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Tin Lead BGAs in leadfree paste ( pb-free paste )

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#40222

Tin Lead BGAs in leadfree paste ( pb-free paste ) | 6 March, 2006

We have a customer transitioning slowly to lead free due to necessity rather than a need to comply i.e. some of their BGAs can only be purchased Pb free only. So for this we would want to produce them a leadfree soldered PCB (not necessarly RoHS complient though). However the question has arisen can you use a tin lead BGA with leadfree paste (assuming there will be no temp issues)as there are a number of BGAs on the PCB and they have stock of some SnPb ones that they dont want to scrap. The PCBs are double side SMD with a few hand soldered PTH parts.

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#40238

Tin Lead BGAs in leadfree paste | 7 March, 2006

Craig - You should not use the leaded parts, as you are, in all likelyhood, going to see massive voiding. We just went through this same deal where I work, because a manufacturer sent us the incorrect parts. You can go with lead free parts with regular solder easier (the lead free BGAs just act like a ceramic would) but not the other way.

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#40247

Tin Lead BGAs in leadfree paste | 7 March, 2006

I have to disagree with you, BillyD. If Craig searches the forum, he will see that the concept of a Pbfree BGA with SnPb paste has been debated many times. You say it is fine. I say it is NOT. There have been several studies done that clearly show the long term reliability of a Pbfree BGA soldered with SnPb paste is compromised. It may look good when it comes off the line, but it does not stand up in temperature cycling. Here is a link to one of the best papers, by David Hillman: http://www.aciusa.org/leadfree/leadfree_impact_of_reflowing_a_pbfree_solder_alloy_using_a_tin-lead_solder_alloy_reflow_profile_on_solder_joint%20_integrity.html

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#40256

Tin Lead BGAs in leadfree paste | 7 March, 2006

muse- I agree with you. You should never mix chemistries if you don't absolutely have to. If you HAVE to, you can go with a lead free BGA with leaded paste with much fewer defects than the other way around.

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James

#40260

Tin Lead BGAs in leadfree paste | 7 March, 2006

Billy,

I have to disagree with you. We inadvertently soldered a Pb BGA with SAC305 paste and then had it micro sectioned and spectrum analyzed. Imagine our surprise when we saw the spectrum analysis showed Pb. The micro sections showed none to acceptable voids, one small void. We tracked down the problem later but I would have no reservations about soldering Pb BGAs using SAC305 paste. The thing I find most troubling is the manufacturers of BGAs are still using soldermask defined pads, I think this is one of the biggest issues with BGA reliability.

My $0.02

James

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#40261

Tin Lead BGAs in leadfree paste | 7 March, 2006

I think it has a whole lot to do with the heat applied, and the paste type. It was a seriously populated, very thick board, so we had to go big with the heat just to make 240-ish. That produced voids maybe 45 or 50% of the ball size. We knew immediately as, it was the only BGA on the board with any issue. This will probably happen a lot in the future. One big question is, what impact does theis intiative have on brokers?

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#40267

Tin Lead BGAs in leadfree paste | 7 March, 2006

Craig. i spoke with a process engineer today and he told me that if you mix pb and non pb it may work but may cause premature failures if in a thermal cycleing environment. failure cause may be micro fractures.

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Loco

#40342

Tin Lead BGAs in leadfree paste | 10 March, 2006

You have 2 groups giving us 'evidence' here: 1. the research companies who will do anything to scare us, just to make sure more research will be done, if they said everting was fine they'ld be out of a job in a sec. Ex: look at that paper, a leaded profile for a partly leadfree component will never work.

2. the new breed of component manufactures who want to assure us that mixing PB and LF will work, so they dont have to bother making all their BGAs in 2 versions LF en PB. The article by motorola posted here a couple of months ago is a good example.

The truth? only time will tell...

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Kerm

#40353

Tin Lead BGAs in leadfree paste | 11 March, 2006

Hi Craig:

I agree with muse. The problems almost always will show up later on duringg life cycles in terms of reliability problems. While the solder joints may look acceptable right after production there is no guarantee of meeting the reliability requirements from customers. Paste fluxes that are designed to work with SnPb metallurgy paste work best at the temperatures that are ultimately used for Sn Pb reflow operations and geranerally will range between 215 to 225 C or maybe a littel higher so that reliable solder joints are produced. Taking this flux chemistry to temperatures that far exceed their designed working range will only serve to set up the reliability of the assembly to early failure. You indicate that your company deso not want to scrap a number of BGAs that are SnPb....you may want to consider having them sent to a BGA repair to have the SnPB solder balls deppulated and replaced with Pb free ones if you do not have capability to do so in-plant. The cost to do so mat be charged back to the customer if they agree. If you have a large number of them, then, you might want to consider obtaining a BGA re balling tool to do it in-plant and save enough money to pay for it (see http://www.bgatools.com and see a DVD that shows the tool).

kerm

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#40355

Tin Lead BGAs in leadfree paste | 12 March, 2006

Hi,

We have been soldering lead free BGA's onto products with lead based paste, and we have had no increase in defects at all. We have used a standard lead paste profile, so we are not altering the temps anything from what's recommended by the paste manufacturer, and we don't rely on the lead free solder balls on the BGA from collapsing. And this is on one of our most reliable products.

So lead free BGA's are fine as far as we can see, and I agree that a lot of this research is designed to scare. Remember when you had to trash every bit of equpment you had because BGA came along, but when you actually started working with BGA they were one of the easiest parts to use?

We also paid to attend a popular lead free consultants seminars on lead free about a year ago, and he had run a lot of production on lead free, and he also said from his experience that soldering lead free BGA's onto lead was fine.

However this thread was about soldering lead based BGA with lead free paste, and that was not recommended at all. I don't actually understand why anyone would want to do this, because the product would not technically pass as a lead free product, so why not solder with lead based solder paste?

If someone wants to solder a lead based BGA onto a lead free board, with lead free solder paste, then it's not going to be a lead free product. So it would be a fraud to have a lead free product that still had some lead free parts on it.

Could you split production and send any remaining lead based parts on a lead solder build, and then get them into the US market, while doing lead free runs with lead free parts to Europe and Japan?

That's what we are going to do as we might have some small qty's of lead based parts we will want to use up before the deadline, but we need to change over in May the latest otherwise our stock might remain in the reseller channel before being sold before the deadline. This lead free deadline is really earlier for us because we need to make sure there is no lead free finished product in the sales channel before the cut off date.

Regards,

Grant

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#40380

Tin Lead BGAs in leadfree paste | 13 March, 2006

The original issue was Co. does not need to go Pbfree, but some BGA's they use are only available Pbfree. They don't want to throw away Pb BGA stock, so some boards might have a combination of both Pb and Pbfree BGAs. The question basically was Should they solder with Pbfree solder (because of the Pbfree BGA), even though there would also be Pb BGA on the board? The first poster said no, this would give voids in Pb BGA. I responded that soldering Pbfree BGA with Pb solder would give reliability issues, basically implying the opposite, that they should use Pbfree solder on a board with mixed BGAs. The Co. never implied that they would try to claim Pbfree. They just want to use up their stock in the most reliable way possible.

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