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Solder in via holes causing printing problems

#31917

Solder in via holes causing printing problems | 6 January, 2005

We need some opinions about a situation that is causing problems in our process. The pcb's in question are .125 thick with a HASL finish. The boards are coming in with a large amount of solder trapped inside the via holes. Sometimes this solder is actually protruding out of the hole forming a nodule that prevents the stencil from forming a gasket on the board, making it impossible to print.

What causes the solder to be trapped inside the via holes on some boards and not on others? Does the thicker board have something to do with it? Does it have something to do with the soldermask design? We've seen it on other boards as well, .062 thick, but it seems worse on these .125 thick boards. We are also seeing the solder trapped in NPT holes that are there to provide breakaway in the array tabs. How can solder stick in holes that have no copper in them???

Any ideas would be greatly appreciated. Pictures can be seen at http://www.sipad.com/smtnet_1_6_05.htm

Matt Kehoe mkehoe@sipad.com

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#31918

Solder in via holes causing printing problems | 6 January, 2005

On solder in vias: It looks like the via are not solder masked well and pick-up solder either: * Solder coating ... OR * During leveling

It's possible that the design specifies openings in the solder mask for these via.

On solder in the unsupported holes: Some residual material left behind from some direct metallization [DM] processes may be the culprit. After drill, holes are treated to accept electrolytic copper plating and may even get 'flashed' with a thin electrolytic copper plate. Non-plated holes are typically tented and protected from additional copper/tin plating when the dry film is applied for the pattern plate process. (tin for etch protection). During the etch process, most if not all of the copper plating or DM that may be in the non-plated holes gets removed. If not all is removed or if the residual DM is left in the hole, it is now susceptible to plating process.

On pads with no solder: Doesn't say much about the effectiveness of your fab's final inspection process does it?

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RDR

#31922

Solder in via holes causing printing problems | 6 January, 2005

It appears that this particular PCB supplier has a non-optimum HASL process along with other issues. It is much harder to completely remove the solder out of vias on thicker boards. Looks like more air pressure was needed to remove all of the excess solder. Solder will stick into NPT holes when the process isn't optimized, it sticks from friction and not from wetting to the PCB material unless of course there is plating in the holes.

Russ

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Chunks

#31924

Solder in via holes causing printing problems | 6 January, 2005

It appears your board house is having a problem with the solder resist (mask). From the couple pix it appear that the holes are being plugged by the mask, so your HASL is not able to be blown out during the hot knife process. Time to call your board vendor and show him what he's giving. No doubt they're having process problems and passing the buck to you. Sh*t in = sh*t out.

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