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SAC305 Lead Frame Component with Leaded Process?

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#56520

SAC305 Lead Frame Component with Leaded Process? | 16 September, 2008

I had a vendor change thier lead finish from tin/lead to a SAC305. We built up several assemblies before we realized the change. The reflow process is a standard leaded profile peaking at 220 C. What are the reliablity concerns with mixing the tin/lead solderpaste with the SAC305 leadfree component that have not seen reflow temps over 220 C? Thanks for any comments!

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#56534

SAC305 Lead Frame Component with Leaded Process? | 16 September, 2008

Well, typical content of Pb in Sn/Pb polating is a few percent (just to mitigate Sn whiskers growth). Therefore, the lead finished with SAC305 won't be much different from those finished with Sn/Pb. You can make one or two cross-sections for sanity check to see the structure (and make sure Pb is evenly distributed in the bulk of the joints) and if it looks normal, then that should be it.

Regards,

Vlad

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#56535

SAC305 Lead Frame Component with Leaded Process? | 16 September, 2008

Yes, this scenario is fine. However, BGAs are a different story since the ball is the bulk of the solder joint. In this case, you MUST run a "hybrid profile". Look it up in the fine SMTNet archives (keyword: hybrid profile). I have "been there and done this", by the way.

Further, for the non-BGA component that switched to the SAC plating, Vlad is correct. The Pb is such a small %-age of the joint that the effects to switching to SAC are minimal for most components. However, with some RoHS components and too-cold a profile, the intermetallic (IMC) may be weak. It is recommended to go a little HOTTER, to ensure full reflow of the SAC305 joint and full IMC formation. Transitional Sn-Pb solder paste well help you in this scenario (keyword: transitional solder paste.) Been there and done this too.

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#56536

SAC305 Lead Frame Component with Leaded Process? | 16 September, 2008

Well, as we are talking about lead finish, I don't think the question was about BGA. :-) AS far as the intermnetallics is concerned, then you are right, it might require a bit more heat delivered to the joints, but it also could be perfectly fine. That is why I suggested to do X-section to do kind of qualification.

Vlad

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#56537

SAC305 Lead Frame Component with Leaded Process? | 16 September, 2008

Vlad, yes, he made no mention of a BGA, but I thought I'd just throw it in there. Many people are concerned with the "backward compatible" scenario - running RoHS parts with a Sn/Pb solder paste. The scenario's a no-brainer with most components, but it's huge for BGAs.

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#56540

SAC305 Lead Frame Component with Leaded Process? | 16 September, 2008

Thanks for the response! I thought is was odd the component vendor chose SAC for a lead frame finish. I am in the Military world, and things have been difficult to control from a lead-free standpoint. I have not been able to find any studies on SAC lead-frame finishes reflowed with Sn/Pb solder paste. Everything I come across relates to SAC reflowed at the higher temps. I am aware of the intermetallic concerns at regular Sn/Pb reflow temps, but have not seen any data. Thanks again for the help!

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#56542

SAC305 Lead Frame Component with Leaded Process? | 16 September, 2008

Even BGA may be no braner if it's done properly :-) I had a few cases with that and afterall it worked fine. :-)

I agree, it was a good think to mention.

Vlad

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#56554

SAC305 Lead Frame Component with Leaded Process? | 17 September, 2008

There is one thing you should keep in mind, though. It's better not to have any area of the leads covered with only SAC after re-flow, just to make sure you Sn-whiskers won't have any chance to grow :-)

Vlad

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