Itead Studio OpenPCB service: In search of OS PCB part 2

Yesterday we tracked down the designers of two random open source PCBs we received from Itead’s OpenPCB service. Today we look at two more PCBs: a reprap board, and one of our own.

Itead has a PCB exchange option when you order boards. For 10 cents extra you get 2 different open source PCBs from other designers. We decided to take our chance and see what kind of PCBs we receive.

We sent two 5×5 designs, so we received 4 extra boards from the exchange. In this series we cover which boards we received, and we hunt down the makers and get the story behind the board.

This board was easy to track down, it is clearly marked ‘http:///’. It’s a controller-PCB for a Prusa Mendel Reprap machine. Here is the blog and wiki about this project. Of course we sent the developer an email about this board and this is his reply:

.. It’d be my pleasure, though to tell you about the board and project.

The board you have is 1 of 2 designed to control a RepRap 3D printer, a device that literally prints 3D objects out of plastic. A RepRap printer works by extruding plastic out a tiny nozzle, slowly building one layer at a time on a ~100^2cm print bed. The electronics to control the printer’s motors and heaters and read its sensors are not particularly complicated, but currently they are quite expensive (nearly $200). The board is part of the GenL electronics package I’ve been designing for RepRap printers. The entire kit will cost as little as $75 unassembled.

GenL electronics are split across two boards, the smaller of which you have. That board houses a socket for a Teensy microcontroller and the supporting low-volatge sensor circuitry. The 5-pin connectors are for opto-interruptors that detect when each of the X/Y/Z axes have traveled to the end of their range. The smaller 2-pin connectors are for thermistors that monitor the temperatures of the extruder nozzle and the printer bed (which is often heated to keep the part from warping while it is printed).  The large 20-pin header near the center of the board connects it (via ribbon cable) a companion high power board. The high power board houses the stepper motor controllers (for driving the X/Y/Z/Extruder axes) and the power transistors that drive the heating elements. ..

The last board did surprise us a quite bit as it looked very much the same as one of the boards we send to manufacture. Actually it was! We sent in four design, and they picked one of our own to send.

The story about this board is to test a new footprint for QFN we designed and add a new accessory to our beloved Bus Pirate. This will add an extra 8 I/O to the Bus Pirate (we are working on the support in the software) and is ideal to test LCDs or logic-chips with the BusPirate. The chip is connected through I2C and has a similar opendrain with selectable pullup voltage as the buspirate. When we designed this board we made a couple of (cough cough stupid) errors that needs to be corrected in the future revision, but we did have a chance to test the new QFN footprint.

We put a regular throughhole pad in the middle of the exposed pad (EP). This will make the EP easy reachable with a normal iron from the backside and thus QFN soldering is now possible with a regular finetipped iron! Yeah! At least some kind of succes ;) We used too much heat so the pad got off on the backside, but the solderconnection was fine. There were two reasons it came off: we applied too much heat and the pad was too small.

The topview of the assembled PCB, in the middle the PIC16LF1823. It’s a 3V3 part that can’t survive 5V. On the right, the Bus Pirate v3 connector, with the 3V3/5V pullup selector behind it. Also shown is the I2C slave address selector.

Finally on the backside (notice the missing pad), there is a female IDC header which accepts those handy breadboard wires without much hassle. Also the powersupply is conveniently accessable. We like this with those small breadboards!!

PCBs are added to our PCB store and can be yours!

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    1. Well it saved me from writing another post about the new IOX8 project. Perhaps I resend the board with all the stupid mistakes removed and posted about that (only the good story)

  1. Instead of a Through-Hole Pad, try a Via, or even several Vias. It’s actually common practice to put another thermal layer on the opposite side of the PCB from the chip and use the vias to conduct heat away. Then you end up with a thermal mass that’s larger than what would fit under the chip.

    1. I noticed that on some boards and expected it mandatory for ‘power’ chips.

      Downside of via’s you also need to remove the soldermask if you want to use the via’s as soldering help and I think the via is to small to conduct the heat from the iron. According to the datasheet from microchip there is no need for via’s. I don’t have any experience though and this was the first try and was fairly succesfull. I guess when the chip grow several more holes (whether via’s or th pads) are necessary.

      1. It seems that Eagle always removes the solder mask from vias by default – either that or I have some setting flipped that I don’t know about. One PCB fab house even asked permission to change the via presence in the solder mask, but I can’t remember whether they wanted to add them or remove them. Apparently, it affects the cost of making the solder mask. I may have even had a surface mount board where one side had no holes in the solder mask except for vias, and perhaps they were hoping to just screen a solid sheet. I’ve done too many boards to remember which one it was, though.

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