Monthly Archives: May 2013

Maker Faire 2013, here we come

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As designers, here we often give PCB function the highest priority while making the mounting/enclosure options (see above) an afterthought. This is especially problematic if the design will be constructed more than once or twice, prothesis where custom drilling/milling/manual processing will become very expensive and hard to reproduce. Having worked custom mold-injected enclosures, ampoule custom metal enclosures, and off the shelf enclosures, here are some things I have learned along the way:

  • Check with your PCB assembly house about component placing near PCB edges, a lot of places will typically want >1.27mm. This is a process issue for removing individual boards out of panels and being able to clear the cutting tool. More is better as it makes it easier to handle the board after de-panelizing without touching mounted components.
  • Label component keepout areas on the PCB print if you need to clear anything, especially when using ridges to support PCBs in plastic cases. Locations of these keepouts can be obvious when initially designing the PCB, but you will soon forget and may accidentally place components there in a future revision. Changing a PCB fab is usually a lot cheaper than changing the injection mold, so you will be redesigning your PCB in situation.
  • Make sure that your board only fits into the chassis one way. This can be done by using a non-symmetric mounting hole pattern or by having specific cutouts for mechanical pins or ridges.
  • Comply with standard drill sizes (and keepout areas) for standard mounting hardware. Tim Hausherr has a good overview here. It is a lot easier to be able to use standard fasteners instead of drilling arbitrary holes in PCBs and then trying to find something that fits.
  • Specify a torque spec and use an electric driver with torque setting when driving any kind of threaded fastening hardware. If you don’t do this, you can expect to get anything from cracked PCBs from overtightening to loose hardware/PCBs in your chassis.
  • If you expect a lot of vibration, avoid using threaded fasteners and use clips instead. This is especially true for mounting speakers to chassis. It is tempting to use threadlock, but this is a bad idea from a serviceability standpoint (see below).
  • Expect that you will get failures in the field and that you may have todisassemble units coming back from the field. This is the primary reason I don’t like using threadlock unless it is going into standoffs which are stamped into the chassis and will not turn when you try to back your fasteners out. This is also the reason to give a second thought to conformal coating/potting your circuits unless you have a specific performance reason to do this (IPXX rating, isolation). Coating your PCB/filling your case with epoxy will not stop people from being able to reverse engineer the physical aspects of your design.
  • Use 3D models if your connectors, or at least mark their outlines on an assembly layer, so you can align them with your chassis. The large gap between the USB and Ethernet connectors on the rpi makes it challenging to mount PCB in an off the shelf rectangular case.

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We finally have everything squared away for Maker Faire 2013 and we should be able to show off an open-source LED controller. That design is not complete, physician
however, I still wanted to be able to give something away. I settled on creating a PCB business card which doubled as an SMT prototyping board on the back. I did a few iterations and settled on something that was all surface mount. There are pads for an SMT mini-USB port, a 32qfp,80tqfp and 64 pin IDC side-connector. All of the quad footprints will accept smaller packages with the same pitch and the IDC pads are designed in such a way that the 0.8mm PCB can be soldered between the pins of a standard 2.54mm/0.1″ connector. There is also a grid of pads that have 1.27mm pitch in one direction and 0.65mm pitch in the other direction so they can accommodate a SOIC one way and TSSOP the other way. The finish is ENIG with a high performance white solder mask so it should stand up to soldering without discoloring.

Stop by the mobius.io table and just ask if you would like to get one of these. Gerbers for this card are here: mobius_io_card2013 .