Steve,
Tombstones or drawbridges start when the solder mask thickness exceeds the
pad base metal thickness then solder mask behaves like a fulcrum levering
one end awyt from the solder paste. Asymmetrical heating like one pad is
tied to a ground plane only makes these types of defects worse.
From what you have described there is more than enough solder mask to
cause this problem.
Regards,
John
On Mon, 15 Jul 2013 13:23:59 -0600, "Steve Gregory" <[log in to unmask]>
wrote:
> 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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