I'm not sure if your suggestion of vertical clearance is endorsed or just
some of that famous UK wit.  Your example is one of the few times this would
be helpful. Through hole on top of SMT.  Other combinations would drive the
test and inspection guys nuts (bonkers?).

I too have the clearance blues (greens) but more often when adding dual
footprint patterns to cover parts availablility issues.  Similar to placing
a dip for prototype and an overlayed SOIC footprint for production. I
recently placed two 0805 footprints so they formed a common pad high or gnd
selector for a zero ohm placement.  I tried every combination of positive or
negative clearance rules to get rid of the clearance violation only to give
up and contruct a special three pad footprint with a three pin resistor
symbol.  A poor way to do it because the new symbol shows up as a single ref
designator and single value.  I had to make a separate symbol for pull up
and one for pull down placement.  

-----Original Message-----
From: Ian Wilson [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, February 06, 2002 2:44 PM
To: Protel EDA Forum
Subject: Re: [PEDA] Setting component clearance rules

On 01:52 PM 6/02/2002 -0500, Abd ul-Rahman Lomax said:
>At 02:00 PM 2/6/2002 +1100, Ian Wilson wrote:
>>Negative clearances on components do not work - would be nice if they 
>>did.  Alternatively a rule that allowed component clearance violations to 
>>be ignored for specific components or regions would be helpful.
>Here is what we should have. There should be two placement outline layers. 
>Track on these layers would represent the limits of reserved board space; 
>a closed outline figure would be required. Crossing of this track (not 
>mere contact) would generate a placement violation, as would enclosure of 
>one component's space by that of another except if the following feature 
>is added:
>A further refinement would be that additional closed figures would be 
>allowed *within* the outline, representing free space under the part. 
>(crossing of such tracks would be an error.)
>The placement outline layer would be an associated layer, i.e., it flips 
>with the part.
>The placement outline would also be used by any autoplacement routines.
>The design rules should also allow exceptions, as Mr. Wilson suggested.
>Abdulrahman Lomax
>Easthampton, Massachusetts USA

But then why stop there - the design rules should allow us to check that 
the component fitting under the one with some "off-pcb" space is not too 
high.  I may be able to fit an 0805 under a ROM socket possibly even an S0 
in some parts but not a fat tant.  Or what about fitting small components 
under the curve of a large axial cap?  The height available varies with 
distance from the centre line of the component.

I seem to remember that one of the PCB packages has a design rule system 
that can deal with addition and subtraction etc.  Using this (and with some 
extra design rule functions like DistanceToComponentBorder, and/or Overlap 
etc that give us measures off the component(s) being DRC'ed) we could get 
more elaborate.  Cost would be a significant slow down in DRC and great 
scope for complex design rules not doing what we actually wanted.

Ian Wilson

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