Tented Via Options and Tradeoffs

Just read an inquiry concerning tented via-in-pad where the vias in a thermal pad under a chip are tented on top to prevent solder wicking. We've always left them open on top and sometimes on the bottom, and then windowed the paste on the pad. What are the tradeoffs between open vs. closed or plugged?

D.N.

Expert Panel Responses

Open vias can often create voids in the solder joint. During reflow, the gas in the open via expands, and some could enter the solder joint, resulting in voids. Closed or plugged vias can prevent this source of voiding.

Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions

I've been in the soldering materials/applications industry for 25 years. Since joining Alpha, Ive been the global product manager for preforms, wave soldering flux, solder paste and more recently the Director of the soldering materials reclaim business.

Open via holes in the thermal pad will of course allow wicking of solder to the bottom side of the board. Wicking can be reduced or slowed by printing the solder paste around the vias, but some solder may still wick to the bottom side. The solder joint on the thermal pad will be reduced in thickness which may lower the standoff height of the component. One potential positive to this is that voiding in the solder joint may be reduced through this wicking action.

Tented via holes are often partially plugged and therefore could allow some solder to wick into the via holes. This partial plugging is random and will change from board to board. As a consequence solder wicking will occur randomly in different vias on different boards.

Plugged and plated vias are sealed and therefore the solder cannot wick to the bottom side of the board. This style of via tends to have more thermal mass and does a better job of wicking heat away from the component. One drawback is that voids in the solder joint tend to form or collect around the plugged vias. Plugging and plating the vias also adds cost to the circuit board.

Some newer design ideas for vias in thermal pads include placing solder mask on the thermal pad which blocks solder flow to the via holes. These designs essentially break up the thermal pad into solder mask defined zones where the solder paste is reflowed.

Tony Lentz
Field Applications
FCT Assembly

Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

Voids are common where the applied solder paste wicks into the vias in the thermal pads under the part. There should be adequate margin built in to the design so that some of degree of voiding is allowable. This is one of the attributes where "acceptance criteria will need to be established between the manufacturer and user".

If solder mask is used to tent vias that will be under the part it may prevent some of the solder from going into the hole, but it would itself be a small void. I'd also have concerns about the adhesion of such a small mask deposit on a copper plane.

The safest option would likely be to either plate the holes closed or fill with a suitable thermally conductive hole fill material.

Kevin Mobley
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group

Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.

Reader Comment

Tented via with solder resist might be of course helpful for preventing solder wicking,but it could be also exposed to hole corrosion problem during field operation. PCB finish process such as ENIG,OSP or immersion tin is accomplished followed by solder resist tenting process, so during finish process chemical trapped into hole and if not removed completely in cleaning step resulting in hole corrosion later. Hole plugging or 100% hole opening would be recommended and if not available hole cleaning process after finish should be reinforced.

Jinho Lee, KETI

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