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February 2019

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Subject:
From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Wed, 20 Feb 2019 12:41:15 -0800
Content-Type:
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Hi Steve-

Nice looking fabrication job on that board. The holes are well centered and
the fine resolution of the QFN pads is pretty decent considering the
roughly 0.003" thick copper.

Looks like this board has the vias unfilled, but paved over with mask in an
attempt to "tent" the holes. Lots of people are doing this despite how
gross a cross-section of that technique looks. Maybe the PCB fabricators
are trying to go to thicker masks to make this practice look a little
better!

Wayne Thayer

On Tue, Feb 19, 2019 at 9:14 AM Steve Gregory <[log in to unmask]> wrote:

> Hi All,
>
> Seeing that there is a recent solder mask thread, I have a question about
> double masking. We have an assembly that we're building that has a 15.7-mil
> pitch QFP and 0201's on the assembly, so when I ordered my stencil I spec'd
> .004 thick foil and 8-mil wide apertures for the QFP (since there's no
> solder mask web between the pads) and 13-mil squares for the 0201 pads.
>
> The 0201's did great but I had a ton of bridging on the QFP! That really
> surprised me. I started looking at the PCB a little closer and noticed that
> there was a double layer of solder mask on the board. Here's a couple of
> photos:
>
>
> https://photos.google.com/share/AF1QipNeiheoq_Go0K7zsQY_iF6vfJk_YNws-6UlCC6i3z7p5LHn-HSQ31rVj-Cl-EJ-xg?key=bUJEdkRsQzlaOUkzcWdLaXI3dEI2TV91V2lhbHlR
>
> I inquired the board shop why there was two layers of mask on the board and
> was told that because of the 2-oz. copper weight being called out on the
> outer layers, it required two layers to get complete coverage and not have
> it thin out on feature edges. Lines are at 8-mils and space is 6-mils. That
> doesn't seem too extreme to me. Is that really true? I've built boards
> before that had way more than 2-oz. copper that didn't require double
> layers of the mask to ensure good coverage, but those weren't boards that
> had 15-7-mil pitch devices and 0201's on them either.
>
> --
> Steve Gregory
> Kimco Design and Manufacturing
> Process Engineer
> (208) 322-0500 Ext. -3133
>
> --
>
>
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