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Phosphorus Crystal Formation

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#45010

Phosphorus Crystal Formation | 13 November, 2006

We are having intermittent faults with BGA's soldered onto an immersion gold finish board with snpb solder. One potential cause I have been informed is the formation of phosphorus crystals. As I am not an expert in this area I have no idea if this is correct or not. Can anyone point me in the direction of any literature on this subject or offer advice on how to resolve this crystal formation.

Thanks

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#45012

Phosphorus Crystal Formation | 13 November, 2006

Good afternoon,

1. Never heard that phosphorus crystals formation creates difficulties with BGA soldering. Two points to check are reflow process and screen printing process. BGA package is designed to have extremely high yield.

2. Title: Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip ChipTechnologies Author: Dr. Ning-Cheng Lee Publisher: Butterworth-Heinemann ISBN: 0750672188 Publication date: December 2001

BR, Pavel

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greg york

#45026

Phosphorus Crystal Formation | 13 November, 2006

do you mean too much gold brightener that dewetts solder???

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#45027

Phosphorus Crystal Formation | 13 November, 2006

You said your gold processing is immersion. More importantly, what is the process used for your nickel? Electroless correct?

What is the percent by weight of phosphorus found in your analysis?

How many times was the nickel plated? [You should be able to see this in your sections of the failed connection.]

At which metal boundry does the failure occur?

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#45032

Phosphorus Crystal Formation | 14 November, 2006

Thanks for your input folks. I will have to get back to our customer regarding the analysis as it was them that carried it out. If I can get more details then I will post them. For my own benefit, is phosphorus crystal formation to be expected using this baord finish?

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#45035

Phosphorus Crystal Formation | 14 November, 2006

Good afternoon,

Yes, it is to be expected. This is well known �black pad� issue. Google �black pad�, for example: http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/JEM-0109-1262/$FILE/JEM-0109-1262F.pdf?OpenElement

BR, Pavel

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