Anyone Designing Lead Free Yet?

[roseann]

Interested in what materials/fabrication processes designers have worked with any success? How did you come about your decisions? REALLY would like to know about PCB finishes you have chosen and WHY?
thanks

[Carl_at_xrite]

We are beginning our transition after we test some new material.
The finish we have tested and proven has been the ENIG finish.

Enig finish does have its pitfalls but as for shelf life, availability and storage requirements this seems best to suit us.

Immersion tin (omicron)is a very good finish but can be run through a limited number of thermal cycles (ours being 4), shelf life is also not the greatest.
On a positive side if the thickness can be maintained over your thermal cycles without a degradation in finish the flatness and solderability are very good.
this is not widely available and the baths are hard to maintain.

Immersion silver has excellent solderability and coplanararity but the shelf life handling and storage requirements are issues

OSP's again the handling,shelf life and wash requirements would not work for us.

[BPOWERS]

We have been designing to lead free for about 6 months now,for our applications iso410 is our matl choice with ENIG as a finish.

bob

[Tom Frayda]

Hi Carl-

This came up on the PADS listserver and I thought you may find it interesting. I previously was not aware of this failure mode for ENIG.

http://www.alphametals.com/products/solder_solutions/pdf/Global_SMT_and_Pkg_Magazine-The_Black_Pad_Failure_Mechanism.pdf


Tom

[Carl_at_xrite]

Tom,
This is one of the drawbacks I indirectly referred to in my inital post.
Duriing our initial procurement and solderability testing of leadfree/RoHs compliant PCB's with an ENIG finish we set out with the following set of tests and inspections.

1. upon receipt we do a complete visual inspection of all boards to look for poor coverage or something out of the norm.
2. initial solderability test and thermal profiling (we always request a solder sample for use before population).
3. solderability review
4. populate remaining boards and reflow.
5. post reflow inspection and board level test.
6. environmental testing (extended thermal cycles and humidityand unit level functional testing)
7. post environmental test inspection
8. board level test
Seems redundant but the black pad problem and enig's sometimes brittle solder joints call for some due dilligance as fatr as test and inspection.
Also one thing to keep in mind is that no matter what finish you may use or what alloy you may use for soldering there are inherent problems in every one.
Lead free solder over time can actaullt fracture at the fillets, gold grows whiskers as does tin as does solder. (nasa website)
And even if you solve the surface finish question and eliminate whiskers and fractures etc. you m ay still have to deal with filaments on the internal layers of your PCB (CAF) this may be more prevalent at say the BGA substrate level or CSP's but the problem exists.
There is still alot of research yet to be done
I can send you links or pdf's of some of these articles if you are interested.

Carl