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Dbl-Sided Reflow Question

C.K.

#3926

Dbl-Sided Reflow Question | 2 June, 2000

Okay, there's a debate here at my company.

We build a card here that, often times, we get batches where there is excess HASL in the VIA hole barrels.

Here is the problem that this causes:

During the "1st reflow", the excess HASL bleeds out of the VIA's and onto the other side of the PCB, and causes HASL "spikes." These HASL "spikes", in turn, screw up the screen printing for the 2nd side reflow because the spikes cause the stencil NOT to be flush to the board.

Okay now here's the debate....my theory is that the HASL "bleeds out" of the VIA's because there is no flux in the VIA's - hence, no "wetting forces" holding the excess HASL in there. Gravity pulls the HASL down onto the other side.

What do you other soldering gurus think?

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#3927

Re: Dbl-Sided Reflow Question | 2 June, 2000

Hey CK, we�re all sodder googoo�s ... wanna be gurus

Why donchu tell �em not to put sodder in your vias? Er, makit so the sodder won�t flow out of the bottom of the vias and cause dem stalactites?

OK OK, here�s wacha do, modify the second side stencil to add holes that correspond to the via stalagmites (nee stalactites) that way the stencil will lay flat and you�ll get good gasketing. Cool huh? Yeh yeh yeh, that�ll add more paste to the vias and make the board even more unreliable than it was before with a smaller amount of sodder in the via, but at least you�ll get a good print!!!

Anywho, say there�s no flux, no metallurgical soldering action, just a sodder googoo, a board with some sodder in the vias, and a very hot oven. So, the sodder in the via melts, gravity makes the sodder move down the barrel, surface tension tries to hold it back, wetting doesn�t come into effect cause there�s no flux, and the sodder reaches the edge of the via. The crowd grows tense. Does the sodder jump or does it hang? Who will win this pitched battle between gravity and surface tension? This is a classic duel between good and evil!!!

"Helllo!!! I'm not a doctor, but I play one on TV. Do you have hemorrhoids? I do and so do many people just like us!!! What works for me is ... "

Auuuggggh!!! Gravity can�t over-come the capillary/surface tension. The sodder forms a nicely rounded glob at the opening of the via, but it just can�t pull away to make that plunge from the rail to the floor. May be if only CK had run the oven just a little hotter, the surface energy of the sodder would have been high enough to allow gravity to win the day. Er, if CK had just run the conveyor just a little bit slower, would that have been enough?

Say tuned boys and girls, next week on "A Propeller Head, A Heating Element, And Sodder" we�ll replay this gripping tale with flux in the via ... Doesn�t that sound ...

As a preview of that show, we�ll talk about: * If there�s no flux in the via, why does the sodder flow? I thought it was the flux that removed corrosion to increase wetting that allowed sodder to flow. * And wudda wanna bet there�s still flux residues in those vias, anyhoo? * Yehbutt, who cares if there�s flux in the via?

Have a good weekend Dave F

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Ramon I Garcia C

#3928

Re: Dbl-Sided Reflow Question | 3 June, 2000

Hi I don't speak english very well but some time I had A similar problem but the diference that we only procesed a single side, my pcb have one via hole barrel close to the pad then the solder flow throu the hole, my problem was an insuficient solder, is ityour via hole barrel close to the pads? maybe solder quantity it's little high, well I think.

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Dreamsniper

#3929

Re: Dbl-Sided Reflow Question | 4 June, 2000

CK, just curious...is the via located close to a pad or land? if yes, what's the gap between the via pad and the component pad/land edge ? what's the Via hole size? Have you checked where does the solder that bleeds out of the via come from? Please elaborate more.

regards,

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Roni Haviv

#3930

Re: Dbl-Sided Reflow Question | 4 June, 2000

Hi C.K. ,

You can ask your PCB manufacturer to cover all vias with solder mask from CS (It called "pluged" vias). It's done from one side to prevent any close area that can cause "eraption" of solder because of pressure under reflow.

All our boards have pluged vias and we never had such problems.

Regards Roni

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C.K.

#3931

Re: Dbl-Sided Reflow Question | 5 June, 2000

Dave:

Thanks for you Earl-Moonesqe explanations. To answer your 3 questions below:

* If there�s no flux in the via, why does the sodder flow? I thought it was the flux that removed corrosion to increase wetting that allowed sodder to flow.

The "sodder" flows because it's "Eutectic" and the excess solder melts at 183C and has nowhere else to go...the IMC is already formed from the HASL process and when remelted, with no flux, has nowhere to "rewet" to, if you will. Kind of like touching up an already-formed solder joint with an iron....the excess solder just deforms but doesn't rewet to anything...

* And wudda wanna bet there�s still flux residues in those vias, anyhoo?

Yes, you're right!! There might be excess flux residues in them via barrels....but why arent' there any wetting forces?

* Yehbutt, who cares if there�s flux in the via?

I agree...who cares? Dat sodder's still bleeding out of that damn hole.... my only explanation?? As my cubemate would say - PFM (pure f*cking magic)..

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C.K.

#3932

Re: Dbl-Sided Reflow Question | 5 June, 2000

Dream:

Yes, VIA's are connected to actual SMD pads via traces - roughly .030" from the pad. The VIA is .013" in diameter, and the excess solder bleeds out of the actual VIA barrel after the 1st reflow....

-CK

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#3933

Re: Dbl-Sided Reflow Question | 6 June, 2000

CK: Sure bud ...

* If there�s no flux in the via, why does the sodder flow? I thought it was the flux that removed corrosion to increase wetting that allowed sodder to flow.

The "sodder" flows because it's "Eutectic" and the excess solder melts at 183C and has nowhere else to go...the IMC is already formed from the HASL process and when remelted, with no flux, has nowhere to "rewet" to, if you will. Kind of like touching up an already-formed solder joint with an iron....the excess solder just deforms but doesn't rewet to anything...

=> You bet. It goes liquidous and tries to flow down hill flux or not.

* And wudda wanna bet there�s still flux residues in those vias, anyhoo?

Yes, you're right!! There might be excess flux residues in them via barrels....but why arent' there any wetting forces?

=> The wetting force will make it easier for the moten sodder to flow over the sodder coating in the via. * Yehbutt, who cares if there�s flux in the via?

I agree...who cares? Dat sodder's still bleeding out of that damn hole.... my only explanation?? As my cubemate would say - PFM (pure f*cking magic)..

=> Naw, it aint no PFM. It's sposed to do that. It's just that you didn't expect it to do that.

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po

#3934

Re: Dbl-Sided Reflow Question | 9 June, 2000

If you spent alittle more time on your bar graphs/time lines and less on the floor, the answer would be obvious!

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