Choosing Solder Mask Expansion Value
Dan D.
copper pad and the dielectric material that usually coats circuit
boards. Thus, the material of the gap is bare fiberglass.
What is the purpose of this gap, and how do I determine the minimum
gap size? For a 64-pin chip, I usually set it to something small like
1mil or 2mil since a greater solder mask sliver between the small pads
prevents solder bridges.
But, if I am not limited in the size of this gap, how big should I
make it? Should the gap increase in size proportional to pad size?
The specific footprint that I am working on now is a 700 x 250 mil
pad. My first thought was to give it a solder mask expansion of 5
mil, but I would really appreciate a more scientific way of
calculating it.
a7yvm1...@netzero.com
> The solder mask expansion value is the size of the gap between a
> copper pad and the dielectric material that usually coats circuit
> boards. Thus, the material of the gap is bare fiberglass.
>
> What is the purpose of this gap, and how do I determine the minimum
> gap size? For a 64-pin chip, I usually set it to something small like
> 1mil or 2mil since a greater solder mask sliver between the small pads
> prevents solder bridges.
Please specify the package of this 64 pin chip. It could be anything,
QFP, QFN, BGA, PLCC, TSOP???
The gap is to allow for tolerance of the placement of the soldermask
over the copper. You don't want soldermask on a pad (usually). The
tolerance has gotten quite good and you can certainly use 1 or 2 mils.
However if you can be happy with a larger opening it does make the
board easier to manufacture, thus cheaper. I only use tight tolerances
if the footprint demands it. ie there's nothing to be gained
specifying molecular accuracy on a DIP 14...
However you need to keep some sane value so the board can be soldered
normally.
The second thing to look out for is the minimum web, that's the bit of
mask left over between pins. If it's below 3 mils, usually that's not
manufacturable and the vendor will remove them. Or they might charge a
premium. Depends.
You can proably tell your CAD package how to handle soldermasks, it
should be able to generate voids between pins if necessary.
It sounds like you might be soldering this manually? No paste stencil?
>
> But, if I am not limited in the size of this gap, how big should I
> make it? Should the gap increase in size proportional to pad size?
> The specific footprint that I am working on now is a 700 x 250 mil
> pad. My first thought was to give it a solder mask expansion of 5
> mil, but I would really appreciate a more scientific way of
> calculating it.
You need to contact the manufacturer and ask for their capacities.
Usually they publish it on their site.
Set the gap to whatever they specify is their standard capacity so you
don't get dinged in price or lead time.
Of course, some parts do require some bizarre soldermask magic so YMMV
with my advice.
Don't forget to keep a clear and open comm channel with your board
vendor. Specify EVERYTHING in WRITING at least twice, once in your PO
and once on your fabrication drawing. (Make sure they are the same
instructions)
Dan D.
Thanks.