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Re-Reflowing PCB's

markhoch

#26017

Re-Reflowing PCB's | 13 October, 2003

I've found a number of PCB's that have already been manufactured that were run using the wrong reflow profile. I verified that the PCB's did not see adequate peak temp, (as per the paste manufacturers recommendation)**. Is it possible to run the PCB's thru again using the correct profile? Would this even be beneficial? ** The manufacturer recommends a peak temp of at least 213degrees C. These boards saw about 208degrees C. They were above liquidus for 65-70 seconds, and the manufacturer recommends 60 +/- 15 seconds.

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#26019

Re-Reflowing PCB's | 13 October, 2003

"Would this even be beneficial?" is the correct question. Please descibe the solder connections.

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#26020

Re-Reflowing PCB's | 13 October, 2003

PCB is 12.25" x 4". There are large MOSFETS on the board. Two large Electrolytic Caps. Two Large Multi-Layer Ceramic Caps. 15 C Size Tantalym Caps. A fair number of IC's, (SO-8's, SO-16's). No BGA's or finepitch of any kind. The board is double sided (reflow), with nothing larger then tantalyms and SMB diodes on the bottomside.

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RDR

#26021

Re-Reflowing PCB's | 13 October, 2003

i think Dave night have meant what do the solder joints look like? (maybe not) Anyway, I personally wouldn't reflow these boards unless you have solder joint failures We (I) used to hardly ever exceed 210 Deg. during profiling until the leadfree stuff started coming in. So, if your baords are good now leave them be. Reflowing again will only make them worse as far as heat stress's.

Russ

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#26023

Re-Reflowing PCB's | 13 October, 2003

Our customer is starting to see some solder joint failures during their extensive testing proceedures. Specifically with an SMD/DIP device that has a very small foot in relationship with the pad on the PCB. (By SMD/DIP device I mean a DIP IC that has been prepped to use as an SMD device.)

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#26029

Re-Reflowing PCB's | 13 October, 2003

PTH DIP that have been prepped to become SM can be very unreliable, depending on how the part is prepped. Consider asking your customer to do a failure analysis of those solder connections.

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#26031

Re-Reflowing PCB's | 13 October, 2003

> Our customer is starting to see some solder joint > failures during their extensive testing > proceedures. Specifically with an SMD/DIP device > that has a very small foot in relationship with > the pad on the PCB. (By SMD/DIP device I mean a > DIP IC that has been prepped to use as an SMD > device.)

Why is it always the guys who do the most extensive testing end up bending DIP's into SMT parts?

If they are having failures it may be that the DIP part doesn't appreciate the reflow environment. Might want to consider the DIP reliability in that regard.

I don't think re-reflowing the boards would result in a significant solderability improvement either given that your peak is already 208C.

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#26037

Re-Reflowing PCB's | 13 October, 2003

How long were the boards above 203*C?

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#26038

Re-Reflowing PCB's | 14 October, 2003

10-15 seconds. Paste manufacturer does not call out this spec. What should it be and why??

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#26039

Re-Reflowing PCB's | 14 October, 2003

To us, time above liquidous plus 20*C is more important than the 60 sec [er what ever] above 183*C [which we think is meaningless] that the fine solder paste supplier suggest.

Based on what you've told us and lacking any failure analysis, we'd guess that you have good solder flow and you have component solderability / solder strength issues with those prepped PTH DIP components.

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#26047

Re-Reflowing PCB's | 14 October, 2003

Thank you very much for your help. It's starting to look as if you're correct.

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Dean

#26051

Re-Reflowing PCB's | 14 October, 2003

Get high magnifiction micro structure analysis of the failed joints. I bet you will see the correct grain structure. What about the other part types? Any failures? I assume this is standard 63/37 Eutectic solder?

These aren't heart monitors or go into guidance systems? Right? If you see smooth fillets and evidence of wetting you are fine.

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#26057

New Information on Re-Reflowing PCB's | 15 October, 2003

These boards also were above liquidus for 75 seconds, and the customer puts the boards thru extensive thermal shock and stress testing. An Alpha Applications engineer told me that because the boards were above 183 degrees for 65-75 seconds that the inter-metallic bond is thicker then normal. And these joints may become more brittle and tend to fail if the customer is doing extensive thermal shock and stress testing. In my opion, when you do extensive thermal shock and stress testing, you are reducing the life expectancy of your product anyway. (Kinda like buying a new car with 50,000 miles already on it!) Aren't those tests destructive? Should our customer really expect boards that've been through such tests to fully function thru an additional "life-cycle" test? Any thoughts????

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