|Ask the Experts|
February 16, 2021
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?
|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.
Director of Reclaim Business
Alpha Assembly Solutions
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.
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.
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
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