Hi John,
Be careful your 'modified fixture' allows unrestrained expansion in the x
and y planes, otherwise the constraint of movement in this plane will want
to pop the board up or down in the z direction. If that movement is
constrained too, you could build up damaging internal stress levels as the
board fights the constraints.
Since your solder joints have probably cooled in a deformed shape to
match the warped board, I think your plan to go up to the liquidus point
of your solder is a good one. Alternately, you could try and heat the
board to a lower temperature (110 degrees say, for FR-4 with a Tg of 125 -
just enough so the FR-4 is soft enough to remould in a flatter shape) very
slowly (so that board deformation change is less than the creep rate of
solder) and allow the creep properties of the solder to allow SJ movement
to the new board shape. Others on this list (Werner are you there?)
probably have some good input on those 2 different strategies.
In either case, I think you'd want to control the cool down rate very
carefully (i.e. do it slowly), to try and keep the temperature through the
thickness (z direction) of the board as uniform as possible. For bare
boards, the most severe warpage is typically noted around Tg (say 120-125
for difunctional FR-4) because there are dramatic changes in the
mechanical (young's modulus and shear modulus) and thermal (CTE) material
properties around this temperature. Thus, even relatively small
temperature differentials can create temporarily highly unbalanced lamina
stacks. So if you go up to 220, slow your cooling rate to its lowest
value between 145 and 105 degrees C. (for 125 Tg material).
Good luck!
Andy
----------
From: [log in to unmask] [SMTP:[log in to unmask]]
Sent: Thursday, October 09, 1997 4:28 AM
To: [log in to unmask]
Subject: [TECHNET] Fixing Warped Assemblies
Address,
I have a recent lot (100 pieces) of assemblies with enough warp in
the
longest edge to keep the assembly from plugging into the backplane.
As a flag, the warp should have been caught by operators following
Wave, but was not. I have a solution to ensure future assemblies are
not affected. I am also in the process of modifying the Wave fixture
for low mass assemblies and the Wave Process Flow Procedures.
There are three options I have come up with to (temporarily) fix the
warp problem, such that the field engineers can insert the
assemblies.
OPTIONS:
1. Bake assemblies in oven at X temperature for X time with applied
weight; or
2. With the modified wave fixture, which ensures rigid stability in
all regions, re-solder assemblies via wave soldering; or
3. Run assemblies through the Convection Reflow Oven at X temp for X
time with the modified fixture holding the assembly in place
I have opted to go with Option 3 and would like to hear others
experience(s) or any other avenues of thought. I am thinking the
assemblies shall be run at a maximum of 220?C for 55secs max. (ramp
up
temp/time is not included) at 1.12cm/sec using a Heller 1500 Reflow
Oven. My insanity is to bring the fab temperature as close to the
fabs Tg to minimize if not eliminate any Warp Memory. Granted, this
is another thermal excursion, but lets approach as if there are no
alternatives and components can withstand.
Thank you
John Gulley
Inet Inc.
Plano, TX 75075
972-578-3928
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