From:	NAME: WILLIAM G KENYON              
	FUNC: Chemicals/Electronics           
	TEL: 302-652-4272                     <KENYONWG AT A1 AT ESVAX>
To:	NAME: [log in to unmask] <"[log in to unmask]"@ESDS01@MRGATE@ESVAX>


There is not an industry standard for the added surface area of 
the components.  The intent of the military at the time the 
equivalence factors were developed and published (Naval Avionics 
Center Technical Report MRR-3-78) was to have all the component 
makers determine the area of the components including the leads 
and publish that information on component spec sheets.  Then all 
the engineer would have to do is to do the (length x width x 2 
sides) + the sum of the areas of all the components from the 
drawings.  That is why you see the populated test boards in that 
document- note that one PWA had just resistors, another had just 
transistors, etc. So when the working group did the data analysis 
on the test boards, we had to make the assumption that all the 
different types of components in the data set would be used on 
the mil. PWAs.  Thus the equivalence factors reflect such a 
component mix.  However the component people - who rarely come to 
IPC or Mil groups where the board and assembly people congregate 
- never got with the program.  

So you are using the l x w x 3 version of the "rule of thumb" 
which would mean that you are including both sides of the board 
plus making the assumption that the area of one side of the board 
is approx. equal to the area of the components.  This may have 
been a pretty good fit when everything was ICs and PTH, but may 
(may be, not must be) different with surface mount.  

If you have some board designs of the '78-80 vintage and the SMT 
version of the same PWA, it would be a nice contribution to check 
out the area x 3 rule.  Pls publish it on TechNet if you do the 
numbers.  

If Doug Pauls doesn't have a copy of the MRR 3-78 available, I 
should be able to find my original and photocopy it.

Bill Kenyon
Global Centre for Process Change, Inc.
302-652-4272/-5701 Tel/Fax