PCBs

A consignment of PCBs arrived from ExpressPCB.com this morning.  I've chosen to use their MiniBoard Pro service so the consignment consists of 3 production PCBs.  All 3 are the same; the middle one looks different because it has been turned over.  Each production PCB contains 12 SmartSPR PCBs that must be separated by hacksaw before assembly of the other components.

The 12 SmartSPR PCBs are not all the same.  Two of them (lower right of rightmost and leftmost PCBs) are for use with through-hole soldered MOSFET devices.  The other ten PCBs are for use with surface mounted MOSFET devices and one of them (top left) includes provision for an in-circuit serial programming (ICSP) header to facilitate programming of ATtiny MCUs with Arduino sketches.

One of each PCB type has been separated by hacksaw, ready for assembly and testing to prove that the PCB layouts are correct.

The surface mount MOSFETs are not so difficult to solder as you might suppose but it's a good idea to solder them first, before the PCB becomes cluttered with other components.  One technique is to:

1. Place a little (very little) solder on the source (GND) pad of the PCB.
2. Hold the MOSFET carefully in position while you tack it in place with the soldering iron.
3. Apply heat to the exposed edge of the MOSFET being careful not to damage the plastic case while melting some solder that will be drawn under the IC's edge.
4. Solder the gate pin to its pad.
5. Inspect the tacked source pin and complete the solder joint if required.

Both of the above units have surface mounted MOSFETs, two different types as it happens so that both types can later be subjected to field trials.  The ICSP unit on the right includes an indicator LED to assist in debugging of new sketches.

Later, the printed circuit assemblies (PCSs) have been bench tested to verify the PCB layouts and basic operability and they are now ready for field testing to show that the components, especially the power MOSFETs are able to tolerate the sort of stresses, particularly heat that will be encountered when they are called upon to do real work.  These 3 units used a 3-wire design that proved to be unsatisfactory in field testing and has now been superseded by the present 4-wire design.