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February 2008

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Subject:
From:
Dwight Mattix <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dwight Mattix <[log in to unmask]>
Date:
Tue, 5 Feb 2008 09:15:18 -0800
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At 09:06 AM 2/5/2008, Ryan Grant wrote:
>Thanks John.  I'm not too worried about laser vias because they have a
>bottom.  If one doesn't pre-fill the via like you mention, a solder void
>will occur with BGA's.  However, the void is usually small.  Although,
>I'll have to see what the relative void size is on a 0.8mm pitch CSP...

Depending on uVia quality and aspect ratio it can be a problem even at 
.8mm.  If the uVia is too rough or narrow at the top it won't wet out and 
the expanding gas has to go somewhere -- usually late in the cycle when 
combined gas law is working to create a very large bubble in the solder ball.

I've seen large enough voids in solder ball from poor uVia quality or uVias 
with overhanging/thick copper at the knee that it caused adjacent balls to 
bridge.  I've got some pictures archived l and could post them if you're 
interested.

dw

>What about through vias that are not filled with epoxy?  Have you
>soldered on top of those?
>
>-----Original Message-----
>From: John Burke [mailto:[log in to unmask]]
>Sent: Tuesday, February 05, 2008 10:00 AM
>To: 'TechNet E-Mail Forum'; rgrant
>Subject: RE: [TN] Open via in pad - any success?
>
>When I did that on a single sided via (laser drilled) I pre-filled the
>vias
>with a separate stencil/reflow operation to "level the field" followed
>by
>water cleaning followed by regular assembly techniques
>
>
>
>John Burke
>
>(408) 515 4992
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
>Sent: Tuesday, February 05, 2008 8:38 AM
>To: [log in to unmask]
>Subject: Re: [TN] Open via in pad - any success?
>
>Thanks Paul,
>
>If you and two others can do it, I guess it can be done...  I'm glad I
>asked before I made a fool out of myself.
>
>So now the question is, how did you do it (if you can tell me)?
>
>Whenever I've tried it, either gravity would cause the solder to sag out
>the bottom, or the via would suck solder away from the joint.
>
>Thanks,
>Ryan
>
>________________________________
>
>From: Paul Edwards [mailto:[log in to unmask]]
>Sent: Monday, February 04, 2008 6:43 PM
>To: TechNet E-Mail Forum; rgrant
>Subject: RE: Open via in pad - any success?
>
>
>Ryan,
>
>It really depends on the size and depth of the via, the via to pad
>diameter ratio....
>
>Q: has anyone been successful with a via in pad that is not plugged?..
>A: YES, both through and blind vias
>
>Q: If so, was it double sided?
>A: YES if you mean a double sided PCB, and YES if you mean it was a
>through via..
>
>Q: ...was one side connected to a BGA ball?
>A: YES
>
>Paul
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
>Sent: Monday, February 04, 2008 4:55 PM
>To: [log in to unmask]
>Subject: [TN] Open via in pad - any success?
>
>Hi All,
>
>I just got told by a design engineer that LaserJet printers are built
>using via in pad where the via is NOT plugged.  He claims the via is
>filled with solder first, then goes through the normal SMT process.
>
>He intends to use this to connect a BGA ball on one side of the board to
>a cap on the other side using an mechanically drill via that is not
>plugged.
>
>Before I call BS on this, and tell him "do not pass go until that via is
>plugged, flattened and plated over", has anyone been successful with a
>via in pad that is not plugged?
>If so, was it double sided?
>And, was one side connected to a BGA ball?
>
>Thanks,
>Ryan
>
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