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What gap to use on solder mask?

Discussion in 'Electronic Design' started by Rileyesi, Jun 7, 2004.

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  1. Rileyesi

    Rileyesi Guest

    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.
     
  2. Leon Heller

    Leon Heller Guest

    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
     
  3. Rileyesi

    Rileyesi Guest

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

    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.
     
  4. Activ8

    Activ8 Guest

    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.
     
  5. Activ8

    Activ8 Guest

    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.
     
  6. Activ8

    Activ8 Guest

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

    Rileyesi Guest

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

    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).
     
  8. Activ8

    Activ8 Guest

    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.
     
  9. 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.
     
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