Notey starts to become real...

I had always intended to do two revisions.... a prototype and a production one.  This is because the probability of no mistakes seemed pretty low.

The schematic for the prototype v0.1 board needed to be done very quickly as there was a very limited amount of time.  I made use of application notes to decide how to lay out the USB power switchover and support circuitry, but had some snap decisions to make with regards to minimising development effort...

Bluetooth compromise

As previously discussed, I wanted to use the nRF8001 for the high level of simplicity... and had originally intended to do my own implementation - it does not require that many support components, and there is at least one balun specifically tuned to the requirements of the nRF8001 to give best RF performance (a lot safer than using a bunch of individual passives, and probably better), plus a chip antenna.  However, once again, time constraints were against me.  Doing my own BTLE solution was introducing another potential thing to go wrong, even if there were some good reasons to do it (higher performance, lower cost).... they were a number of suppliers of pre-tested nRF8001 modules using a PCB antenna and all the soldering and passives already done.

I had originally tested with an Adafruit module but for production decided to go with Olimex as they offered a quantity discount.  There was another advantage to using an off the shelf BTLE module... it meant the main PCB could be made much smaller, which reduced cost to somewhat offset the module expense.  From looking at the Olimex board, the best way to connect to another PCB was through the double sided test header pads.  The two boards are connected through a simple 2.54" header... a small routing indent in the PCB was made for the plastic connector spacing to sit, so that the profile was not made any bigger than necessary.

The Olimex BTLE module

Prototype Schematic and PCB Design

I did the board in Target 3001 as I usually do... this was my first design with the new V17 - a few things are better but it's still as quirky as ever and not exactly the most stable piece of software I've encountered, but it is my favourite of all the ones I've tried.

For the first attempt at a schematic, I definitely erred on the safe side when it came to most design decisions, but for the PCB I did need to make the board as small as possible to keep the cost down, and also limited to double sided copper.

v0.1 Schematic

v0.1 PCB Layout

The PCB layout was a bit of a challenge... Anna's wedding ring was difficult, but this was at least as dense and producing quite a few of them!  While 0402 components are small, they have the disadvantage that you cannot run traces between the pads very practically, so you don't save as much as you might think.

I normally try to avoid 0402 capacitors, but to keep the board to a 40x40mm size, there was no option to just use 0603... the EPD alone requires 16 support capacitors and numerous discrete transistors.which makes the design quite tight.  One good thing about 0402 is that it makes it easier to put the caps exactly where they are needed, which simplifies the layout... on a small board, the last thing you want is to need multiple vias just to route a decoupling cap... while most of the support caps do not require super low inductance as are not for high frequency use, it is still good practice to try and minimise it.

While I could use both sides of the board for components, it didn't seem practical to reflow the board on both sides (the risk of components falling off on the second reflow unless you use a solder paste with a different melting temperature), so the display side would all be hand-soldered.

Rendering of (mostly!) completed v0.1 board

So I sent off the board files for a super-quick turnaround and got them back the same week...

Prototype PCBs

PCB with the nRF8001 for size/location reference

So onto aligning the stencil... this is a real pain to do when you have multiple parts with 0.5mm pitch, particularly on the QFP as it has to be right in both directions.  When I was happy it got stuck down with kapton tape using some other 1.6mm thick PCBs as spacers.

Stencil alignment

Applying the paste was tricky.  It helps to stir the solder paste to warm it up a bit as then it flows a bit better.

Applying solder paste

The end result of the squeegee action was a bit rough and ready.  I wouldn't quite call it a "high definition" result but it was a first attempt after all.  Made me think quite a few solder bridges were likely...

PCB with paste applied

Well, I'd find out soon enough.  Time to place some components - I used my tweezers to carefully start putting all the major components into position.

Just Enough Essential Parts?

And into the reflow oven it goes... now it's a waiting game...

Low temperature reflow profile in progress

A few minutes later, the oven beeped and I opened the drawer to have a look..

Dodgy reflow...

Ah.  Not quite the outstanding success I was hoping for.  A few of the SOT-23 transistors have reflowed, but almost nothing else.  Gah.  Okay, so the low-temp profile clearly isn't hot enough, either due to the oven temp sensor being off or due to the paste having different characteristics.  There didn't seem to be any harm in giving the old-school leaded solder profile a try just to get something up and running - the temp is still much lower than lead free and should put less stress on the components.

Dodgy reflow of a different kind

Well... it's definitely reflowed, there's no question of that.  Unfortunately my fears regarding the paste and the tight pitch spacing was very much confirmed - lots and lots of solder bridges!  This had to be sorted out by hand with some flux and some desoldering wick.  The profile and footprints will definitely have an influence on this, but it could be that the consistency of the paste is not well suited to such fine work.  In any case, I needed to try and get a prototype board working...