Not entirely sure I understand your question, Here's my take.
If the component is overhanging pad then the actual contact area pad to
component is going to be less with solder but pretty much the same for
adhesive (the solder will only bond metal to metallisation, but the
adhesive will stick to whatever it is in contact with).
However the lap sheer of Ag adhesives is (roughly) only about a half
that of solder, but this may not be so significant if you have fillets
as then you have increased bond area and the bond is also in tensile, Of
course if you have a TCE mismatch then the extra compliance of solder is
important as this will accommodate it better than most adhesives as
these are more rigid, but you could have a flexibilised adhesive
compared to a very thin solder bondline which changes things somewhat
and the stress may be in peel, of course this is at room temperature at
higher temperatures the solder will soften but the adhesive won't till
it reaches Tg, but also .... I think you get the point.
Good luck on modelling: In this respect don't forget that Ag adhesives
are composites, so the properties given in data sheets [which are based
on classical tests on bulk samples], may not actually reflect the
reality of the material when used in thin bondlines and to/across
interfaces.
For these sorts of reasons most adhesive applications of this type are
application specific and subject to actual testing.
Regards
Mike Fenner
Indium Corporation
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Doug Pauls
Sent: Tuesday, December 09, 2003 8:14 PM
To: [log in to unmask]
Subject: [TN] Adhesive strength vs overhang
Good afternoon all,
Internally, we are having discussions on the amount of allowable
overhang of passive SMT components (R and C). Our specifications are
based on solder joint connections and the solder specifications are not
in question. We have an application where a silver filled adhesive is
used to make the connection between SMT metalization and mounting pad.
The solder joint requirements were applied to these adhesive joints and
I am wondering as to the validity of this. The adhesive is epoxy based
and, in my mind, more rigid than solder. I think we are unnecessarily
rejecting parts that are acceptable.
So, before I go off and do a complex designed experiment, I thought I
would check with the Technet crowd first. Do you know of any studies
that show the adhesive force (e.g. pull strength) vs. the degree of pad
overhang?
Doug Pauls
Rockwell Collins
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