At 11:40 AM 7/11/97 U, you wrote:
>Subject: ASSY: Encapsulant/underfill of TSOP              1997.07.1111:33 AM

>I am posting this on behalf of a co-worker who is investigating the
>reliability of TSOP solder joints.  I understand that the leadframe material
>(Alloy42 vs Cu) and particular geometry of TSOPs reduces the board level
>reliability (relative to other leaded devices).  We intend to include an
>"encapsulated/underfilled" TSOP in the study.  
>
>Any recommendations as to which type of material to use to restrain the solder
>joints ?  If you have a specific material which you have used, this would be
>of interest (spec, name, etc).  Is it better to underfill the complete
>component (my opinion) or only the joints ?
>
>Thanks in advance.
>Charles Elliott
>NPI Process Engineering
>Newbridge Networks
>[log in to unmask]

Charles,

I have seen problems with solder joint reliability on TSOP packages with
Alloy 42 leads after exposure to thermal cycling.  The idea behind
encapsulating the solder joints is to uniformly disperse the fatigue strains
which are ordinarily localized without the epoxy encapsulant.  It is
important to select an epoxy with a coefficient of thermal expansion (CTE)
close to the CTE values of the solder and of the Alloy 42 lead.  In order to
do this, you will probably find that you need to use a filled epoxy.  One
source you may want to investigate is the Norcast 3230 series of epoxy resin
systems. (Norlabs phone number is 1-800-677-5650.)

I do not know of any added benefit from underfilling the package in addition
to covering the solder joints.  Underfilling the package would make rework
extremely difficult.  It is difficult already with just the solder joints
encapsulated.  You also need to consider the environment for your
application.  Encapsulating the TSOP solder joints may be overkill.

Good luck,

Brian Hyde
Reliability Engineer
L-3 Communications
Aviation Recorders Division
E-mail: [log in to unmask]


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