Saturday, November 12, 2016

Brewing Up a Storm

This post was originally written in early July - during the design phase of myStorm. Unfortunately this and other posts were accidently deleted, but have now been partially recovered.

From now this blog will continue on my Wordpress site

This week we saw the arrival of the first batch of myStorm pcbs from Toby in Shenzhen.  The gerbers were sent on Thursday 21st July and pcbs  - plus solder stencil arrived the following Thursday lunchtime - quick work.

Meanwhile Alan and I had been out shopping for the BOM parts - with enough components to build up a small prototype batch of about 5 boards.

We got together on Friday evening at the Surrey and Hants Hackspace, and managed to spend a couple of hours painstakingly placing the 80 or so 0402 parts onto the freshly screen-stenciled - solder pasted sample pcb.

In fading light we retired with more than half the SMT components placed, with a view to completing the task in the fresh morning light - when both our eyes and hand co-ordination would benefit from a good night's sleep.

The placement continued for another 2 hours in the morning and by 11:30 we had a fully placed board. After some basic checks for IC alignment - we turned on Alan's home made hotplate - an aluminium block, with a cartridge heater and a cheap temperature controller. Setting the temperature to about 220C - within 5 minutes the board was starting t show positive signs of solder paste reflow.

The pasting operation had been not 100% successful,  - the definition of the solder pads around the pins was far from perfect - but with the application of heat - and the miracles of surface tensio - the solder flowed beautifully - leaving perfectly soldered pins and 0402 components.

A quick bit of remedial work to fix any "tombstoned" components, I was onto the last leg of this demanding race - and soldering the PMOD and Arduino connectors into the remaining throough-hole locations on the board.

Initial Tests

Check, check and check again - then correct the things you missed.

First we checked for supply to ground shorts - and the 3V3 was connected to 0V  - because a single 0402 100nF decoupling capacitor had moved sideways across its pads and was shorting across them. Easily fixed - but not so easy to spot  in the first place. A few solder bridges between pins on the 64 pin LQFP ARM package - again easily fixed with the soldering iron and a dab of water-solvent based SMT  flux.

Then it was time to apply the 5V power and test the power rails. All appeared well with 1.2V for the Vcore and 3V3 as the digital supply to the ARM and the FPGA.

On first power-up some of the LEDs glowed dimly - and the power light glowed yellow.

Next we had to find a way of getting some life into the ARM chip - an STM32F103RC - the same as used on the Nucleo dev board. I opted to get "Blinky" running - using mBed to program the board.

Downloaded the ST Link Utility - which included the latest drivers and the programming application.  Once this was down loaded it was relatively easy to use the blink example, change the port Pin to PA_15 and flash the LED!

A first success!

Tomorrow - there are 2 more prototype pcbs to populate and bake.  Once you have done the first one - fixed the snags - the rest is easy.

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