Hi Steve-
Reducing the thickness of the stencil by the excess mask height should lead to depositions being comparable to what you had on the previous, successful runs.
If you don't mind spending a bit of money, and the problem is only on one side of the board, you may want to use this to test out SIPAD. Since with that process the solder is reflowed without components and then flattened, the incidence of tombstoning will be immensely reduced. I have used this with very dense SMT boards down to 0201 passives (0.275mm round apertures). (Below that, the flattening process doesn't work so well.) To use SIPAD, it works best if they apply their long term sticky flux to the deposits so you can just peel some wax paper off and start placing. But that particular flux is rosin based, so it might not work on your line--you'd have to deposit sticky flux of your chemistry on it instead. A nice bonus of the pre-deposited solder is I found I could reduce reflow temperatures by 10C (doesn't need as much fluxing/cleaning action).
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of stephen gregory
Sent: Monday, July 15, 2013 8:55 PM
To: [log in to unmask]
Subject: Re: [TN] Soldermask thickness causing 0402 tombstones?
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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