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Part Sizes for Hand Assembly

Scadys prototypes are placed by hand under a stereo microscope, printed through a stencil and reflowed on a hot plate. The package sizes on the board decide how long that takes, how many joints need rework and whether a fault can be found with a meter probe.

Passives: 0603 unless space forces 0402​

  • 0603 is the rule. An 0603 part is quicker to place by hand and easier to inspect. It is also large enough to probe at its pads during fault-finding.
  • 0402 is used only where space forces it, such as the fan-out around a processor, where an 0603 decoupling capacitor cannot sit close enough to its pin. Where 0402 is used, it goes on the audited Scadys land patterns RESISTOR:RES-0402 and CAPACITOR:CAP-0402.
  • Nothing smaller than 0402 is used for a passive.

0402 parts can be hand-placed, and the MDD500 carries many of them, but they are slow to place, hard to inspect and hard to probe. The MDD500 build is the reason the rule is 0603.

Packages that are excluded​

  • Leadless packages under 2 mm, such as X2SON, DSBGA and any leadless part at 0.4 mm pitch, are excluded. A regulator offered only in X2SON is replaced by a SOT-23-5 alternative.
  • QFN and DFN packages are accepted at a body of 2 mm or larger and a pitch of 0.5 mm or coarser. A finer-pitch processor, such as the 0.35 mm pitch QFN104 of the ESP32-P4, is carried only where no alternative exists, and its assembly is planned for specifically.

Small dual transistors in SOT-363 packages are accepted. They bridged on earlier boards because their land pattern merged the solder-mask openings between pins, and with a mask dam between every pair of pads they assemble cleanly.

Paste printing​

The paste print decides most of the solder faults on a hand-built board. Paste is spread across the stencil with a thick coated paper card, such as a business card, held vertical and swiped horizontally with light pressure. A stiff plastic card pressed down onto the stencil forces paste under it and around the apertures, and that bridges fine-pitch pins. On the first MDD500 board, the top face was printed with a pressed plastic card and its 0.35 mm pitch processor bridged in many places. The bottom face was printed with a paper card, and its 40-pin connector and two small sensors reflowed correctly first time.

A hand-placed board also takes hours, and paste slumps as it waits. A fine-pitch part is placed early, and the time from print to reflow is kept short.

Orientation of small polarised parts​

A small polarised part, such as a SOD-523 ESD diode, carries a single mark whose meaning differs between parts. On the TI ESD321, the mark identifies pin 1, which is ground, while habit from rectifier diodes reads a mark as the cathode. On the first MDD500 board the orientation of the two USB ESD diodes was argued from the datasheet during placement, and both were fitted backwards. A meter in diode mode across the part would have settled it in a minute.

Wherever a part's orientation is in doubt, the part is measured with a meter in diode mode before it is placed. A footprint for a part whose mark does not mean "cathode" also carries an explicit pin-1 or ground marking in silkscreen, and the bring-up sheet checks every polarised part's orientation before first power.