> -----Original Message-----
> From: Abd ul-Rahman Lomax [mailto:[EMAIL PROTECTED]
> Sent: Friday, 16 January 2004 12:25
> To: Protel EDA Forum
> Subject: Re: [PEDA] Gerber output problems
> 
> 
> At 10:54 PM 1/14/2004, Drew Mills wrote:
> >I completed the layout of a small PCB (99SE) designed for 
> enclosure in a
> >moulded case, then realised I needed an additional hole for 
> mechanical
> >locking. So I added a free pad, with no copper - just the 
> correct hole size.
> >Everything looks good in PCB, but now, when I output the 
> gerber files I need
> >to panelise in Camtastic, there is no trace of the hole, 
> except for a tiny
> >pin-prick on the top soldermask layer. Why is it so?
> 
> Here is what is going on. The soldermask layer is a 
> calculated layer, it is 
> generated from the pad size. In order to make a "pad with no 
> copper" you 
> made the dimensions of the pad zero. The solder mask is 
> generated as an 
> oversize from the pad size, so that it clears the pad. That's what is 
> creating the "pin-prick," it would probably be a pad with a 
> diameter of 
> twice your solder mask clearance, perhaps 20 mils, about the 
> size of a pinhole.
> 
> What did you expect to see on the gerbers?
> 
> I usually create mechanical holes as a pad with clearance, 
> the pad being 
> made the size of, for example, the MMC of a screw head for a 
> screw going 
> into the hole. That way my DRC guarantees that a screw can't 
> bite into a 
> track.... I've often left it just like that, but there is 
> some thought that 
> hole plating in mounting holes can create problems with fragments of 
> copper. I'm not sure how much of a real problem it is, but if 
> you don't 
> want the hole plated, then you can request that from the 
> fabricators (it 
> may be enough to uncheck the Plated box on the Advanced tab 
> of the pad edit 
> dialog). Unplated holes can be a bit of extra cost, they have 
> to be treated 
> specially.
> 
> If you don't want the pad to be there, you can made the pad 
> size smaller 
> than the hole. I wouldn't make it zero, though. Invisible 
> pads give me the 
> creeps.... If you want solder mask to be clear of the hole, 
> you can set a 
> rule for that pad. You might give the pad a name like "MH" 
> for "mounting 
> hole" and then you can create design rules for the pad 
> Free-MH. This would 
> allow you to define sufficient clearance rules to prevent 
> possible shorts, 
> if that is relevant for this design, as well as a solder mask 
> expansion 
> that will make the solder mask opening be larger than the hole.

Abdul, being familiar with this design (Drew works in the same office as
me), I can comment on the clearence problem - there is not one. 

The "missing" hole is for a plastic locating pin in a blow moulded housing.
Hence no need for copper to allow for screw head size, as there is no screw.

Tom L.


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