Pb-Free BGA on SnPb Assemblies

You need a paste designed specifically for this. They are available. Most have 2% silver in them and higher temp flux so you can reflow at "no-lead" temps. The silver aids in the intermaetalic bond and the high temp flux allows you to reflow the no-lead part without burning the flux off too early. I use the Henkel Loctite 218.

Trying to raise your temps with standard paste will result in very weak solder joints. You cook the flux off too early.

"Is there a "special" profile anyone uses for the best of both worlds?" Tried every profile out there and failed. You need to change your paste.

"What are the advantages, if any, to use a Pb-Free BGA on a SnPb assembly?" There are none. Just makes it easy for your purchasing dept to buy no-lead parts.

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We have this issue as well in our house. A customer calls out a lead-free bga and wants to place on a leaded assembly. All we do is send it out and get the BGA reballed with leaded balls.

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We've never, to my knowledge, sent out Pb-free BGAs to be reballed for use on an SnPb assembly. In fact, we developed an in-house reballing process and we haven't felt the need to do that either (for salvaging an occasional reworked part, maybe).

Our standard procedure is to use a "hybrid" profile (search the forum), which is basically a regular profile with an elevated peak temp of ~225 or so and we've never had an issue. It's quicker, easier and cheaper than re-balling and definitely worth a look. With the RoHS initiative, the supply chain issue (limited availability of SnPb parts) was predicted so the hybrid profile was conceived and is common in the industry.

We're a tier 2(1 maybe?) CM whose name most here would recognize and we're extremely quality-driven so this advice isn't coming from some mom-and-pop, low-budget shop. My advice: don't make it more difficult and expensive than it needs to be. Good luck.

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I am with Rwyman. This "hybrid" profile is also used for reworking the BGA's.

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CSullivan,

On the great SMTNet archives, there was much discussion about how to run in this scenario. Here is one such thread:

http://www.smtnet.com//forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=10483&#Message42337

Also, I wouldn't "worry" too much about "damaging" tin-LEAD components, as there's almost no such thing. The majority of the component manufacturers already HAVE a "RoHS" plating.

As others say on this thread, I would worry more about your paste flux and alloy. Some paste manufacturers make a "transitional" formula that can survive TAL's of 217�C for at least 30 seconds, and peak temperatures as high as 240�C. You meet those requirements and you will collapse the BGA spheres seamlessly. Talk to your paste rep. about their offerings.

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