Mechanical constraints

Status: envelope CONFIRMED by user. Board outline recommended below, needs your sign-off before layout starts.

1. Width budget — where the 100 mm goes

This is the calculation that sets your board size. Two options, and they differ by 10 mm of board width.

Option A — insulate on the OUTSIDE (recommended)

XPS foam is wrapped around the outside of the box. The 100 × 100 mm figure then refers to the structure, and the finished payload is ~150 × 150 mm with 30 mm foam. Check whether your mechanical lead intends 100 mm to be the structure or the finished exterior — it changes everything below.

Option B — insulate on the INSIDE

70 × 70 mm is tight for this component count. It is doable but the floorplan gets cramped, the RF and memory zones lose their separation, and routing gets harder.

Recommendation: Option A. Insulating outside costs nothing and buys you 20 mm of board width, which is the difference between a comfortable layout and a difficult one.

2. Recommended board outline

Board:          90.0 × 90.0 mm, rectangular, 2 mm corner radius
Mounting holes: 4 × Ø3.2 mm (M3 clearance)
Hole positions: 80 × 80 mm square, i.e. 5 mm in from each edge
Keep-out:       6 mm radius around each hole (standoff footprint)
Thickness:      1.6 mm

Alternative if you want CubeSat stackability: the PC/104 form factor used across the CubeSat industry is 90.17 × 95.89 mm with holes on an 82.55 × 88.90 mm pattern. Adopting it lets you buy off-the-shelf CubeSat standoffs and stack future boards. Slightly more awkward numbers, real long-term benefit. Your call.

3. Height budget — the 300 mm

The tallest single item is the SBM-20 Geiger tube at 108 mm long, which does not fit horizontally in a 94 mm internal width. It must be mounted vertically.

Below the payload, hanging free: the 868 MHz half-wave dipole (16.4 cm total) and the PT1000 probe on its 300 mm lead.

Placement rules that come from this layout

• MS5611 vent path must reach outside air, be shaded, and be away from the SPS30 fan outlet. A small labyrinth vent on a side face works.
• 868 MHz antenna and GNSS patch at opposite ends. A 20 dBm transmitter at 868 MHz beside a receiver listening at 1575 MHz is the classic self-jamming failure.
• Geiger tube far from the battery and the radios — not for RF reasons but because it is the one high-voltage item and it should be reachable for inspection without disturbing anything else.
• IMU as close to the payload's mechanical centre as possible. Off-centre mounting turns rotation into apparent linear acceleration. Note the offset in firmware if you cannot centre it.

4. Mass budget

Under 1 kg. Comfortably inside the common regulatory threshold for unregulated balloon payloads (often 4 kg), but confirm the Uzbek requirement with the same authority handling your frequency approval — a balloon payload is an aviation matter as well as a spectrum one.

5. Thermal design

With ~1 W of dissipation inside a box insulated with 20–30 mm of XPS, the interior typically sits 30–50 °C above ambient. At a −60 °C ambient the electronics likely see −10 to −25 °C, not −60 °C.

Design consequences:

• Design the board for −25 °C operation with margin, not for −60 °C.
• Design the external PT1000 probe and the battery for the real −60 °C.
• Measure internal against external temperature and log both. Validating this prediction is itself a small, clean engineering result worth a graph.
• Do not add a heater in v1. Add the two temperature sensors, fly, and decide from data. A heater adds current, complexity and a failure mode for a problem you have not yet confirmed exists.

6. Still needs your confirmation

1. Does 100 × 100 mm mean the structural box, or the finished insulated exterior?
2. Is the structure a tube or a square box, and what material and wall thickness?
3. Is the payload rigid-mounted to the balloon train, or does it hang on a swivel? (Affects spin rate and therefore the M6 antenna-pointing analysis.)
4. Where does the parachute attach, and does its hard point pass through or beside the board stack?