Peter,
We have done countless numbers of BGA devices since 1996 and have seen these
and virtually every defect imaginable during rework. I just read Earl's
response and he brought up most of the issues that you should review during
your process. Now to answer your question, how to minimize the chance of
bridging.
There are several things you can do after you've verified lack of board and,
just as important, part warpage. Usually the 1st thing we do is make sure
that we're not heating the part and site too quickly. If you are, you may
be warping the part or site during the cycle. We have profiles sometimes at
.5 degrees C/sec to soak to limit warpage.
The next thing, actually the 1st thing, you should ensure is that the site
is in good repair. If you are missing solder dams between the pad and vias
or mask between the pads, this will decrease your susceptability of having
sucess.
I do have a question for you. What type of part is this? PBGA, TBGA, or
other? Weight of the part will make a difference on ball collapse
obviously. A little known, "trick" of the trade to control ball collapse is
to use a material to limit how much the part can collapse. We use kapton
tape on difficult parts at the corners. I know this is a little extreme,
but we have made many happy customers by doing parts this way when they had
concluded, after many hours of failing to successfully rework a part, that
it can't be done. The thickness of the material could be as much as 50% of
the ball thickness. This material is placed on the board just inside the
outline of the part silkscreen, if there is any, at the corners. This
technique is usually used with larger, heavier parts, not with parts as
small as 35mm. It sort of substitutes for the high temp column in a CCGA
for purposes of support and standoff.
To answer your other question, it is not too common for this size part to
short if it is a PBGA and generally, you won't need to use solder paste to
do this process.
A side note...every wonder why so many shorts are in the corners during
rework? Thermocouple several areas through the bottom side of the board
into the spheres on a part soldered onto your board. Make sure you
thermocouple at least 2 corner areas, 2 areas on the center sides and 2
towards the center of the part. As you remove your part with your rework
equipment of choice, hopefully not a hot air gun!, observe how the corners
are up to 10 degrees hotter than other areas. This delta will often cause
corners to reflow first, especially if you have less thermal load in the
board on one corner. What causes this? A square or rectangular nozzle.
Think about it, if you blow air through a square nozzle, where is most of
the air pushed, to the corners. If this air is heated then most of the heat
will be located there. Just one more thing to try to control during the
rework cycle.
And one last thing, I reallize I am new to giving input to this forum and
apologize in advance for sometimes not always agreeing with others when it
comes to rework issues. I have been doing, modifying and trying to
perfect(if there is such a thing!) rework processes most of my adult life.
My opinions and suggestions might not agree with everyone elses, so keep in
mind, these are just suggestions and not documented procedures.
Bill Westmoreland
President, BEST, Inc.
Phone (847) 797-9250
Fax (847) 797-9255
Email [log in to unmask]
http:www.solder.net
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Peter Lee
Sent: Monday, September 09, 2002 3:16 PM
To: [log in to unmask]
Subject: [TN] BGA rework - excessive collapsing
Technet,
We are having problems replacing 35mm BGA, 1.27mm pitch on a OK industry
BGA rework station. The resulting X-ray revealed a solder bridge at one
corner.
There was some warpage on the BGA but not significant. PCB was
reasonably flat. Thermal profiling was done to ensure all temp. were
optimum. I did notice the BGA balls collapsing quite a bit (at least
half of the ball height) after rework.
Anyone has suggestion on how to minimize the chance of bridging? Is it
common for BGA of this size? We don't have the capability to paste the
board for BGA rework.
Rgds,
Peter
----------------------------------------------------------------------------
-----
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
----------------------------------------------------------------------------
-----
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
