Early results for the new chamfer polarity pcb and carrier.
There are a few things I need to adjust from here to get it as good as
it can be, but, I just got the pcbs that match two earlier carriers (one
with chamfer, one with a pin, but without the notches in the corners),
and they already prove out a few of the things I wasn't sure about.
There's some new pics added to the bottom of this album.
https://photos.app.goo.gl/zynHU4LSpd5BZBYC9
Specifically:
https://photos.app.goo.gl/wgwpDoLmUk5npqHb6
https://photos.app.goo.gl/1XRMmWijtPeiVrYM8
https://photos.app.goo.gl/JponommBdc6Dx93T9
https://photos.app.goo.gl/HUouhuZSJ5odtf4Z9
This pcb ends up proving not only the design if the fab produces it
perfectly, but also the result when oshpark produces a slightly sloppy
version.
Both the solder mask and the router path are out of alignment on this
batch. Drills are slightly off too. And, it still works, so, good!
Happily the new shallower edge contacts still came out ok even though
the router ended up cutting too far in on one side, and too far out on
the other side. I was worried about exactly that.
One thing I have to tweak is I have to back the solder mask a little
further away from the edges of the drill holes on the edge contacts and
the jtag and power pin holes.
Ideally there is no reason for the top or bottom pads or annular rings
to be exposed on any of those, just the vertical walls on the edges and
inside the holes, and I would like the top & bottom surfaces to be covered.
But if you let the solder mask get too close to the actual edge of the
hole, it ends up going down inside the hole and putting varnish on what
you wanted to be a clean electrical contact surface. Even worse if the
mask is also out of alignment.
This shows it clearly.
https://photos.app.goo.gl/7MDvhFCffRVPPYsS6
Otherwise I really like how the edge contacts came out. They're
everything I was hoping for in terms of optimizing to consume the least
amount of pcb real-estate, and making a flatter contact surface for the
socket pin to touch right in the middle, as similar to a flat chip leg
as we can get with a round hole, while still mechanically sound enough
to survive the end mill. That is thanks to using the rectangular pad
shape. The corners of the rectangular pad provide anchoring, which
allows the straight lines to come in right up to the edge of the hole,
where normally you need to leave a pretty good lip all around the hole.
I would really prefer a little better polarity enforcement. The pin is
really good on that point. It stands out and is both unmistakable
visually, and unforceable mechanically.
https://photos.app.goo.gl/jxJSohH4htAMZHpY7
https://photos.app.goo.gl/bU9ES9h62Ju7nJge7
https://photos.app.goo.gl/aACmi4ZzkRyiUFFu9
At least this updated pin version makes the hole smaller than my earlier
version (smaller than these pics. this "c9" pcb is old and the hole can
be smaller than it is here.) and by having the hole instead of the
chamfer, it allows all 4 corners to be full. The outer shape of the pcb
is a plain rectangle with no funny notches or anything, and if you
wanted a component or a wire pad tucked right into a corner you can have
it. And even the hole doesn't get it the way of top-side components too
much. A chip or other larger component is free to span right over the
top of it. I think I could even position the LP2980 exactly over top of
the new smaller hole.
The chamfer version, the chamfer just isn't very big, and can't really
get any bigger because then you eat into the pin1 edge contact. But,
it's good enough I guess. Here is the worst example I was able to make
by trying to put the pcb in the wrong way on purpose. Worst meaning, the
most successful attempt to do it wrong. I can almost successfully jam it
in this way, but, only almost. So, ok. Getting rid of those notches in
the corners and the hole in the middle is pretty valuable and I guess
worth it. This way the pcb ends up having more usable real-estate and is
just simpler to make. Like, you don't need to actually use my template
kicad file to start with. All you need is the simple dimensions.
https://photos.app.goo.gl/AQ7tut5rHz8pLBts9
https://photos.app.goo.gl/JsNTD6U11mzja7J97
Other times when I didn't try so hard to get it wrong on purpose just to
test the worst case scenario, it was pretty obviously wrong.
https://photos.app.goo.gl/HhEdPCDPbMcLfup16
All in all, I think the advantages of the chamfer board outweigh the
disadvantages so I'm switching over to use it for everything, not just
as a special alternative option. It's valuable to have a single design
that all pcbs use, so that the parts can all swap. Like a seller, or
just an individual, could order a bunch of carriers and not have to care
if they were going to make more rexs or more teeproms or whatever other
new boards come along.
Also, holy cow that Sculpteo 60um service is remarkable! (this is a
different version I ordered a week or more ago that just came in)
https://photos.app.goo.gl/jKbdbJVe9PRFeT8Z8
https://photos.app.goo.gl/19c8tLpV999i2Z1G8
The Shapeways parts are already proven to work just fine though, so, the
extra cost and extra accuracy from the 60um option isn't necessary. But
dang it's pretty cool.
--
bkw
