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Subject:
From:
[log in to unmask] (Robert Blomstrom)
Date:
Thu, 19 Oct 95 14:59:24 EDT
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I recently read of a design paractice of 
7mil lines / 4mil spacing

>THE COMPANY I'VE BEEN WORKING AT HAS BEEN DESIGNING WITH .007 WIDE TRACES AND
>.004 SPACING FOR YEARS. THIS CAN'T BE THAT MUCH DIFFERENT THAN THE REST OF THE
>COUNTRY. AND PERHAPS THIS IS PUSHING THE FAB SHOPS TO THEIR LIMITS.

>FAB SHOPS HAVE ALWAYS MADE INTERNAL ADJUSTMENTS TO OUR GERBER DATA, TO TRY AND
>COMPENSATE FOR THEIR OWN ETCHING VARIABLES. I KNOW FOR A FACT THAT SOME SHOPS
>ADD .001-.0015 TO ALL APERTURES WHEN PHOTOPLOTING, JUST TO TRY AND MEET OUR
>PWB SPECS. BUT, BY DOING THIS, OUR .004" SPACINGS ARE GREATLY REDUCED. THIS
>ALONE CAN REDUCE FAB YIELDS. THAT'S NOT THE DESIGNERS FAULT, JUST A FACT.


The question here is what should be the trace width / spacing of the final board?

If you design a board with, for example, 7mil lines and 4 mil spacings you 
can't expect ANY fab house to use those numbers to build with. That has nothing
to do with fine line capability, just the physics and chemistry 
behind the etching process. In order to etch a material you print a pattern 
onto a panel to mask with. Then you subject that panel to a chemical solution. 
The liquid etches down into the panel to expose the laminate as spacings,
BUT while the solution is etching DOWN it is also etchng IN towards the center
of the masked trace line. 
THEREFORE, if you print with a 7 mil line to mask with, you will NOT get a 7 mil line
to end with, but something less than 7 mils. 

SOOOO..... compensation is done to reverse enginneer the final line width since 
           designers usually forget to state the final line width spec when they submit 
           artworks to the fab house. The fab house in turn tries to build so that
           the final line width is equal to the supplied line width. 
           In doing so, the fab house will compensate and use up that 4 mil spacing 
           that was provided for in the design files. 

    One solution when doing tight line/spacing is to indicate to you fab house 
    what FINAL line width and spacing is tolerable. 
    AND in return, you could find out your fab house's compensation factors and 
    reverse enginneer those values back into your line / spacing sizes so that 
    you could tell the fab house not to bother compensating so that you will get 
    exactly the line size and spacing you expected when designing the job. 

ie: design with a 10 mil line / 4 mil space to achieve a desired 8 mil line / 6 mil space.



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