Steve,
 
I have read this before but what concerns me is when do you know how much
stretch is to much stretch.  Not knowing anything about the BGA discussed in
your example, .46 mm to .68 mm seems like a lot of stretch.  I would think
that stretching the solder joint may lessens its ability to survive a drop
test.
 
joe

-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]]
Sent: Tuesday, July 01, 2003 12:40 PM
To: [log in to unmask]
Subject: Re: [TN] double sided reflows and BGAs


Hi Joe!

I have to agree with Bev about taller joints being a good thing. In fact,
one of the seminars I attended at APEX this past year (Solder Joint
Reliability of BGA Assemblies given by Jean-Paul Clech) stated precisely
that.

SRS (Solder Reliability Solution) Models predicted that you would see over
1000 thermal cycles more before failure with a double sided reflow BGA than
with a single reflow. Tests actually proved that.

Tests were done on a 540 I/O cavity-down Enhanced BGA under thermal cycling
conditions that were cycles from 0° C. - 100° C., with dwell times of 5
mins. at 72-cycles per day.

Single reflow stand-off height was .46mm, double reflow stand-off was
stretched to a height of .68mm. Single reflow BGA's made it to 8328 cycles,
while double-reflow BGA's made it to 9271 cycles.

-Steve Gregory-




Hello Tech Netters,

New board designs are incorporating BGAs on both sides of the board.  During
the 2nd side reflow, it appears that some of the larger BGAs installed on
the 1st side reflowed which are now upside down are dropping slightly I
suspect due to gravity.  Dropping in the wrong direction, away from the pwb.

Who can shed some info on how to either stop the BGAs from dropping, or
apply some type of adhesive to the corners after 1st side reflow to secure
the BGA in place?  I am worried that securing the corners with an adhesive
my later cause a reliability problem since you are now securing the corners
to the pwb.

thanks
joe




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