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Mix Pb and Pb free alloy

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JohnM

#41754

Mix Pb and Pb free alloy | 24 May, 2006

I have a product with mixed in Pb free and Pb BGAs. I decide to use SnPb paste with Pb free reflow profile to make it work for both types of BGA. Do I have a reliability solder joint issue by using this method? How is your opinion on this case? Can I adjust the reflow profile to have solder paste melting first then BGA ball to correct the voiding joint in this case(SnPb paste with Pb free BGA or CSP)? What is the case of Pb free BGA using SnPb paste since the volume of Pb free BGA ball is more than volume of SnPb paste deposit? How is the solder joint reliability of Pb free_BGA using SnPb paste and Pb_BGA using Pb free solder paste? Appreciated your help.

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inds

#41758

Mix Pb and Pb free alloy | 24 May, 2006

John, there have been quite a few discussion on Mixed Assemblies in this forum.. do a search.. you will get whole lot of information.. in addition

- IF you are using Lead-free reflow profile...make sure the rated temp for Sn-Pb components is not violated.. - You can go down on the temp if the joint is mixed and still get good mixing of paste and ball along with ball collapse.. - Reliability will always be a concern in Mixed Assemblies. - So far I have not been able to eliminate voids in mixed assemblies... I maybe wrong, but I think voids will be difficult to eliminate in mixed assemblies.. it may require precise reflow profiling..

hope it help..

inds

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#41760

Mix Pb and Pb free alloy | 24 May, 2006

Yikes! Sounds like a hairy situation there. I've successfully collapsed a Pb-Free BGA (using a Sn/Pb solder paste), with a peak temperature of 220 Deg. C and Time Above SAC's Eutetic point (217) of 17 seconds.

Beware of overcooking your smaller mass chip components. If they get up to that 240 or greater range, you'll see things like poor wetting, charred or burnt up flux, or excessive IMC.

The verdict on backward compatability of BGA's is mixed. Google it!

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#41761

Mix Pb and Pb free alloy | 24 May, 2006

Inds,

It's about time that lead-free components can stand higher temps "myth" stops. The only difference is the plating, the component itself has not changed.

Can you give us an update on this threat? If you�re the same inds http://www.smtnet.com//forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=8916&#Message41349

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JohnM

#41766

Mix Pb and Pb free alloy | 24 May, 2006

So, it looks like the backward compatible of BGA statement is not guarantee for solder joint reliability. It doesn�t matter what temperature and what melting sequence we have in order to avoid the solder void issue. Is Pb diffused homogeneously through the grain boundaries of Pb free BGA ball? Will the weight of Pb less than 20% not degrade the joint?

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Chunks

#41783

Mix Pb and Pb free alloy | 25 May, 2006

Loctite makes a product called MP218 that works well with mixed alloys.

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#41787

Mix Pb and Pb free alloy | 25 May, 2006

Patrick, I believe you are mistaken about this being a myth. Many component manufacturers changed more than plating to make their component withstand higher temps. Some changed mold compounds, others changed plastic types used. Many of these changes were also done so that the MSL could stay the same for Pbfree processing. Those that didn't change these other substances often saw their MSL degrade by a couple of levels (ie MSL went to MSL 3). At the very least, manufacturers had to retest/requalify their device for the higher temps. Some components are "leadfree" and still cannot withstand higher temps, but most can. I have mostly seen that problem in connectors that 'always were' leadfree, but the plastic body is not able to handle higher heat.

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