Steve,
There was another paper that AVX never turned into an app note and when I
return from international travel in two weeks I will look it up and see if
I have a copy that is electronic.
John
On Mon, 15 Jul 2013 17:54:46 -0700, stephen gregory
<[log in to unmask]> wrote:
> Hi John,
>
> I think I found your paper, was it 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 of my issue, 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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