Prototron Soldermask

Via Filling. Why fill a via and what are the fabrication constraints

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By Mark Thompson | Published on: May 06, 2019

Part 1: Why fill a via?

There are several different reasons you may choose to fill a via. Via-in-Pad, the most common, is when you have a via that sits smack dab in the middle of an SMT/SMD, or you may be filling a via so you can have a mask clearance on ONE side and the absence of a clearance on the other side, such as a BGA.

For years fabricators have used mask material in lieu of an epoxy filled via for applications where a mask clearance is necessary on one side but not the other. This would require creating a “pilot” or a void in the mask on the side where clearance exists, then performing a secondary “plugging” process to fill the hole from the side where NO clearance exists.

This “mask” process became quite time intensive in a CAM department, especially where there may be Via-in-Pad’s on both outer layers. Today, that is No longer necessary. Today, we have epoxy fill.

The process goes something like this: Let’s say you require the via’s in a certain region of the board to be filled due to either Via-in-Pad or a situation like described above where a mask clearance exists on one side of the board but not the other. Here we drill JUST those via’s that require filling FIRST, the holes are then plated, filled and cured with epoxy. They are then “planarized” or ground down flat to the surface, and finally the epoxy will get an additional plating to encapsulate them completely in copper. Once filled, any additional through holes or other via’s that do not require an epoxy fill, will be completed.

Having done this, does it “matter” if there is a clearance on one side and not the other like with a BGA or with a SMD/SMT on one side and not the other? NO, at that point it is as if the hole does not exist although it clearly does.

Part 2: Why use a conductive epoxy/paste for via filling?

I think the word "conductive" is a misnomer in this case.

The via/hole has already gone through a plating process making it “conductive” and the electromagnetic fields are present in the BARREL of the hole NOT with what you are filling the via with.

So why use a conductive silver/epoxy or copper/epoxy fill for holes? The answer is SIMPLE. You desire to transfer HEAT from one side of the device to the other side.

Part 3: What are the constraints of via size for epoxy fill, silver/epoxy fill, or copper / epoxy fill?

Anywhere between .008 drilled holes to .020 are the sizes that can be filled. Anything smaller such as .0059 holes on a .047-.050 thick board are tricky to fill completely and anything OVER a .020 hole frequently does not fill all the way. So any size between .008 and .020 are IDEAL for this application.


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