Partial modules - what the hell are they?!

As you may be aware, I have recently started using a PCB assembly service for my 'high turnover' modules such as sequencers and envelope generators. The reasoning behind this is basically 'time is money' - by integrating the fabrication and assembly processes not only do I save a whole bunch of time (building an 8-step sequencer is a time-consuming process, largely due to the presence of 11 SOD-323 diodes) but I can also take advantage of the price breaks that the fabricators get for buying components at far larger volumes than I'll ever use.

This means I can reduce prices whilst preserving my margins - as an example, I can now take a fiver off the cost of an 8-step sequencer whilst retaining the same margins as before.

That's quite a saving - the savings on some other modules are even larger.

Have I mentioned that PCBs are cheaper to ship than pre-built modules? They certainly are - I can send a bunch of PCBs and panels in a board-lined Jiffy bag as a large letter, whereas larger modules require 'small parcel' sized boxes.

A partial module is exactly what it says it is: a part-build module. In this particular case it means that all of the surface-mount components have been pre-soldered during the fabrication process, leaving only any through-hole components (jacks, pots etc.) to be fitted.

Some people like to do this themselves, so I will be selling certain modules as 'partial' builds, whereby it is up to the customer to fit whatever through-hole components they see fit. This means I can reduce prices further because I don't have to source additional components or account for the time spent assembling them. Everyone wins.

A quick guide to the sort of stuff you're likely to need:

  • Potentiometers: Alpha RK097, or anything with a compatible footprint. You'll also require appropriate knobs. If trimmer pots are involved, the footprint will be for a Bourns 3296W or equivalent.
  • Jacks: PJ301M-12 (perhaps better known as the THONKICONN jack) - some modules may call for a stereo jack, in which case the stereo jack footprint will be clearly indicated on the board silkscreen.
  • LEDs: any 3mm LED will work - red LEDs are generally better because they have a lower voltage drop, although in a lot of cases the LEDs are buffered anyway. The short leg of the LED should go through the square pad (generally GND). My LED of choice is the Vishay TLHR4000, which has a forward voltage drop of 2V.
  • Switches: be careful here - I use Dailywell DW-series switches which have solder pins at 2.54mm centres. More often than not, I use ON-ON switches, with the centre pin being common.
  • Capacitors: where through-hole electrolytic capacitors are required, take care to get the polarity right (there'll be a small + on the silkscreen for the positive lead). Ceramic and film capacitors are not polarised.
  • Diodes: same rules as electrolytic caps - the cathode band on the diode should correspond with the circle or stripe on the silkscreen. Usually, the 1N4148 will do the trick. If the module requires Schottky diodes then I generally go for the 1N5819 (note that if I've specced a Schottky diode then you can replace it with a 1N4148 or similar, but there'll be a larger voltage drop across the diode).

Some modules require transistors and/or voltage regulators in a TO-92 package - take care not to get these mixed up! You won't need anything exotic - for NPN transistors, the 2N3904/2N2222 will do the trick, whilst for PNP you can use the 2N3906. All of these are cheap and easy to get hold of. I rarely, if ever, use MOSFETs.

Voltage regulators will generally be 78L05 or equivalent (for modules which require a 5V line, such as anything involving a microcontroller), whereas the sequencers will require a 78L08 (8 volt) regulator

Does this mean you're no longer hand-building modules?

Nope.

For high-turnover stuff it's more efficient, in terms of both time and cost, to turn the main assembly over to the fabricators (see above regarding economies of scale, etc.). I will still be hand-building short-run modules and custom orders, not to mention some 2HP modules which aren't amenable to the assembly process on account of their size - most fabs put a lower limit on the size of PCBs they can handle and 2HP modules generally creep in just under this. Annoying, but there you go.

Whilst the bulk of the assembly will be done by the fabricators, final assembly and testing will still be done 'in-house' by me. Plus I'm still reponsible for all of the design work.

The easiest way to tell if you've got a hand-built module is to look at the components - if it's mostly through-hole then chances are I built it unless you bought a PCB and did it yourself. Hand-built SMD modules will use 0805 passives throughout, whereas fabricator-assembled modules will use 0603 or a mixture of 0603 and 0805 components.

This means that fabricator-assembled modules are still repairable, even though they use smaller components, and the same conditions apply vis-a-vis repairs. Geezer eyesight means I can't solder 0402 and smaller components reliably so these are only used in exceptional circumstances. Ditto for things like TQFN chips and whatnot.