Waheed,
A simple two layer board would consists of top and
bottom copper layers. These are layers where you draw
your pads and wires.
On top of this (both sides) you place a mask, which is
mainly for protection. On these layers you have
openings only for the pads (TH and SMD).
Paste layers are used when you have to solder SMD
components using reflow process. On this layer you
should see only the SMD pads. It's mainly applied for
the top layer but if the PCB is double reflow, they'll
ask you to provide solder paste on bottom parts, too.
The dimensions of the pads in paste layers could be
the same size as the pads in copper layers or a little
bit less. Assembly companies could adjust them
accotding to their needs. 
So, the paste layers are not important for PCB
manufacturing but for assembling the PCB.

Hope this helps.
Mira



--- Waheed Bajwa <[EMAIL PROTECTED]> wrote:
> Well Ian thats a lot of useful information and of
> great help to me... and
> thanx for the link too...
> 
> Just a few clarifications...
> 
> So the overlay layer and the silkscreen layer is the
> same thing...
> Looks like the mechanical layers are not invovled in
> the manufacturing of
> PCB and just for reference only? no conduction?
> 
> I get what is Solder Mask layer but what I don't get
> is why do we need paste
> layer after this? I blv both are almost the same
> thing... There would be
> holes in the solder mask layer and there we can
> apply the paste even if its
> the case of surface mounted components... Please
> explain...
> 
> Best Regards,
> Waheed Bajwa


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