While answering another technet question on buried via, I had one of my 
own.

In the late 1980's, I recall seeing a study (IBM-Endicott?) which 
demonstrated that the most reliable via thickness with respect to Z-axis 
expansions was something like .5 - .7 mil.  Was this just wishfull 
thinking?  

If this is so, why have we not pursued it as a standard?  If it is a 
better thru hole, wouldn't I (now a consumer of PWBs) want that?  And 
wouldn't every PTH house like to reduce their plating times by 30%?  

Is a possible answer:  The extra copper is to ensure coverage.  I have 
known boards with copper thickness of .4-.6 mil to have problems with 
blow holes in the assembly process.

George Franck
Raytheon E-Systems
Falls Church Va

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