Electronic – Transitioning to lead-free reflow and soldering

Honestly, I wouldn't try to solder my own BGA's. I know this doesn't directly address your issues, but hear me out.

It takes a lot of work and effort to solder a BGA. There's a lot of trial and error. A lot of messed up test boards. But then it's soldered. Now what?

Now you have to prove that it's soldered correctly. For that you need one or more of the following: JTAG test (US$10k, never has 100% coverage), optical inspection (US$20k for the equipment), or X-Rays (US$500k). The cost of doing these tests is too much for the normal hobbyist, and is even beyond many small companies.

Skipping those tests, you proceed with debugging your PCB. And let's say that the BGA is a complex CPU. Inevitably you'll find a bug. The CPU will randomly crash. Is it your software, your electrical design, or the soldering on the BGA that's causing the problem? Debugging this, in light of some possibly problematic soldering, is going to be terrible. It will add a lot of time to your debug process, possibly months, and you'll loose a lot of hair on your head. And then you can repeat this for the next major bug.

Without confidence that your soldering is perfect, you will always have this dark cloud over your head. Every little bug that shows up "could be a BGA soldering problem". This is made worse if you have multiple engineers working on the same PCB since the software guy will be questioning the hardware guy, etc.

Then, even if the BGA soldering is perfect, did the chip get too hot? Did you destroy the chip by getting it too hot? Even on modern assembly lines this is an issue. But with the proper equipment you can adjust and measure the temperature profiles to at least get you in the right ballpark. On one board I did recently, the BGA's were being damaged. The solder balls looked great, but under a very nice X-Ray machine we could see that the gold bond wires melted from the heat.

I've been there. Not at the hobbiest level, but professionally as we were bringing up new boards while the assembly shop was learning to do BGA's. We had no JTag. No optical inspection. And the X-Rays were terrible. Our PCB had 11 BGA's on them. That was 2 years of hell I don't wish to repeat.

So, here's my recommendation:

Get someone who has the proper equipment, training, and experience to solder your BGA's. There are a lot of contract manufacturers that'll do a single BGA. It takes money, but that's way less than the time you'll spend trying to debug your own soldering.

If you must do it yourself, then you should get the proper equipment, training, and figure out how to get the experience required. For this to pay off in the end, you need to have a large enough company and need to justify the huge amount of time and money that you'll put into this.

But I would never try to just kludge something together. That's a recipe for, um, bad stuff.

42/58 Tin / Bismuth is not unknown as a low temperature solder but has issues.

While widely used for some very serious applications (see below) it is not a mainstream industry contender for general use. It is not obvious why not given its substantial use by eg IBM.

Identical to the Bi58Sn42 solder you cite is:

• Indalloy 281, Indalloy 138, Cerrothru.

  Reasonable shear strength and fatigue properties.

  Combination with lead-tin solder may dramatically lower melting point and lead to joint failure.

  Low-temperature eutectic solder with high strength.

  Particularly strong, very brittle.

  Used extensively in through-hole technology assemblies in IBM mainframe computers where low soldering temperature was required.

  Can be used as a coating of copper particles to facilitate their bonding under pressure/heat and creating a conductive metallurgical joint.

  Sensitive to shear rate.

  Good for electronics. Used in thermoelectric applications.

  Good thermal fatigue performance.

  Established history of use.

  Expands slightly on casting, then undergoes very low further shrinkage or expansion, unlike many other low-temperature alloys which continue changing dimensions for some hours after solidification.

Above attributes from the fabulous Wikipedia - link below.

According to other references it has low thermal conductivity, low electrical conductivity, thermal embrittlement issues and potential for mechanical embrittlement.

SO - it MAY work for you, but I'd be very very very cautious about relying on it without very substantial testing in a wide range of applications.

It is well enough known, has obvious low temperature advantages, has been widely used in some niche applications (eg IBM mainframes) and yet has not been welcomed with open arms by industry in general, suggesting that it's disadvantages outweigh advantages except perhaps in areas where the low temperature aspect is overwhelmingly valuable.

Note that the chart below suggests that flux cored versions seem to be specifically unavailable either as wire or as preforms.

Comparison chart:

The above chart is from this superb report which however does not provide detailed comment on the above issues.

Wikipedia notes

• Bismuth significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of Sn16Pb32Bi52 with melting point of only 95 °C, which diffuses along the grain boundaries and may cause a joint failure at relatively low temperatures. A high-power part pre-tinned with an alloy of lead can therefore desolder under load when soldered with a bismuth-containing solder. Such joints are also prone to cracking. Alloys with more than 47% Bi expand upon cooling, which may be used to offset thermal expansion mismatch stresses. Retards growth of tin whiskers. Relatively expensive, limited availability.

Motorola's patented Indalloy 282 is Bi57Sn42Ag1 . Wikipedia says

• Indalloy 282. Addition of silver improves mechanical strength. Established history of use. Good thermal fatigue performance. Patented by Motorola.

Useful lead free solder report - 1995 - nothing to add on above subject.