What gap to use on solder mask?

[Rileyesi]

What is a good value to use for a gap to be used when generating a solder mask
and a flood fill on a PCB? By this I mean, I can tell the software how much
room to leave around the pads and traces when I fill a region (like for a
ground plane) as well as for the solder mask.

Is .020 for the flood fill and .010 for the solder mask too tight or should I
leave myself more room. To me, what that means is that the ground plane will
come to within .020 inches of the pads and traces and the solder mask will come
to within .010 inches.

Thanks.

 

[Leon Heller]

What is a good value to use for a gap to be used when generating a solder mask
and a flood fill on a PCB? By this I mean, I can tell the software how much
room to leave around the pads and traces when I fill a region (like for a
ground plane) as well as for the solder mask.

Is .020 for the flood fill and .010 for the solder mask too tight or should I
leave myself more room. To me, what that means is that the ground plane will
come to within .020 inches of the pads and traces and the solder mask will come
to within .010 inches.

The Pulsonix software I use defaults to .005" oversize when plotting the
solder mask file. You really ought to check with your board supplier. Some,
like PCB-Pool, generate their own solder mask, as they put several
customers' designs on one panel.

Leon

 

[Rileyesi]

What is a good value to use for a gap to be used when generating a solder

mask

The Pulsonix software I use defaults to .005" oversize when plotting the
solder mask file. You really ought to check with your board supplier. Some,
like PCB-Pool, generate their own solder mask, as they put several
customers' designs on one panel.

Leon

Good suggestion, Leon. Thanks. The on line service I use calls for .005
minimum for solder mask.

For the fill (i.e. for the ground plane) I suppose I could use the minimum
distance between traces.

Thanks again.

 

[Activ8]

The Pulsonix software I use defaults to .005" oversize when plotting the
solder mask file. You really ought to check with your board supplier. Some,
like PCB-Pool, generate their own solder mask, as they put several
customers' designs on one panel.

Leon

One vendor wants .0008 over. Misprint? My layout app has them all 5
mil over like yours for the default padstack, but the footprint libs
footprint libs have them the same size as the pad. Either way, I
have to edit the gerbers or change things in layout.

 

[Activ8]

What is a good value to use for a gap to be used when generating a solder mask
and a flood fill on a PCB? By this I mean, I can tell the software how much
room to leave around the pads and traces when I fill a region (like for a
ground plane) as well as for the solder mask.

Is .020 for the flood fill and .010 for the solder mask too tight or should I
leave myself more room. To me, what that means is that the ground plane will
come to within .020 inches of the pads and traces and the solder mask will come
to within .010 inches.

Thanks.

Your pour isolation depends on your app. For RF, you end up with
coplanar waveguide and the por isolation figures in to the line
impedance.

For high voltage, you have to consider the dielectric breakdown of
air - about 3 kV / mm dependant on humidity for a bare board and for
one with soldermask, you have to be sure there are no conductive
contaminants on the board.

 

[Activ8]

Good suggestion, Leon. Thanks. The on line service I use calls for .005
minimum for solder mask.

For the fill (i.e. for the ground plane) I suppose I could use the minimum
distance between traces.

Thanks again.

My global prefs are set for 12 mil track to track, track to pad, and
track to via, so you *could* do that.

 

[Rileyesi]

What is a good value to use for a gap to be used when generating a solder

mask

Your pour isolation depends on your app. For RF, you end up with
coplanar waveguide and the por isolation figures in to the line
impedance.

For high voltage, you have to consider the dielectric breakdown of
air - about 3 kV / mm dependant on humidity for a bare board and for
one with soldermask, you have to be sure there are no conductive
contaminants on the board.

This particular board is simply 4 op-amps with a VERY slow moving 0-5 VDC input
signal and a gain of no more than 1.5.

Actually, about 95% of my traces are on the top side of the board (only the
power lines are on the bottom) so I am wondering if I even need a ground plane.
I was just using one to "be on the safe side" and to make the routing easier
(i.e. connecting all the pins that need to go to ground via the plane rather
than needing traces back to the ground of the power supply).

 

[Activ8]

This particular board is simply 4 op-amps with a VERY slow moving 0-5 VDC input
signal and a gain of no more than 1.5.

Actually, about 95% of my traces are on the top side of the board (only the
power lines are on the bottom) so I am wondering if I even need a ground plane.
I was just using one to "be on the safe side" and to make the routing easier
(i.e. connecting all the pins that need to go to ground via the plane rather
than needing traces back to the ground of the power supply).

Oh. In this case it's easier to use bottom side traces for ease of
soldering unless you really want to send the layout to a board house
in which case the plated through holes take care of all that as well
as help keep the traces from lifting off the board.

At these levels and speeds, you can get away with just about
anything like putting each chip in a separate room and connecting
them with jumper clips.

 

[Frank Bemelman]

Good suggestion, Leon. Thanks. The on line service I use calls for .005
minimum for solder mask.

For the fill (i.e. for the ground plane) I suppose I could use the minimum
distance between traces.

Protel uses the global clearance setting as default. But I always increase
it
to 25mil before doing the final fill. Perhaps it is not a problem for a
manufacturer,
but I always get a bit worried when a small clearance is used for the entire
board,
for the same reason you run a board with 10 or 12 mil traces and not 7,
except
for some small areas where you are forced to use 7 mil.