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October 1999

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Subject:
From:
"Joy, Stephen C" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 29 May 1998 15:15:00 -0700
Content-Type:
text/plain
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text/plain (210 lines)
Mpark,
I have run a few trials using via in pad and found that the solder does wick

thru the holes. Even worse if wave is required.
How do you design the sm artwork? If the hole is tented isn't there the risk
of
paste capping an air bubble?
Steve


Jeremy,

I see the wicking of solder thru via holes is problem, thus forms
insufficient solder joint fillets.  To get around the problem with via
holes in SMT pads w/o introducing board cost increase/other
fabrication process complications, use a smallest possible via
hole size, which is 0.25mm (0.010") and mask bottom side via
hole pads with LPISM.  This restricts the flow of solder thru via
holes and at the same time, allows flux and volatile gas to
escape thru them.

Mpark
NII-Norsat International Inc.


>>> Brad Kendall <[log in to unmask]> May 29,
1998  10:10 am >>>
As far as getting the initial solder joint to work using this pad you
may
be successful.  You are obviously aware that you will get solder
wicking
into the via and possibly starving your solder joint, but with a
PBGA with
eutectic balls you should be fine.  However, to rework the joint
through a
12-16 mil via?  If you apply heat to the via it will pull the solder
away
from the part because solder flows toward the heat source.  If
you did
force enough solder through the via to heap up and touch the
BGA, you have
not heated the BGA pad and thus you will not get a good solder
joint to the
BGA.

I guess it is worth a try.  But the rework part is questionable.

Brad Kendall
Hella Electronics Corp.






[log in to unmask] on 05/29/98 08:04:26 AM

Please respond to [log in to unmask]; Please respond to
      [log in to unmask]

To:   [log in to unmask]
cc:    (bcc: Brad Kendall/Hella North America Inc.)
Subject:  Re: [TN] BGA Footprint




Jeremy,
My guess is you will still get some solder wicking down the hole.
Anyone
else try this yet?  But I don't understand why you can't rework your
BGAs,
not that you should have to rework many anyway.  AirVac, SRT,
Conceptronics
and others make very nice equipment for BGA rework.

regards,
Bev Christian

> ----------
> From:         Jeremy
POLLARD[SMTP:[log in to unmask]]
> Sent:         Friday, May 29, 1998 1:00 AM
> To:   [log in to unmask]
> Subject:      [TN] BGA Footprint
>
> Hello,
> am hoping that someone out there has experience with BGA
footprints.
>
> We have completed Rev 1 of a double Eurocard PCB, 10
Layers with 7 x 356
> pin BGA devices on board. While everything went well with this
run and we
> had no problems with ball connections, it is impossible to
perform any
> rework or modifications if required.
> The footprint was the recommended pad with track out to a via
or  " Dog
> bone". On the next revision we would like to do the following,
Elimnate
> the "dogbone", place a through hole in the actual pad for the
ball and
> adjust the solder paste stencil so that the volume of paste
deposited
> allows for the hole through the board.
> I would use a hole size of 0.3mm (12mil) to 0.4mm(16mil)
certainly no
> larger.
> This approach should enable us to rework dodgy joints from
the solder
side
> of the PCB
> It will also allow us access possibly to test the pins from the
BGA.
>
> Has anyone done this ?
> Was it succesfull in a production environment ?
>
> Many thanks foryour help.
>
> regards
>
> jeremy
>
>
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