At 11:08 AM 5/17/2002 -0700, Brad Velander wrote:
>I developed my undying
>adoration for Fab dwgs at one employer where we purchased our PCBs in
>various shops usually located in China, Hong Kong, Singapore, India or
>Korea. Try doing that without very complete and thorough Fab Dwgs. You (and
>your employer) won't survive long without Fab Dwgs.

Reading carefullly, I'll agree with Brad. But what he wrote could be 
misinterpreted. Not all environments require fab drawings. A small 
engineering firm might have a relationship of trust with a fabricator who 
routinely does boards according to the fabricator's standard production. 
This is, in fact, quite common. Formal fab drawings take time to make, 
which means they cost money and a little time-to-market, perhaps. 
Obviously, if one is going to various vendors and trying to get the same 
product each time, one needs formal specifications.

As to the matter of specifying finished sizes vs. drill sizes, I include 
the knowledge of plating thickness and how it affects hole sizes, as well 
as the allowed variation in finished size, in the design rules I set. 
Protel's default 10 mils is pretty naive, rarely is it the best setting, 
especially with, for example, power plane air gaps.

I am giving the fab house a set of files which represent what I want. How 
they get there is largely up to them, unless I specify it. I don't want to 
nail them down to a particular drill size; exceptions to this would be 
rare, I can't think of any I've encountered.

However, it is prudent to specify on any fab drawing that the holes are 
finished or as-drilled, whichever applies. I think fabricators are 
accustomed to seeing finished sizes in drill files, so sending them 
as-drilled sizes might toss them for a loop unless it is very clearly flagged.

I haven't watched the actual process, but I would imagine that fabricators 
take the drill file and assign real drill sizes to the drill codes 
according to what they think their process requires. With finished sizes 
being specified, these drills would nominally be an average of 3 mils 
oversize from finished.

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