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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Reflow Profile Dry Out Stage

Chris

#2205

Reflow Profile Dry Out Stage | 13 December, 2000

I have a product where an IC is soldered using a stencil which puts down way too much paste. The result is massive bridging. Anyway, one of my coworkers claimed he could eliminate the bridging by baking the circuit board in an oven for 5 min at 90 deg C. He did this, and sure enough the bridging was eliminated completely. The solder joints look very nice as well. What did this bake out do? Did it reduce the flux activity and thus reduce the wetting action of the flux and in turn reduce bridging? Was some of the flux consumed thus decreasing the fluxes ability to reduce surface tension of the molten solder? This solder paste is a no clean and the reflow profile follows the manufactures guidelines. I also have other product which is printed using a thinner stencil and I don't have bridging problems. Incidentally, there is no solder paste bridging after solder print using the thin or thick stencil. I will be ordering a thinner stencil for this product experiencing the bridging. I would just like to know understand why this bake out eliminated the solder bridging present after reflow.

Thanks,

Chris

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Chris

#2206

Re: Reflow Profile Dry Out Stage | 13 December, 2000

I should explain that this bake out was performed after population. The solder paste slumped terribly after this bake out. However, after the back out and after reflow, no bridging.

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Michael Parker

#2207

Re: Reflow Profile Dry Out Stage | 13 December, 2000

It is apparent that you have evaporated the flux before you reflowed. You got lucky when the paste slumped and you didn't get major problems elsewhere.

A more reliable solution is to adjust the stencil apertures for the part that is experiencing bridging. This should be done as part of the modifications when you order that new stencil.

There are many references in the library on the subject that you can research to get your reduction specified. Also, IPC has just released a Stencil Design Guideline. A recent thread here (in the last two weeks) has that info as well.

Good luck

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Steve A

#2208

Re: Reflow Profile Dry Out Stage | 13 December, 2000

Chris,

I am not a chemist, but my best guess would be that you were able to remove some of the solvents from the paste flux without exhausting too much of the activators. The loss of solvents could have cut down on violent boiling (violent boiling will expand the mass of the reflowing solder enough to cause a short). Of course I could be wrong. Dave Torp at Kester Solders might have a better answer. dtorp@kester.com

Steve A

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Reflow Oven

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