Bob,
Consider the possibility that the flux overheated and deactivated prior to
reaching melt temperature.  This can happen on a slow ramp where you spend a
lot of time at a preheat plateau prior to the spike.  In this situation the
paste would wet to the pad but not merge with the eutectic solder ball.
Jim Kittel

 -----Original Message-----
From:   bbarr [mailto:[log in to unmask]]
Sent:   Wednesday, November 15, 2000 7:45 AM
To:     [log in to unmask]
Subject:        [TN] BGA Soldering Problem

I have a PBGA357 that is exhibiting a strange (to me) soldering condition.
Two to three balls (not necessarily adjacent, but usually along an edge) do
not appear to be wetting to the solder paste on the land. This is what it
looks like after reflow: The paste on the land has reflowed to a mound, the
ball on the BGA is compressed in shape but not wetted to the mound on the
land (so the ball must have reached reflow temperature to get compressed.) A
clear line of demarcation between the compressed ball and the mound of
solder on the land is visible. The component fails in test, but sometimes
pressing on it can get it to work. In most cases the balls on either side of
the one not wetted look fine. Process is OA flux with convection reflow.
Profile run several months ago with thermocouples imbedded in one center
ball and one edge ball showed max. temp. of 217 (center) and 220 (edge)
degrees C with time above 183 of 56 seconds. Max slope is 2 (center) and 2.8
(edge) degrees C. I am going to go run a verification profile, but in the
mean time your thoughts are much appreciated.

Bob


Robert Barr
Manufacturing Engineering
Formation, Inc.
Voice: 856-234-5020 x3035
Fax: 856-234-6679
email: [log in to unmask]

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