Plugging vias with conductive paste is limited to holes over a certain size
and aspect ratio. Anything below 10 to 12 mils is not really fillable and,
that aside, an aspect ratio of more than 1:6 (dia:depth) is also tricky.
Equally, if you're dealing with blind vias that need to be filled, the max
diameter I could get filled on a 7 mils deep hole is 24 mils.
My two cents-worth. But what's 2 cents worth? Yesterday I was approached by
a very poor-looking old fellow on a tricycle cart, begging for change to
get something to eat. I gave him the small amount of change I had in my
pocket - just over one Singapore dollar, which included a 5 cent "copper"
coin. The fellow actually called me back to return the copper - it wasn't
any use to him. Who said beggers aren't choosers?
Peter Duncan
"Houston,
Terri" To: [log in to unmask]
<Terri_Housto cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
[log in to unmask]> Aero/ST Group)
Sent by: Subject: Re: [TN] BGA Via's to tent or cover
TechNet or not?
<[log in to unmask]
ORG>
11/10/01
04:46 AM
Please
respond to
"TechNet
E-Mail
Forum."
Warren, some board shops are able to fill the vias with a conductive paste
and then plate over the top.
This avoids the problem of a partially plugged via while giving you a
surface to probe.
The BGA side could use soldermask to cover the plugged via.
Good luck,
Terri
> ----------
> From: [log in to unmask][SMTP:[log in to unmask]]
> Reply To: TechNet E-Mail Forum.
> Sent: Friday, November 09, 2001 3:23 PM
> To: [log in to unmask]
> Subject: [TN] BGA Via's to tent or cover or not?
>
> I have seen different ways to address masking small vias (<13mils) under
> BGA.
> cover them with mask prior to HASL top an bottom ( can't we need to
ict
> probe
> bottom)
> cover top with mask prior to HASL (could cause trapped chemistry in
via
> and
> will not allow solder thru, we like the effect on thermal reliability
> for
> environmental extremes)
> leave the mask opening only slightly larger than via (tent)
> do a post hasl mask to cover over via ( causes 2x the thickness of
mask
> at
> overlap, could be a problem with uBGA ball only 14mils dia to start
> with may
> not touch pads)
>
>
> We use #3 but have problems with rework leaving solder plugged vias that
> look
> like shorts in xray.
>
> If there are some other ideas or thoughts on how to design the via
masking
> under
> the BGAs I would like input. Thanks!
>
>
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