Saturday, February 13, 2016

PCB milling: how to verifiy the accuracy of routed traces

Inspecting an Eagle PCB milled board

Here is how I verify that my pcbgcode settings are good, and more precisely, how accurate the "etched" traces are, in relation to the depth of cut. The latter is not easy to set when using V-shaped mill bits, since the deeper, the wider the traces. The one below, between two pads is only 0.01 inch, i.e. 10 mils, and it was properly milled.

The trick was to overlay the live image from an USB microscope over the Eagle board design, scaling the windows appropriately. I applied 50% transparency to the webcam window (I am using ''cheese'' here), which is trivial with Kubuntu / linux. The microscope is an Andonstar, a great & cheap tool, for which I wrote a micro-review here.

By the way, I will no more use these "titanium" coated V mill bits, as they produce rough edges compared to the cheaper ones I bought earlier on ebay. This board is just ugly. Well, the PCB copper clad is also a super cheap one (I bought a lot "50Pcs 70 x 100 x 1.5mm FR-4 Single Side Copper Clad PCB Laminate Board" for $37 on ebay: I could not expect much but they are NOT FR4 but old style bakelite, not fiber glass! Much more fragile, not really worth).


Also my holes are slightly off-center, but not bad enough to be fatal here. Cheap 0.8 mm drills?


Review: the useful Andonstar USB microscope

The Andonstar USB microscope is definitely a great tool to double check surface mount components among other uses. I bought it from them on Banggood for about $50 (yep, it is not a low cost low quality USB endoscope).

Below I had to replace an AMS1117 regulator from a RAMPS that I blew (I plugged a 2-wire endstop to the wrong pair of 3-pin, which mean I grounded the power supply and got magic smoke...). I could then check how ugly I did ;)

The zoom range of an Andonstar USB microscope! And excellent tool for its price.
My main opinion is:
  • great wide zooming range (see the pic below)
  • integrated leds that are very close to the camera sensor (no external device would give enough light in the shot on the right given how close the lens is from the board)
  • the light intensity can be tuned down to zero. Sometimes the object is too reflective and it "flattens" the image. Then, an external lateral light beam is better.
  • a stable support with fast height adjustment (the stick can be raised/lowered quickly from the "main" tower) PLUS a fine thumbwheel on top of the camera barrel.
Drawbacks?

  • none really for the price
  • make sure to align "north" with "north" by precisely rotating the barrel, else it is hellish ;)
  • still the main pole has a bit of free play left, which can be felt when tuning with the thumb wheel at high zoom ratios (the small wheel is very firm). And there is no easy way to hack this.

Update: this slightly more expensive $85 version (A1-200W-B) addresses this with a definitive improvement. It uses the same microscope, but the already good stands looks way better. I would buy this one if I had to buy again.
The same USB microscope/endoscope but with an improved stand ($80 here)

Anyhow it is a must have in my opinion. I use it very often to inspect PCBs, to read SMT component values, or whenever my eyes are tired...

Final note: this microscope is featured in a video by Dave on EEVblog (a nice source of information by the way).

Note: the above scope is excellent for super tiny operations and also for finely checking the results. But I often rely on a "two eye", cheaper magnifier like the one below ($9!). It works well enough for 1206 SMT soldering jobs for example. Having the depth is much more comfortable, and it is easier to switch from the view on the board to the view through the lens than to look at a flat video on the screen while your hands are elsewhere. It does work (hence this review), but it is not intuitive at first.
Much cheaper ($9), but more comfortable magnifier,
for bigger jobs like large SMT soldering.
You can look through the lens with both eyes so you get the sense of depth back.
But it certainly does not zoom as much as the Andonstar.