It's been a few years since I was in the screen print room, but here is my
opinion.
The problem with legend ink dams are:
1. it is thermal, so where it lays is where it stays.
2. line width minimums are around .004-.005", less will give skipping.
3. Legend ink bleeds. Depending on the ink viscosity, room conditions and
curing characteristics you can add .002" to the line width.
4. If no mask on pad is the quality requirement, then the registration
tolerance on a .020" pitch and .012" pad will be +/- .002" for legend
printing.
Combine all of these variables and you have quality problem. I doubt that
anyone is using this process, and if they are, ink on pads is acceptable. If
they are and are using a photoimageable legend ink, they are paying too much
for the boards.
If your process has solder bridging with nothing (or solder mask) between
pads then there is another problem, and it isn't the pcb design.
If ink on the pad is no issue, make it primary solder mask. Design the mask
.008" wide for the .012" space. This will usually give clean pads, but there
will be some low level of fallout, based on the process controls of your
supplier. It may not be as thick as legend, but it will be more reliable and
have comparable yields for bridging.
Steve
-----Original Message-----
From: Mike Gish [mailto:[log in to unmask]]
Sent: Friday, June 04, 1999 9:14 AM
To: [log in to unmask]
Subject: Re: [DC] Silkscreen feature in between fine pitch
The article was published in the June issue of Printed Circuit Design. The
article is named "Being first
matters" Describes how Thomson Electronics uses several methods to "do it
right the first time" etc.
Including software that allows for pitch compensation during autoplacement.
Solder thieving pads for wave
solder, and silkscreen features on fine pitch components to stop solder
bridging. (How they impliment this
is what we have all been trying to figure out) By the way your questions
about registrations was my exact
question! You are right on target!
Mike Gish
Sr. PCB Designer
Ditron Manufacturing, Inc.
Phoenix, AZ
Jack Olson wrote:
> Well, not that my experience means anything, but I have never seen that
> technique.
> I would think silkscreen registration is poor relative to the small space
> between fine-pitch pads, and even a slight error will screw up your
> assembly/soldering.
>
> I would like to know where that article was published,
> or hear from others who do it successfully.
>
> Jack
> (my employer stands behind every opinion I have.... <grin>)
>
> -----Original Message-----
> From: Roberto Torres [mailto:[log in to unmask]]
> Re: Silkscreen feature in between fine pitch
>
> No one else uses the below mentioned silkscreen technique?
>
> On Wednesday, June 02, 1999 10:52 AM, Mike Gish Wrote:
>
> I just finished reading an article that
> described adding
> silksreen
> features in between pads on fine pitch
> components to create
> an
> additional solder barrier between pads.
> Does anyone have
> any experience
> doing this?
>
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