Hi John,
I think I found your paper, was Surface Mount Zero Defect Design
Checklist?
http://www.avx.com/docs/techinfo/smzero.pdf
It talked about soldermask thickness and draw-bridging on page 4.
I said I would have a picture, and here it is. It's from one of the bare
boards that was still back in the stockroom. It's from one of the
datecodes that we are having problems with:
http://stevezeva.homestead.com/Triple_Mask_Layers.jpg
You can see that this has THREE mask layers on it! Why would anybody do
that? Like I said the fab note said SMOBC, Black Matte. Is there
something about black mask that I don't know about?
It looks to me like the fab shop screwed-up and put the wrong mask down
in the first place, then tried to fix it by covering it up with the
black mask.
I have to get a new stencil because the part number was changed for an
oscillator from Crystek, the footprint changed. I don't need to put
paste down for the center pad anymore. So if I'm getting a new stencil
anyway, is there anything I can do that might help me with the
tombstones? Maybe reduce the apertures for the 0402's? I've got a
sneaking feeling that the customer will want to keep using these boards,
and I just wonder if there's anything that I can do with the stencil
that might help.
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Maxwell
Sent: Sunday, July 14, 2013 9:23 PM
To: [log in to unmask]
Subject: Re: [TN] Soldermask thickness causing 0402 tombstones?
Hi Steve,
Absolutely, solder mask between pads are the leading cause of
drawbridges to be precise for 0402 sized chips It only gets worse when
we go to smaller case sizes. I actually got best of conference at SMTAI
in I believe 91 for my paper on designing assemblies for sub miniature
chips. Covered it then and we are still designing boards with solder
mask in all the wrong places:-) I stared then get rid of it then and
followed up with a couple of papers a few years back when I was at
Johanson Dielectrics for HV parts.
Regards,
John Maxwell
On Jul 14, 2013, at 8:02 PM, stephen gregory
<[log in to unmask]> wrote:
> Hi all,
>
> Have any of you ever had thick solder mask cause you tombstones? I'm
thinking that maybe that has happened to me. We built a board last week
that we built before without any problems. It does have quite a few
0402's. We had a few random tombstones last build, but not anything like
we had this time! Tombstones all over the board! Nothing changed from
the last build except the datecode on the PCB's. They were supplied to
us from our customer.
>
> I'm pretty sure that the board was double masked. The fab drawing
calls out for SMOBC, and then when you look farther down in the notes
section it calls out for a matte black mask. It looked to me that there
may have been a green LPI first put down, then a matte black mask put
over top of it.
>
> I don't have pictures now, but I'll get some tomorrow. I've found some
> links that talk about mask causing
> tombstones:http://blog.screamingcircuits.com/2007/11/soldermask-and.ht
> ml
> http://communities.mentor.com/mgcx/message/2726
>
> So I know it can happen. I just want to check with you guys to find
out if it happens often.
>
> Thanks!
>
> Steve
>
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