Lon,

I think the other technetters have hit it on the nose as far as what is
going on.  You have some CTE mismatch problems that induce more stress than
the bond can withstand.  SMT adhesive in the cured state has high tensile
strength, and is brittle, when you actually need it to be more ductile.
What I have done in the past to overcome this problem is this.  Rather than
putting all of the adhesive dots under the component, I would put a few
large dots at the edge of the component.  Rather than bonding the underside
of the package, I am now bonded to the side and underside (especially if I
can get adhesive past the mold line at mid package).  This gives you much
more shear strength on the bond, and also a heck of a lot more tact (green)
strength during placement equipment handling.

This is why most folks use the double dots for small actives.  A single dot
is usually hidden, whereas the unsightly double dot provides much more shear
strength due to the structure that secures the part from the sides.

Steve Abrahamson

> -----Original Message-----
> From: Lon Weffers [SMTP:[log in to unmask]]
> Sent: Thursday,June 21,2001 6:50 AM
> To:   [log in to unmask]
> Subject:      [TN] FW: dispensing glue
>
> Dear technetters,
>
> One of my colleges has a problem with glueing a plcc on the solderside of
> the pcb.
> They glue with a Hereaus glue and when they check the joint directly after
> the oven it is stuck.
> When the pcb's are cooled down the component falls of the board.  The glue
> spot is on the board.
> What causes this problem??
>
>
> Lon Weffers
> Matas Electronics bv
> www.matas.nl
>
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