Susan
SMA adhesives are thermosets. This means they become hard on cure and this is not
reversible.
A couple of things go on with SMA adhesives when you heat them.
1] Their rheology is temperature dependent, so as you warm them the viscosity goes down.
2] As you warm them they cure so the viscosity goes up [to infinitely high]
In a properly designed adhesive 2] is much more important than 1], but it is conceivable
that the thinner more mobile material will have a chance to wick into little cracks before
it cures. Once cured the material is no longer mobile. So if you find it in a crack it got
there before it cured.
On probabilites this means that the crack was already there as otherwise you  are looking
for a "latent" crack which opened with the heat of the curing process allowing the glue in
before it became immobile. [You could maybe verify this by running a few loose components
through your oven and inspecting under a scope]
If this problem is not a one off then you should be looking at mechanical handling: eg
feeding or most likely placement height/pressure.

Mike Fenner
Bonding Services & Products
T: +44 1295 722992
F: +44 1295 720937


----- Original Message -----
From: <[log in to unmask]>
To: <[log in to unmask]>
Sent: 11 October 1999 16:26
Subject: [TN] Surface Mount Adhesives


> Hi All
> Does the "cured" adhesive used to secure surface mount components
> to the bottom - wave solder - side of a board flow during wave solder
> operations?
>
> I am trying to find out how/when the adhesive got inside the crack on a
> cracked
> resistor.  My impression was that the cured adhesive would not move????
>
> Susan Mansilla
> Robisan Lab

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