Scott,

They may have encapsulated the leads with epoxy to enhance solder joint
reliability.  IBM Endicott wrote a paper on the use of this method for TSOP
solder joints exposed to wide temperature ranges.  Coefficient of Thermal
Expansion (CTE) mismatches between materials cause highly localized fatigue
strains which can initiate cracks in the solder joints.  The encapsulant
firmly holds the package to the board and uniformly disperses the strains
over the encapsulated area between the package and the board.

Brian Hyde


At 09:49 AM 2/20/97 -0500, you wrote:

>I am working on a quality team to improve our surface mount
>assembly processes and was critiquing some SMD boards from other
>manufacturers and was wondering why some of them are putting a
>clear over-coating on fine pitch (.025" and smaller) parts after
>they are attached to the board.
>
>It seems to me that this would limit re-work (if required) unless
>there is some way to remove the coating.  It would also mean
>that no probing of the parts could be done in a troubleshoot area.
>
>There must be some good reasons for this clear coating applied to
>the leads.  Please let me know of any reasons all of you have!
>
>Thanks,
>Scott Gaskins
>[log in to unmask]
>
>-- 
>[                     Scott Gaskins - ext. 5368                        ]
>[ ----===>> GE-Fanuc, North America. ----- Hardware Design   <<===---- ]
>[  mailto:[log in to unmask] |"Truth is nothing but a path traced]
>[  http://gein.cho.ge.com/~wsg     |  between errors."  - F.A.Mesmer   ]
>

--------------------------------------------
Brian P. Hyde
Reliability Engineer
Lockheed Martin Advanced Recorders
E-Mail: [log in to unmask]
--------------------------------------------

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