3. Assembling the Printed Circuit Board

On first inspection there appear to be two minor problems with the assembly of the PCB:

1.  The first is the size of the components which, as I have already mentioned, are 'Surface Mount Devices' (SMDs).  I have a little experience with these so I wasn't too worried about ordering the kit BUT beginners beware!  They are a) very small (e.g. one of the resistors is just 2mm long) and b) unlike through-the-hole components SMDs are soldered directly to the board.

That's a millimetre scale!

Anyway, with a little care these are usually easy enough to solder and test so long as you have the right tools.  In my case this means:

• Fine solder wire
• Flux dispensing pen (we used the 'No Clean Flux Dispensing Pen' available from Maplin Electronics).
• 1mm de-soldering wire/solder wick
• Fine tipped, long tweezers
• Magnifying lamp (ours has a little area on the lens which is extra-powerful - very useful!)
• Jewellers eye glass
• Maplin ZD-916 soldering station (with iron set to between 300-320 degrees C)
• Damp sponge (or similar) for cleaning the tip of the soldering iron

Incidentally, I found that for building and testing the kit it was also useful to have:

• A 5-volt power supply of some description fitted with a connector capable of plugging into the power socket on the PCB (mine has a diameter of 3.45mm), centre positive.
• A multimeter or other means of measuring voltage, resistance and capacitance
• 1000 ohm resistor
• Oscilloscope - this is not essential but it is useful for testing
• Test leads (Maplin code FE23A are just about ideal)
• 24 SWG tinned copper wire
• Some dichloromethane solvent (we used 'EMA Plastic Weld') to assemble the plastic box
• Small (No. 2) paint brush to apply the solvent

2.  The second problem is that the components are so small that several of them don't have any markings on at all, or the markings that are there aren't immediately identifiable.  With the documentation there are two images which give you some clues.  This is a layout picture...

Here is a picture of the schematic for the board...

..and here is the schematic for the connectors which is very helpful:

So I decided to do a little detective work and use a process of elimination.  Here is my list of components and their associated markings (in the order that I soldered them onto the board):

• 10K (3 x chips each containing four 10k ohm resistors) -> 103
• APM4953 HT1238M -> 4953
• 470 ohm resistors -> 471
• C1 (shown as 106) -> Unmarked single capacitor measured as 8uF
• C2,3,4 (shown as 104) -> Unmarked tape of 3 capacitors measured as 93nF
• R1,5 (shown as 1k ohm) -> 102
• R2 (330 ohms) -> 33 on the schematic
• R6 (100 ohms) -> 10 on the schematic
• Q1,2,3 (transistors) -> unmarked 3-pin devices
• U5 (74HC138D) -> 74HC138
• U1,2,3,4 (TM1818) -> TM1818

Now turn up the heat on the iron by about 20 degrees

• D1,2 (SS14) -> IN5819
• USB Connector
• Power connector
• 0.1" and 2mm connectors - all fairly straightforward
• Large and small LEDs (the long positive leads go through the SQUARE hole)  The large LEDs are actually sort of illuminated legs that the PCB will stand on when it's sitting the right way up on a flat surface.  I guess that the small LEDs are simply decorative - we will find out!

The reason the multimeter is handy is because if you're not sure of the identity of a passive component it is reasonably safe to measure the resistance/capacitance of the device as a check.

The soldering looks worse because of the flux residue - honest!

So, anyway we soldered this lot up (as per the picture above) and then installed all the little stand-offs (there were two left over in the pack).

TIPS
Here are some tips from my experience with the assembly:

1. Be EXTREMELY careful to get the ICs round the right way.  Removing them and turning them around would be very difficult without the right equipment - and even then it's not easy!
2. The legs of the ICs are very close together and there's a good chance you might accidentally solder two together.  DON'T PANIC.  It's very easy to fix this.  Get some 1mm 'Desoldering Wire' - basically a wick made of copper designed to 'suck up' molten solder.  Put some flux on the braid, remove any excess solder from to soldering iron, place the braid against the legs which have been joined by solder and heat for 2 or 3 seconds with the iron.  The excess solder will melt and get absorbed by the copper.  Magic!
3. Be careful about the order in which you solder components to the board.  Generally you should start with the smaller ones and work your way up but there may be exceptions.  For example, the connector USB1 has some hard-to-get-at legs to the rear so you might want to solder diode D1 afterwards.

Having completed the assembly the only remaining problem is the amount of sticky flux left on the board.  I tried to clean this off with Maplin Flux-cleaning Spray but all it seemed to do was spread it around.  Really, I think it need to be placed in a bath of solvent and given a good go over with a toothbrush.

Okay, the board is now assembled - it's now time to plug it in and see if the whole thing goes up in a plume of smoke...