Ok, I am scratching my head on this one....but I have been on an extended 
vacation and might not be seeing this clearly (but I doubt it), so I thought I 
would post it for your comments.

A company here in the Philippines is doing final assy on a circuit board built 
in Thailand which consists of adding an anodized aluminum slug beneath a 
previously soldered TO-220.  The design calls for the "sandwich" to be mounted 
flush to the PWB using a screw and nut. 

My assumption is that it is intended to have the generated heat be dissipated 
into the PWB as opposed to being mounted on heat sink fins and dissipating into 
the air.  Not the best design but not uncommon.  A thin layer of thermally 
conductive adhesive or gasket is typically used.

When I watch the actual assembly, I realize that the designer has added one more 
twist to the sandwich......a piece of 0.14 mm thick Mica sheet between the 
aluminum slug and the PWB.  The only thermal compound is a small dot between the 
slug and the Mica (basically adhering the slug and Mica alignment). 


One thing to keep in mind that this is a "high volume" product and the cost of 
thermal compound far outweighs the cost of the slug, and increasing cost 
(complete layer of thermal grease between all layers of the sandwich) is not an 
option.  


Mica is an electrical requirement but I am questioning the use of the anodized 
aluminum slug in this design.  I am suggesting that we run a test with the slug 
removed entirely.    

As I mentioned, I really haven't had much cerebral challenge over the past year 
and a half, so all comments are appreciated.

Phil

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