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April 2002

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From:
"Maddox, Allen T" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 4 Apr 2002 09:43:29 -0500
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Like I said before, we're going through this now. It seems that the EMI
gurus are preaching to Engineers to put the via in the pad. This is
something that we, as designers, are going to have to figure out how to deal
with. It's a challenge and goes against everything I've learned over the
years But, if the board doesn't work it doesn't make a difference how
manufacturable it is. There has got to be a way to make everybody happy. Do
we have to go to blind micro Vias?

Allen M.

-----Original Message-----
From: Mark Charlton [mailto:[log in to unmask]]
Sent: Thursday, April 04, 2002 7:40 AM
To: [log in to unmask]
Subject: Re: [TN] Vias in pads


Steve,

We're concerned about the solder wicking through to the other side and
presenting "balls" of solder on the second side.  This is a 120mil thick PCB
and would take a bunch of extra print strokes to fil the via, then the
stencil won't gasket and would probably be damaged when printing second
side, damage by squeege blades.  I like the prevailing opinion: REDESIGN.

Mark


-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, April 03, 2002 7:06 PM
To: [log in to unmask]
Subject: Re: Vias in pads


Hi Mark,

I hope you've seen my pictures and seen what happened with me. But something
you might try that I didn't think of when I ran my ten boards, is to do a
double print stroke when printing the board...this may "force" some of the
paste down inside the via and give it a bit of additional paste to fill down
inside the via so that it doesn't suck so much from the fillet... maybe
worth a try.

-Steve Gregory-




Group,

I have a challenging assembly that I need some of your combined professional
experience to resolve.

I have been presented a 120mil-thick polyamide (sp?) dbl-sided PCB, gold
finish with the majority of the C's&R's being 0402's, some 20mil QSOP and
TSOP, QFP.  That's somewhat challenging in itself but to compound matters,
there are MANY instances of vias in the SMT pads.  The vias go through the
PCB and are NOT tented/plugged.  Many of these vias are on only one side of
2-terminal parts.  I don't have the complete set of gerbers and drill
drawing so I can't determine the drill size of these vias.

Is there ANYTHING I can do at this stage of the game to produce a quality
product in terms of stencil design, reflow profile, etc?  The customer is
aware of the potential problems this presents.  Their primary concern is the
solder wicking through to the other side.  Mine too, in addition to
solder-starving and tomb stoning.

Your thoughts are appreciated.  Oh yeah, of course it's R/F.

Mark Charlton
Engineering Manager
MSI of Central Florida, Inc.
7703 Technology Drive Ste. 102
Melbourne, FL  32904
(321) 725-0626 x.24
(321) 725-0627 fax
[log in to unmask]

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