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Blind Via With Smaller Solder Mask Opening

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NB168 View Drop Down
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    Posted: 18 Sep 2013 at 6:04pm
Hi,
 
Have received a design file from customer for layout modification, this is a HDI designer and the previous designer has created blind via (L8-L7) for the 8 layers board with small solder mask opening. The drill for this blind via is 0.1mm, pad is 0.3mm and the solder mask opening is 0.2mm. These vias are under the BGA.
 
Any idea what is the intention of previous designer of opening the solder mask under the BGA, now the board is fabricated and we are worry of short under the BGA and need to re-do the fabrication with the solder mask close and affected our schedule.Confused
 
Any suggestion on rework on the existing board or the rationale of the open solder mask of the previous designer.
 
Thanks,
NB
 
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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2013 at 7:04pm
All vias under BGA's need to be "Tented" with solder mask to prevent solder bridging under the component.
 
I did the same thing once and the bare boards got to assembly and rejected. Tried to go back to the fabrication shop but they turned me down saying that the solder mask has been cured and there was nothing they could do.
 
So I contacted a different assembly shop and they said "No problem, bring in the bare boards and we'll touch up the exposed vias with a special mask material that we'll manually apply and oven cure".
 
I only wish I would have written down the name of the mask material, but I didn't because I knew from that day forward I would never have exposed vias under a BGA. It cost extra and delayed the delivery, but I did not have to rebuild the boards.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote NB168 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Sep 2013 at 2:05am
Hi Tom,
 
Do you remember the assembly shop that you sent to for touch up? Possible to send the contact please. Many thanks.
 
Best Regards,
NB.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 19 Sep 2013 at 7:44am
It was in San Diego, CA about 10 years ago. I don't remember their name. They said it was no big deal.
 
I'm sure you can Google an answer.
 
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