solder, it might not be enough or it might be irrelevant and the problem will just re-occur again. If there are problems occurring reliably on the same parts across many boards (but not other parts!) then it smells like it might be a non-alloy related issue.  You need to get the boards in your hands and find out everything.

If you choose to try removing and replacing the solder: don't just base your decision on leaded vs lead free.  Go for a solder that you know works.  There are many more things to go wrong with solders.

SMTech:

--- Quote from: Stray Electron on August 09, 2022, 03:26:20 am ---  Lead free solder is absolute shit in my opinion.  A friend of mine has a job at KSC resoldering circuit boards for satellites with leaded solder after they had countless failures with lead free solder.  He's been at it for about four years now and there is no end in sight. I worked for a large aerospace company that built systems for the .MIL and we never stopped using leaded solder. That should tell you something.

--- End quote ---

We've been using exclusively lead free ever since the legislation introduced it in 2006. We have never seen a failure we can attribute to the alloy. Our assemblies go into all sorts of industries and all sorts of environments, but probably never see severe temperature extremes which some alloys(low temp ones in particular) certainly don't like. We also do some prototyping/low volume work for people in the military space and not once have they specified lead-free be used for their assemblies.

The OP however has a bigger problem, identifying what joints are failing and why, physical or thermal stress would be the obvious ones and are not something lead is magically immune to.

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