At 08:23 AM 9/25/01 +0200, Edi Im Hof wrote:
>At 11:47 24.09.01 -0700, you wrote:
>>At 08:21 AM 9/24/01 -0500, Mark E Witherite wrote:
>>>         The issue with the virtual short is that it must be cut to test 
>>> the two nets.
>Has somebody tried to make the virtual short not virtual but real?
>I mean, put it on the _solder_ side with a small gap (4..8mil) and let is 
>solder together on the solder wave. Or put it on the top side and expose 
>both pads and the gap on the top paste mask layer and solder it on the 
>reflow process.
>Does one of this works reliable?
>This would allow to test the bare board and connect them with no 
>additional costs.

Well, the original virtual short depended on (1) the gap being much smaller 
than the film resolution, (2) being much smaller than what can be 
fabricated. If a gap makes it through the first two obstacles, it is 
because, somehow, incorrect apertures were used (and Protel behavior in 
aperture matching is not thoroughly tested to my knowledge, or, at least, 
how it behaves is not documented for us as far as I know). The most likely 
cause of incorrect aperture usage would be a fabricator who notices the two 
flashes adjacent to each other and who assumes that a gap was intended; 
thus he or she modifies the film to make it a larger gap.

However, if a relatively large gap were used in the first place, it would 
need to be a gap that can reliably be fabricated; exactly how large would 
depend on process, but 4 mils might still be too small, or the fabricator 
might need to adjust the process, adding cost. 7 mils or more should not 
add cost.

A 5 mil gap with no mask between may well short in the wave; in reflow I 
don't know; I would not trust it to short. Why should it? Someone with more 
recent practical experience may provide better information.

If a board was for high production and was very cost-sensitive, or space 
was *very* tight such that even a very small resistor would be a problem, 
it might be worthwhile to work out the process parameters and follow what 
Mr. Im Hof suggested. Wave solder direction might be critical, for example. 
Otherwise a zero-ohm resistor is very cheap, requires no special process or 
attention, and has other advantages with regard to potential modifications 
of the short character to add resistance or inductance.

Abdulrahman Lomax
Easthampton, Massachusetts USA

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