Tech Netters,
We have a product with BGA's and via holes under the devices that goes
through a surface mount place then reflow then through-hole stuff and then
wave solder. We "mask off" under the BGA areas with a thermal barrier tape
(fiberglass covered by kapton). This keeps the liquid solder from getting
up into the vias and also prevents the balls of the BGA from reflowing
during the wave soldering process. Also, the "masking" keeps the flux from
driving up through the vias and under the BGA (we use no clean processing).
In higher volume production we would use a selective soldering pallet to
reduce labor.
Once the tape is removed, we have open and clean holes for the In Circuit
Test to contact. I would prefer solder filled holes, but with the correct
probe selection, we get reliable connections to the test-points.
One draw-back of this process is the open holes when the board is placed on
a vacuum actuated In Circuit Test fixture. The open holes pose an ESD
threat to the PCBA, so we use push-down pins on the fixture and no vacuum.
Robert Gordenker
Sr. Test Engineer
ESI Vision Product Div.
(734)332-7004
[log in to unmask]
-----Original Message-----
From: Phil Dutton [mailto:[log in to unmask]]
Sent: Thursday, September 23, 1999 12:58 AM
To: [log in to unmask]
Subject: [TN] Via holes under BGAs
Hello,
Just wondering what current practices are working with
respect to through
hole vias under BGA devices.
We have a PWA that has BGAs and is also wave soldered, so we
are concerned
about the possibility of solder balls under the device.
Currently I am using solder mask encroachment (not tenting)
with LPI mask
on all vias. Via holes are 0.012".
Also, am I worrying too much about the wave solder
travelling up this size
hole?
I've seen suggestions of;
1) 100% plugging using multiple screening passes from the
component side.
(looks like a good idea)
2) 'tenting' with LPI from the component side - (no cleaning
problems under
the BGA, but voids on wave side.)
3) 'tenting' with LPI from the wave side - (void problem
under BGA)
4) dry film solder mask with top and bottom tenting - (most
shops use LPI,
film too thick etc.)
Of course, another solution is to use blind microvias for
the BGAs, then
there are no through holes under the BGA, but we are a bit
worried about
the increased cost.
Any comments or suggestions would be welcome.
thanks,
Phil.
Phil Dutton C.I.D.
Senior CAD Technician
IPC Certified Interconnect Designer
Vision Abell Pty Ltd
Second Avenue, Technology Park,
Mawson Lakes. SOUTH AUSTRALIA 5095
===============================================================
Phone : (08) 8300 4400 (reception)
Fax : (08) 8349 7420
email: [log in to unmask]
Internet Page: http://www.vsl.com.au
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