you said:

>Interesting. So I infer this: If I have a 10" x 7.2" PWB design to be
>fabbed as 4-up on an 18" x 24" panel, each unit represents 72 sq inch per
>side, 144 sq in both sides.

Correct

>I guess you mean image = unit...
>So, is the cost estimated based on the panel area, or the unit area? That
>would be 6 square feet (both surfaces) for the panel, or 4 sq ft for the
>four units...

Now take this cost and use earlier stated estimate for OSP(0.2 to 0.5). This 
gives you a processing cost of $0.80 to $2.00 per panel.

For the size images/units you are using, I have seen finished exposed copper 
areas generally in the 0.7 to 1.1 sq ft range(after solder mask)(total,all 
four images/units). This yields $0.96 to $2.60 per panel based on earlier 
stated estimate for HASL($1.40 to $2.40)

Estimated cost savings for this hypothetical board, not very much. Of 
course, it all depends on real numbers, not estimates. We have done some 
organic coating work, but not enough to confirm or refute suppliers cost 
estimates.

This is in answer to the original question as to why board shops do not pass 
cost savings from OSP's on to it's customers. There might not be any 
significant cost savings, especially if it's volume compromises less then 
10% of total product. There are, however, significant processability 
improvements that can be gained at the assembly shop, if it follows the 
rules for OSP's. A 2 - 5% first pass yield at assembly would seem an 
impressive cost savings.

Charles Stuber
Chemical Process Engineer
Automata
703-450-2600
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http://www.automata.com