Design Log DEC 057 daughterboard mounting hole specification and standoff bom ownership - thepinkmile/Enigma-NG GitHub Wiki
DEC-057 - Daughterboard Mounting Hole Specification and Standoff BOM Ownership
Date: 2026-05-05 Status: Accepted Amends: DEC-039, DEC-023
Decision
This is the single authoritative reference for mounting hole specification and standoff BOM ownership across all boards in the Enigma-NG system.
1. Daughterboard Classification
Only the following boards are classified as daughterboards (host-mounted via standoffs onto a carrier board):
| Board | Carrier Board(s) | Attachment Method |
|---|---|---|
| Actuation Module (AM) | CTL, EXT | M2.5×3.5mm SMT standoffs (MH5-MH8 on each carrier) |
| JTAG Daughterboard (JDB) | CTL | M2.5 standoffs (MH13-MH16 on CTL - to be created at CTL schematic capture) |
All other boards (PM, CTL, STA, REF, EXT, ROT, USM, ENC) are chassis-mounted boards and are not daughterboards. The Encoder Module (ENC) is explicitly not a daughterboard; it uses standard M3 chassis mounting per GRS §4 and DEC-039.
2. Mounting Hole Specification by Board Category
Daughterboards (AM, JDB):
- Mounting holes: M2.5 NPTH, pads connected to local
GNDnet (notGND_CHASSIS). - Rationale: Daughterboards share their ground reference with the carrier via the board-to-board
connector. Tying mounting holes to
GND_CHASSISwould create an unintended second galvanic chassis-bond point, violating the single-point chassis ground topology defined in DEC-039.
Chassis-mounted boards (all others):
- Mounting holes: M3 PTH, pads connected to
GND_CHASSISperdesign/Standards/Global_Routing_Spec.md §4and DEC-039.
CTL special case (carries both categories):
| Holes | Size / Type | Net | Purpose |
|---|---|---|---|
| MH1-MH4 | M2.5 4mm standoffs | GND |
CM5 SoM mounting |
| MH5-MH8 | M2.5 3.5mm SMT standoffs | GND |
AM daughterboard dock |
| MH9-MH12 | M3 PTH | GND_CHASSIS |
Chassis mounting |
| MH13-MH16 | M2.5 | GND |
JDB daughterboard dock |
JDB dock standoffs on CTL are assigned RefDes MH13-MH16 - to be created at CTL schematic capture stage.
3. Standoff BOM Ownership
The carrier (host) board is solely responsible for specifying and sourcing the standoffs used to attach a daughterboard. The daughterboard itself shall not contain standoff BOM entries for its own attachment points.
| Carrier Board | Standoff Entries Owned | Qty per Board |
|---|---|---|
| CTL | AM dock standoffs (MH5-MH8, 9774035151R, M2.5×3.5mm SMT) | 4 |
| CTL | JDB dock standoffs (MH13-MH16, M2.5 - to be created at CTL schematic capture) | 4 |
| EXT | AM dock standoffs (MH5-MH8, 9774035151R, M2.5×3.5mm SMT) | 4 |
The AM and JDB Design_Specs shall each include an explicit note that their mounting standoffs are defined and sourced on their respective carrier board(s), with a cross-reference to those carrier boards' Design_Specs.
4. Summary Table - All Boards
| Board | Category | Hole Size | Net | Standoffs in BOM? | Carrier |
|---|---|---|---|---|---|
| PM | Chassis | M3 PTH | GND_CHASSIS |
N/A | - |
| CTL MH1-4 | SoM dock | M2.5 | GND |
Yes (CM5 screws) | - |
| CTL MH5-8 | Daughterboard dock | M2.5 SMT | GND |
Yes (AM standoffs) | - |
| CTL MH9-12 | Chassis | M3 PTH | GND_CHASSIS |
N/A | - |
| CTL (JDB dock) | Daughterboard dock | M2.5 | GND |
Yes (JDB standoffs, MH13-MH16) | - |
| STA | Chassis | M3 PTH | GND_CHASSIS |
N/A | - |
| REF | Chassis | M3 PTH | GND_CHASSIS |
N/A | - |
| EXT MH1-4 | Chassis | M3 PTH | GND_CHASSIS |
N/A | - |
| EXT MH5-8 | Daughterboard dock | M2.5 SMT | GND |
Yes (AM standoffs) | - |
| ROT | Chassis | M3 PTH | GND_CHASSIS |
N/A | - |
| USM | Chassis | M3 PTH | GND_CHASSIS |
N/A | - |
| ENC | Chassis | M3 PTH | GND_CHASSIS |
N/A | - |
| AM | Daughterboard | M2.5 NPTH | GND |
No - on CTL and EXT | CTL, EXT |
| JDB | Daughterboard | M2.5 NPTH | GND |
No - on CTL | CTL |
Rationale
A single, fully declarative decision removes ambiguity and provides a canonical single-document reference for reviewers and future design agents.
Impact
design/Electronics/Actuation_Module/Design_Spec.md: DR-AM-03 - cite DEC-057 for both CTL and EXT as carrier boards in the standoff ownership note.design/Electronics/Extension/Design_Spec.md: DR-EXT-10 and BOM MH5-MH8 already correct; add cross-reference to this DEC-057 standoff ownership note.design/Electronics/JTAG_Daughterboard/Design_Spec.md: Mounting note - cite DEC-057 for standoff ownership rule.design/Electronics/Controller/Design_Spec.md: Confirm CTL BOM contains standoff rows for AM dock (MH5-MH8) and JDB dock (MH13-MH16) - to be verified at schematic capture stage.design/Standards/Global_Routing_Spec.md §4: No change - this DEC supplements rather than contradicts GRS; cross-reference may be added at GRS maintenance time.