Even though we know these things well, it's easy to get the explanations 
bollixed up.

At 08:19 AM 4/9/01 -0700, Peter Bennett wrote:

>Solder mask and Paste mask are indeed both negative - if you draw
>something on either type of layer, you do make a hole in the resulting

Not necessarily. A hole in the paste mask allows paste to pass through. So, 
yes, by placing the primitive, we are making the hole, i.e., we are 
ultimately creating a place for paste mask.

I'm not sure how paste masks are being made everywhere. If they were 
silkscreened, one would want to expose the silkscreen resist through a 
positive plot of the paste mask layer. (It might go through a negative 
phase, but this might vary from shop to shop; it's been years since I 
actually saw it being done.) Wherever the pastemask pad is, there will be 
black on the film, so the resist underneath will be left unhardened and 
will wash away, so the solder paste can go through it.

If the mask is being made from steel, then it would depend on whether the 
mask was made with a positive or negative process. If it is being directly 
photoplotted with a laser onto the steel, one would plot a positive, and 
the laser would cut the mask openings. If it is being photo-etched by a 
harden-resist process, one would image a negative onto the resist, 
hardening the resist where-ever the pad is *not*, as above with a silkscreen.

Bottom line for us: place a primitive, create solder paste at that 
location. So, to us, solder mask layers are positive layers, not negative 
as stated.

A solder mask is the opposite. place a primitive, *remove* solder mask. So 
solder mask layers are negative layers. Primitives create no-mask.

these explanations are true, but "exposed" means no-solder-mask, whereas 
the "hole" means solder paste:

>However, for the Solder mask, this means that the copper or board under
>the mask will be exposed at that point.

i.e., there is *no* solder mask.

>For the paste mask, this means that solder will be applied to the board
>at that point, through the resulting hole.

i.e., there *is* paste.

Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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