Tim,

You definitely have to bond before you coat. I dare say your coating is a
similar material to the adhesive, but if you bond to the coating, you only
have a thin layer of material to attach to that itself relies of how well
it is keyed to the board surface. Your heavier components will encourage
the  coating to start peeling off the board as they pull at their bonds.

The bare board surface offers a much thicker, stronger surface to which to
bond that won't peel from another surface.

Peter


Tim McQuay <[log in to unmask]>     24/05/2002 01:44 AM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to Tim McQuay

             To: [log in to unmask]
             cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
             Subject: [TN] Conformal coating / gluing








Hi Folks:
We have a debate regarding sequence of operations when we glue down
components and apply conformal coating to the same board. The IPC-A-610C
does not cover this and I have not been able to locate any other info.
Basicly we want to know, do we glue first or do we coat first. Can someone
please tell us if the bond between silicone glue and conformal coating is
as
strong as between silicone glue and PCB uncoated surface. We are concerned
that heavy components, such as the large caps on some of our boards, would
not be well supported if bonding is done after conformal coating.
We have our own bias but we wanted to hear an outside opinion and/or see a
standard on the topic. The adhesive and coating we use are listed below.
        3-1744 (DOW), SILICONE ADHESIVE - for staking big components
        KE3421 (SHIN-ETSU), SLCN BRD COAT - conformal board coating

Thanks
Tim McQuay
Training Coordinator
Vansco Electronics
Fax: 204-474-1674
e-mail: [log in to unmask]

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