Via tenting and plugging

Brad Schick schrieb:

I want to put vias in the central pads of several QFN/MLF devices to

improve grounding and dissipate heat. To reduce costs, the vias are

0.3mm which is fairly large compared to a small QFN20 part's central

pad. To prevent solder paste from draining away during reflow, I believe

I need to plug those vias.

 

I'm considering plugging and tenting the vias from the bottom-side only

so that there is no top-side solder mask to interfere with good reflow

bonding to the chips. Another approach might be to only plug the vias

from the top (no tenting), but I'm not sure how accurately this can be

done in "prototype" runs.

 

Two questions:

 

1. Do either of those approaches seem practical?

I think plugging from the bottom is no good idea if you want to solder

on the top layer. Just think of air residues under the chip during

soldering. For a good soldering result, you need a good and planar

surface on the top layer. This can probably only be achieved by plugging

from the top side and then adding a fresh metal surface (by electroplating).

I would recommend to talk to your board house, they should know.

And I must confess that I didn't practically use this method yet.

2. If I go with bottom-side tented vias, how can do this in Eagle? I

have only been able to turn off the stop mask entirely, not for just one

side. I was looking for a way to draw a polygon on the board that would

"subtract" any stop masks inside its bounds, but I haven't found a way

to do that.

That's easy: Turn the stop mask completely off, and then manually draw

whatever mask you want into the appropriate layer. This must be done for

each via separately, but hopefully there are not too many of these

special ones...

Tilmann

Brad Schick schrieb:

I want to put vias in the central pads of several QFN/MLF devices to

improve grounding and dissipate heat. To reduce costs, the vias are

0.3mm which is fairly large compared to a small QFN20 part's central

pad. To prevent solder paste from draining away during reflow, I believe

I need to plug those vias.

 

I'm considering plugging and tenting the vias from the bottom-side only

so that there is no top-side solder mask to interfere with good reflow

bonding to the chips. Another approach might be to only plug the vias

from the top (no tenting), but I'm not sure how accurately this can be

done in "prototype" runs.

 

Two questions:

 

1. Do either of those approaches seem practical?

I think plugging from the bottom is no good idea if you want to solder

on the top layer. Just think of air residues under the chip during

soldering. For a good soldering result, you need a good and planar

surface on the top layer. This can probably only be achieved by plugging

from the top side and then adding a fresh metal surface (by electroplating).

I would recommend to talk to your board house, they should know.

And I must confess that I didn't practically use this method yet.

2. If I go with bottom-side tented vias, how can do this in Eagle? I

have only been able to turn off the stop mask entirely, not for just one

side. I was looking for a way to draw a polygon on the board that would

"subtract" any stop masks inside its bounds, but I haven't found a way

to do that.

That's easy: Turn the stop mask completely off, and then manually draw

whatever mask you want into the appropriate layer. This must be done for

each via separately, but hopefully there are not too many of these

special ones...

Tilmann