Dbl-Sided Reflow Question

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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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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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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