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From [log in to unmask] Wed May 15 14: |
50:14 1996 |
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Wait a minute. Are you vacuum sealing into one of the corrugated
backed stacks with all the little holes punched in it. Or are you
vacuum drawing a bag (with desiccant inside) and heat sealing the
open end of the bag. The latter way, at least so I've been told, can
induce corrosive gases that can actually impair the shelf life. (From
the heat sealing operation.) Now I haven't actually seen this but
many military accounts that I serviced stated this in harmony.
DHH
______________________________ Reply Separator _________________________________
Subject: re:ASSY: Moisture sensitivity?
Author: [log in to unmask] at SMTPLINK-HADCO
Date: 5/5/96 9:39 PM
We request PCB suppliers to do a final baking before vaccume seal PCBs in
a pack of 20 (board size of 6" x 4") with a desiccant pack before
shipment. Suppliers mark the date of baking on cartons for receiving and
internal usage control. We only re-bake the PCBs if date of use is > 3
months from date of baking.
PCBs are stored in original carton boxs (sealed) until go to Pick & Place
machine and this helps to slow down the moisture absortion.
regards.
----------------------[Reply - Original Message]----------------------
Sent by:"Dill, Norm J" <[log in to unmask]>
Our air conditioning system here mixes in a large volumes of outside
air
to help control the inside temperature while economizing on energy costs.
As a result, the inside and outside humidity levels are very similar.
Our PWB s (FR4 & polyimide) are purchased and stored with desiccant
included in each package of 10. At best, this results in slightly higher
costs from our suppliers, extra handling in our stock room and later
baking
steps.
We are considering restructuring our PWB stocking procedures.
Does anyone have pro/con examples of board storage in this kind of
environment? For example, does the desiccant actually keep the boards
from
absorbing moisture during high humidity days? What would be expected if
the
boards were wrapped/stored in slip sheets and bound with duct tape or left
open on the shelf? If the humidity was controlled, could the desiccant
and
some baking steps be eliminated without an increase in warpage problems.
Any opinions, examples or references will be appreciated.
Norm Dill
[log in to unmask]
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