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expired solder paste

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#54268

expired solder paste | 3 April, 2008

Hi All,

I have some question about expired solder paste. What would be some of the effects regarding assembly quality. Is there any changes in flux composition toward the end of shelf life?

Thanks in advance for any replies.

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#54270

expired solder paste | 3 April, 2008

Potential impacts of using expired solder paste are [University of Bolton]: * Changed paste rheology. Result: poor print quality * Loss of solvents. Result: reduced flux activity * Changed flux formulation. Result: More difficult to clean flux residues * Solvent loss. Result: reduced tack force and tack time of the paste * Oxidized solder particles. Result: poor solder flow

Use a solder balling test to quickly determine the condition of solder paste. Print a small disk (around 4-5mm dia) of paste onto a non-wettable substrate and re-flow as normal. A single solder ball in a clear pool of residue indicates good coalescing ability. Large numbers solder balls remaining in the flux residue pool could be an indication of poor coalescing ability and the paste may be unfit for use.

For more on solder paste, look here: http://by108w.bay108.mail.live.com/mail/ReadMessageLight.aspx?Aux=4%7c0%7c8CA4D0F744F2C90%7c&FolderID=00000000-0000-0000-0000-000000000001&InboxSortAscending=False&InboxSortBy=Date&ReadMessageId=b65f21be-a70b-4ac8-abf9-55f044de7d45&n=126001750

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#54271

expired solder paste | 4 April, 2008

All of what Dave says is spot on. I suspect that the most likely corresponding defects on your assemblies would be poor wetting and insufficient solder.

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#54275

expired solder paste | 4 April, 2008

Define "expired" though... 1 week? 6 weeks? 1 month? 6 months?

Unrefrigerated life is generally 30 days. If paste has been stored refrigerated at manufacturer recommended temperatures, what would you guys do, like say, if you owned your own shop, and it was your dime, and if YOU'D be potentially throwing away 10 jars of paste? Personally, I'd run a test first on a scrap PCB. If it wets and solders well, I'd use it. I guess my cutoff would be a couple of months past expiration. I would thaw it, and then "mix" it myself and get a feel for its rheology. A good screen printing guy can do an on-the-spot rheology test just from feel.

If you aren't governed by ISO/FDA/FAA/QS/etc. and you wanna save a few bucks, do a test on the said expired paste....just as I describe above. If during production it's causing problems on real, actual assemblies, throw the stuff away! If you still don't have a warm-fuzzy about this, talk to a Chemist from your paste manufacturer.

I wonder if expired paste smells bad too?

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#54278

expired solder paste | 4 April, 2008

Flux seperation is a common problem observed due to expiration of paste. The symptom is a yellow layer of flux when you open the jar. Excess flux separation will cause priting problems such as smearing and slumping.

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#54280

expired solder paste | 4 April, 2008

Thanks for the reply.

Just one more question. If there is less solvent but the residues are harder to clean. would there be more contaminant left on the boards? Has anyone did a cleanliness test?

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#54291

expired solder paste | 7 April, 2008

can we use the expired solder paste and put in a solder pot of wave machine?

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#54293

expired solder paste | 7 April, 2008

Yes you can, although we don't think it's a good idea. Look here: http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=48159

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