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Using parts with Tin Bismuth plating in tin lead soldering

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#38395

Using parts with Tin Bismuth plating in tin lead soldering | 12 December, 2005

Hi All was wondering is there anything I should be aware of or look out for if using parts with SnBi (Pb free) lead plating with Sn/Pb/Ag solder paste? I was always lead to believe that mixng lead and bismuth caused problems or are the bismuth levels so low its irrelevant?

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Samir Nagaheenanajar

#38401

Using parts with Tin Bismuth plating in tin lead soldering | 12 December, 2005

Bismuth, when used in the correct percentage will not create any problems. In fact, Bismuth alloy is known to be more reliable when stressed and thermally cycled. However, I learned that in too-high-of-a-concentration ( >4% of the alloy), there is a ternerary phase (not sure if i spelled that right) where the melting point of the alloy is MUCH less than, say, SAC305's.

Sn-Bi has also been associated with...... none other than Tin Whiskers.

There are some in the industry though that think Bismuth is the best thing since sliced bread!

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#38406

Using parts with Tin Bismuth plating in tin lead soldering | 12 December, 2005

Effect of lead on bismuth solders * Lead from hot air leveling (HAL) coatings can diffuse through the grain boundaries of the alloy. * Lead can form a eutectic composition of Bi52Pb32Sn16 in the grain boundaries. * Melting point of this eutectic alloy is 95*C. * Overall result is a large decrease in the strength of the joint. * Fillet lifting of PTH due to pasty range (206-213*C) * Avoid any lead with bismuth solders!!

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Samir Nagaheenanajar

#38415

Using parts with Tin Bismuth plating in tin lead soldering | 13 December, 2005

Dave, thanks for the info, and I, too, have heard all of these things with Bi, when used in SnPb systems.

Industry studies have been done, however, with the following findings:

* Bi addition in the level of 2-5% largely increased the alloy strength (per tensile/stress/strain testing), exceeding that of even Sn63Pb37.

* > 5% Bi results in less plasticity

* Formation of low temperature (ternerary phases) not detected.

I don't advocate the use of Bismuth, per se, but rather I'm just relaying info. that I learned this from a Lead-Free seminar that I attended at Riyadh's SMT Conference not too long ago.

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#38429

Using parts with Tin Bismuth plating in tin lead soldering | 13 December, 2005

Your response is from the point of view of using a Bismuth solder with Pb plated leads. Samir's point of view is from Bismuth in the plating on the leads being used with a Pb solder. There is a very large difference in the amount of Bi that will end up in the final joint, between these two scenarios. As you stated, the Bi alloy that causes problems is Bi52 Pb32 Sn16. The Bi ratio to Pb ratio is 52% to Pb 32% and Tin 16%. The volume of material in the total plating on leads is extremely small compared to the volume of material in the solder of a typical joint. Then consider that the Bi only makes up 1-4% of that small plating material volume. So, yes, the 1-4% Bi in the plating volume will form a very very small amount of this alloy in the total volume of the solder joint, but it is so small that it really doesn't contaminate the structural integrity of the total joint.

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