When you say "tented" is this genuinely tented, or a partial covering? To tent in the old fashioned sense, you need a dry film resist which virtually nobody uses nowadays. Lowest cost (but least reliable) is to "plug" using solder resist, most reliable are dedicated epoxy plugs (assuming non conductive) that can be filled, filled and covered, or filled and capped. these are virtually 100% reliable, but add cost due to the dedicated processes required.
Grant Main
Technical Sales Manager, Fineline VAR Ltd
40 Years in PCBs. Large and small companies in the UK and the US. Experienced Process Engineer, DFM, Materials, Impedance Control, Surface Finishes, etc. Working with Fineline now across the portfolio of manufacturers covering rigid, flex, rigid flex, ATE, Load Boards, MSAP, and other existing and emerging technologies.
When you say "tented" is this genuinely tented, or a partial covering? To tent in the old fashioned sense, you need a dry film resist which virtually nobody uses nowadays. Lowest cost (but least reliable) is to "plug" using solder resist, most reliable are dedicated epoxy plugs (assuming non conductive) that can be filled, filled and covered, or filled and capped. these are virtually 100% reliable, but add cost due to the dedicated processes required.
Grant Main
Technical Sales Manager, Fineline VAR Ltd
40 Years in PCBs. Large and small companies in the UK and the US. Experienced Process Engineer, DFM, Materials, Impedance Control, Surface Finishes, etc. Working with Fineline now across the portfolio of manufacturers covering rigid, flex, rigid flex, ATE, Load Boards, MSAP, and other existing and emerging technologies.
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.
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.
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.
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