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Pb-Free BGA on SnPb Assemblies

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#54552

Pb-Free BGA on SnPb Assemblies | 1 May, 2008

Good Morning,

We are seeing an increased number of our customers that specify a Pb-Free BGA when the rest of the board assembly is SnPb. Of course when we reflow at the SnPb profiles, we don't always get a properly collapsed BGA. So we end up reworking the BGA. We don't run a Pb-Free profile because we don't want to risk damaging the SnPb components.

I am sure we aren't the only ones that are experiencing this. So, I have a question or two:

What is everyone else doing when a customer requires a Pb-Free BGA on a SnPb assembly? Many of the jobs I refer to are consignment, so it's not like we can order the SnPb part from the get go.

What reliability issues do you see?

Are there any extra steps that your company takes to ensure the Pb-Free BGAs collapse properly?

Is there a "special" profile anyone uses for the best of both worlds?

What are the advantages, if any, to use a Pb-Free BGA on a SnPb assembly?

Thanks for any and all input.

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#54553

Pb-Free BGA on SnPb Assemblies | 1 May, 2008

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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#54554

Pb-Free BGA on SnPb Assemblies | 1 May, 2008

Thank you for the reply.

We do build SnPb assemblies as well as Pb-Free. I am aware of the Silver paste. However, what impact is there with reflowing SnPb components with a max temp of say 220C when using the higher Pb-Free temps?

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#54555

Pb-Free BGA on SnPb Assemblies | 1 May, 2008

Have not seen any impact on the components.

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#54559

Pb-Free BGA on SnPb Assemblies | 1 May, 2008

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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#54565

Pb-Free BGA on SnPb Assemblies | 1 May, 2008

As Mark stated in a different reply, we also send our Pb-free BGA's out to be re-balled with SnPb paste so that we don't have to elevate the reflow temperatures. You have to be careful regarding the individual components on your boards as although many components can handle the higher temperatures, we have run into issues with certain switches, RF components, and "specialty" parts, many of which are extremely expensive.

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#54579

Pb-Free BGA on SnPb Assemblies | 2 May, 2008

Sure you can. But I don't have the time, money or resourses to do this. I do not have the luxery of only getting SnPb parts. Wish I did, but I don't.

As for temp specs, I know the specs on all my parts. We profile our ovens every month. We can prove without a doubt that our profiles are in control and pose no threat to the parts. Anyone not knowing this is a fool.

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#54592

Pb-Free BGA on SnPb Assemblies | 2 May, 2008

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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#54608

Pb-Free BGA on SnPb Assemblies | 5 May, 2008

We agree with rwyman 100%.

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dlm

#54661

Pb-Free BGA on SnPb Assemblies | 8 May, 2008

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

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#54663

Pb-Free BGA on SnPb Assemblies | 8 May, 2008

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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#54669

Pb-Free BGA on SnPb Assemblies | 8 May, 2008

I'm a worrier. That's why my friends call me Whiskers.

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#54674

Pb-Free BGA on SnPb Assemblies | 8 May, 2008

But if the BGA was made of spare-ribs, wouldja eat it? I know I would!

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#54675

Pb-Free BGA on SnPb Assemblies | 8 May, 2008

I once took a pair of binoculars and stared at a wave solder for over an hour.

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#54715

Pb-Free BGA on SnPb Assemblies | 12 May, 2008

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