Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Flux evaporation....???

Larry J

#8890

Flux evaporation....??? | 21 October, 1999

Is it possible for flux to evaporate out of the solder paste, and how long would it take? What kind of effect would this have on reflow if all the flux evaporated?

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Brian

#8891

Re: Flux evaporation....??? | 21 October, 1999

Larry

IMHO, I suppose theoretically, with SOME types of paste, it would be possible, but you would not be able to print the paste, place the components or reflow, because a lot more of the chemistry would have volatilised long before the flux and the metal would have become hopelessly oxidised, to boot.

Methinks you worry over nought.

Brian

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Larry J

#8892

Re: Flux evaporation....??? | 21 October, 1999

Brian, my main conceren was it evporating once it was printed on to the board before placement.

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Brian

#8893

Re: Flux evaporation....??? | 21 October, 1999

Same answer

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#8894

Re: Flux evaporation....??? | 21 October, 1999

Larry Elaborating on the specific question, yes, the volatile solvents in the flux can and will evaporate. Evaporate completely in any reasonable amount of time? Probably not. What tends to happen is that the exposed paste dries on the surface and forms a shell around the paste inside. When you reflow and the solvents inside the shell volatilize, the paste deposit sort of explodes and ejects a shower of solder balls. The paste vendors will quote an open or stencil life that can be used as a guide however humidity, temperature, draft and the age of the paste will affect it. Why keep pasted and populated boards hanging around before reflow? John Thorup

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