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Lead-free and Leaded solder in the same reflow

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#39389

Lead-free and Leaded solder in the same reflow | 31 January, 2006

Hi All,

I�ve been doing some research on the issue of running lead-free solder paste and leaded solder paste through reflow at the same time.

The point with this would be to process all standard leaded components with Lead paste while also processing lead-free BGA�s with lead-free paste.

So the reflow profile would be set to run lead-free, a much high than normal temperature than leaded solder paste would normally see.

Has any one seen any articles on this, or have any first hand experience?

Any and all input would be greatly appreciated.

Thanks

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aj

#39392

Lead-free and Leaded solder in the same reflow | 31 January, 2006

how would you apply both pastes.?

anyway, I have succesfully run leadfree bga with lead paste.

You gotta find a profile that kinda meets both requirements.

Taking into consideration the peak temp and time above.

dont have details on hand but I can get the profile data if you want it.

aj...

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Calvin Kolokoy

#39393

Lead-free and Leaded solder in the same reflow | 31 January, 2006

Today's regular Sn-Pb paste chemistries are designed to run at higher peak temperatures ensuring that your lead-free alloys will coalesce with your regular Sn-Pb solder.

That being said, it is do-able to solder both types in one oven. Generally, with your Sn-Pb paste, you'll be at the upper end of time-above-liquidous and peak temps (around the 240-245 range), and with your Pb-Free, you'll be at the lower end...

This overlap in process windows will not always work though, especially if there are large delta's in thermal gradients across your board... (ie your 0402 will get too hot causing other problems, versus your BGA260).

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#39394

Lead-free and Leaded solder in the same reflow | 31 January, 2006

I guess my biggest concern would be what will happen to the inner metallic layer of the leaded solder during the higher temperatures.

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Chunks

#39395

Lead-free and Leaded solder in the same reflow | 31 January, 2006

We just did this and are awaiting the results. Solder balling and a burnt looking flux where visual indicators that we noticed running regular paste at no-lead temps.

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#39402

Lead-free and Leaded solder in the same reflow | 31 January, 2006

Chunks,

For our lead-free eval, we had the same problem with "charred looking" solder paste up the component termination of one of the smaller discretes, coupled with excessive solder balling...this smaller part did see peak temps in the 240 range - the upper end of the leaded solder paste's peak temperature. We had a lead-free BGA on there as well, which collapsed and soldered just fine...but...at the expense of solderability of another part. So, lessoned learned...it is a big balancing act when you solder lead-free parts (especially BGA's) with leaded paste..

I agree with the other fella who said it is possible to solder Lead and No-Lead, but it is board dependent, and also the other guy that said to beware of the excessive intermetallics that may get formed when the regular Lead paste sees too much heat for extended periods.

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Jose Luis

#39420

Lead-free and Leaded solder in the same reflow | 1 February, 2006

Hi all, I'm working in Military Production and then, We have an exception about RoHS (LEAD FREE), but, because we use "normal" components, in this moments, a lot of components have LEAD FREE terminations, and we think that after of 1st July, we wo'nt find any components with Lead. And then, we need to work with a mix technology (components LEAD FREE, Paste with Lead, PCB's, perhaps LEAD FREE...). I read a lot of documents, but I d'ont know real results of real productions. Could you help me?.

Thanks for all

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Joe

#39424

Lead-free and Leaded solder in the same reflow | 1 February, 2006

Hi,

SAC BGA's in a SnPb reflow process can be achieved by ensuring the temp. on the BGA is at a peak min of 220�C and a time above 207-210�C for 60 - 90 Sec. Lower temperatures have shown incomplete mixing of alloys and some voiding.

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Chunks

#39426

Lead-free and Leaded solder in the same reflow | 1 February, 2006

Try an intermediate paste like Loctite/Henkle 218. It's design is to be used in a mixed lead/lead-free process.

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Mike

#39512

Lead-free and Leaded solder in the same reflow | 3 February, 2006

At my last employer, we ran a number of tests to make a "Hybrid" profile work (same profile Joe lists) . It worked great for everything but BGAs. The problem (6 months ago) was using parts not rated for lead free reflow temps. As the market fills with lead free parts, Im sure this issue will disappear. To allow the non-ROHS parts survive, and still get the RoHS BGAs to reflow we ran at the lower temp stated above. We then ran a battery of tests that included thermal cycle and vibe testing, X-ray, and dozens of cross sections performed on the BGAs exposed to the Hybrid profile. The joints survived the thermal cycles, and the vibe tests. But the X-sections exposed a possible long-term reliability issue. They exposed very large lead grain structures, poor intermetalics, and huge voids. Some voids where 70% of the ball. I know that some further work was performed after I left (by some top notch Process Engineers I might add), but I haven�t heard if they ever got it work.

Mike

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Cal Kolokoy

#39527

Lead-free and Leaded solder in the same reflow | 6 February, 2006

Mike did you have these SEM'd to be able to see the IMC's within the solder joint?

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Mike

#39584

Lead-free and Leaded solder in the same reflow | 8 February, 2006

No we didnt. All our images where visual.

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