Monday, November 2, 2015

Making MBed and Arduino compatible Xadow Modules (in Reflow Oven)

I have designed a long chain of Xadow modules by now, including right angled side chains. These include:
  1. Xadow SD (CD4050) - Which allows using upto 32GB Micro SD Card and good read/write speeds due to the driver IC.
  2. Xadow Serial (SC16IS750) - Which adds 1 UART and 8 GPIO ports, with jumpers to allow upto 4 modules in chain.
  3. Xadow MultiSerial (SC16IS752) - Which adds 2 UART's (theres is not enough space for the GPIO's)
  4. Xadow IO (PCA0539) - Which adds upto 16 GPIO ports using I2C
I have tried various suppliers to gather all the components, various PCB manufacturers as well as solder stencils from OSHStencils. Here is the breakdown of my experience so far in prototyping some very small single boards using SMD only components.
Chain of Xadow modules - GPS, Oled, 9-DOF IMU, Barometer, Dual-I2C Uart (Bluetooth), Xadow-M0, Xadow-SD (Left to Right)

PCB Manufacture:

DirtyPCB - They use SeeedStudio for manufacture and fulfilment, but get bulk discounts and I love the Gerber preview option, no extra charge black PCB's and option to get extra PCB's in a batch. I have got gold finished PCB's from them as well these look rather good.

SeeedStudio - My first test PCB's were made here.They also provide an assembly service and a library of commonly used parts. In most cases I have managed to get 80% of the PCB assembled in China with 20% hand soldered at home, mainly crystals and main IC's.

OSHPark - This is the made in the USA solution to Hobbyist PCB's. They produce PCB's in a signature purple colour, which I guess is a soldermask colour no serious industrial PCB maker wants. The ENIG finish PCB's look rather good. The preview function is also handy. However the DRU for checking PCB's is a bit strict in terms of clearance at the board edges, vias are not automatically tented and signals require greater separation leading to a low density PCB overall.

I am yet to try PCBPool and local Australian and New Zealand options, but they are beyond the budget of hobbyists. Once I start manufacturing my designs I shall surely give them a go. Local hackerspace recommended #hackvana and I had a great chat with them as well.

Component Sourcing:

AliExpress - Components are a bit hit and miss, had the wrong ones sent at one point. All the ones tested work.

EBay - Probably same sellers as AliExpress. Similar performance, no bad shipments so far.

Element14 - Next day delivery is amazing for quick prototyping. The part search system has improved a lot and they even have an Eagle library for most of the common parts.

Samples from TI, Maxim and ON Semi - Nothing is as good as free stuff delivered express. All parts are detailed with CAD models (albeit in .bxl).


OSHStencil - The holes are a bit too big, probably because I did not shrink the solder mask enough. Great otherwise.
Homecut - Cut on a Trotec laser Speedy 100 from Mylar and Kapton. The laser has a bi
t too much power, so pads need to be shrunk even more.

Overall it has been a great learning experience and I have probably spent more on research than I would have spent buying a hotshoe IMU like the solmeta off the shelf. However in the end the product does exactly what I would like it to do. Next step - find a suitable 3D printer on 3D Hubs and get an enclosure built, also complete the MBed code to make it all work.

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