Re: gEDA-user: Has anyone used SSOP28.fp?

[Dave N6NZ <n6nz@xxxxxxxx>]

DJ Delorie wrote:
>> Since I have access to a laser cutter at a public machine shop,
> 
> Now that you've admitted that, you're going to have a lot more friends ;-)
> 

Well, I've considered doing an el-cheapo stencil service, but it would 
have to be "plot-N-go" to make it worth while.  There are two issues:

a) pcb paste layer gerber is directly from the pad layer, and I think in 
many cases that lays down too much solder.  I haven't experimented 
enough to be able to say for sure.  I'd like to see some paste layer 
control added to the footprint definition.

b) My current method for prepping a file for the laser cutter is a bit 
of a kludge.  Ideally, I'd like to see the gerber previewer have the 
option of exporting .dxf of the pad outlines from the paste layer.

> How tight is the laser?  I.e. kerf?  Pololu's laser is 10mil,
> commercial stencils use a 1mil cut.  

You can't really talk about kerf as a single number.  The laser works by 
pumping energy into the material until it vaporizes.  You can adjust 
power level, pulse rate, and dwell time.  The cut will "bloom" into the 
material depending on the type of material, the thickness, and color. 
And then there are issues of how tightly the laser cutter is adjusted 
and how it is focused.

> My etched stencils suffer from
> inaccuracy due to the thickness of the brass and long etch times (I
> have thinner brass on order to reduce that).

Well, cutting in 5 mil acetate has issues... the power level very 
tweaky, as there is a fine line between getting a clean cut so the pad 
drops out nicely, versus getting "goo".  (On my first experiment the 
power was *way* too high and I got an entertaining amount of flamage :) 
Referring to my comment above about paste layer pad sizes, I might be 
getting some bloom due to meltage, and I need more experiments to get to 
the bottom of that.

The workshop that I belong to recently got a PCB mill on line.  I'm not 
too excited about it for milling PCB's (why? no plated vias mainly) but 
it strikes me that perhaps it would be a good way to generate brass 
solder stencils.

-dave



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