ShiboSoftwareDev/linux-gameboy-advance

mechanical/ASSEMBLY_DESIGN.md

# Mechanical assembly design

This enclosure is a serviceable handheld assembly, not a cosmetic shell. The
PCB centre and mid-plane are the master datum. The component placements that
define ergonomics and external I/O stay fixed; the enclosure is designed around
those locations.

## Architecture

The rear shell contains the protected pouch, its service door, external port tunnels and four
structural screw towers. Four recessed ISO 7045 M3 x 24 screws enter from the rear, pass
through the existing PCB holes, and thread into 5.7 x 4.6 mm M3 heat-set inserts in front-shell
pillars. Nylon washers isolate the PCB and establish its height. This single
fastener stack closes the case and locates the PCB without the duplicate case
and board screw sets used by the reference project.

Four broad 0.8 x 1.8 mm internal registration tongues locate the shells before
the screws are tightened. They are placed only in checked-empty edge regions. A continuous
perimeter tongue is intentionally not used because it would cross the rigid LCD
at the top edge and several edge controls/connectors elsewhere.

The screen is an independent subassembly. Four M2.5 screws hold a carrier that
supports only the inactive LCD border. A 0.5 mm rear gasket is compressed 0.10
mm between the Z=8.20 carrier plane and the evaluated rigid panel back; a 0.5 mm
front gasket receives 0.05 mm total compression between panel and lid. The
active image, FPC fold, connector corridor and centre component field remain
open. The two bezel rails are joined through routed side load paths rather than
a non-contact centre brace, so PA12 flatness error cannot create a point load
on the display.

## Controls

- A/B/X/Y/Select/Start use individual rear-inserted, rear-flanged caps. Their
  socket ceilings touch the released switch stems; the switches provide return.
  There is no contradictory foam preload acting across an air gap. No separate
  enclosure over-travel stop is claimed: verify switch travel and cap free play
  in the physical control coupon before manufacture.
- The four directional switches are operated by one rocking D-pad. A broad
  rear flange retains the D-pad in the lid, while its central post lands on a
  bonded 1.0 mm Poron PCB pivot pad through a spherical foot. Narrow guides and
  6 mm sockets allow a 2.5-degree rock in eight directions about the actual
  sphere centre; the wide rear flange limits excessive tilt. There is no tiny
  printed snap feature or loose pivot insert.
- L/R are broad captive sliding paddles, not exposed switch stems. Each has twin
  anti-yaw rails, independent hidden retaining tabs and a blind pocket over the
  TS6695 actuator. The evaluated mechanism retains 0.30 mm guide clearance,
  a 0.25 mm actuator rest gap and 0.40 mm usable switch stroke.
- The power slider has raised guards. A split PA12 collar fits directly around
  the real 14 mm C231765 wheel. Twelve small inner ribs engage the supplier
  serrations, thirty-six shallow outer grooves provide finger traction, and a
  concentric shell relief keeps 0.40 mm radial clearance at every angle. The
  corrected, pin-aligned collar reaches about 0.97 mm past the actual side wall.
  The upper wall sector is part of the removable lid, leaving an open path for
  installing/removing the collar. There is no secondary
  axle, shell bearing, friction idler or tolerance-sensitive O-ring preload.

The printed parts target MJF/SLS PA12. The stated gaps are first-article
targets, not claims of production validation. Print a control/collar coupon
and tune compensation for the selected PA12 supplier before ordering a batch.

## Battery and power

A protected two-wire Jauch LP906090JH 1S 6000 mAh pouch sits below the PCB. The bay is
part of the rear shell; there is no separate holder pod and no exterior
battery-cable hole. A bonded Vishay NTCLE413E2104H400 100 kOhm NTC supplies the charger sense
signal. The pocket provides 1.0 mm X/Y clearance per side around
the 93.5 x 60.5 x 9.9 mm reserved envelope (supplier cycled thickness 9.7 mm). A rear foam pad and
four PCB-side isolation pads occupy the available gap; their compression force
and aging still need qualification with the pack supplier. This is not free swelling travel.
Low edge keepers limit lateral movement. A locking three-circuit
JST ZOM inline pair (ZOMP-03V-A-S and ZOMR-03VF-A-S) disconnects positive,
negative and NTC in a side bay exposed by an extension of the battery door,
not behind the pouch. Its conservative mated keep-out is 19.7 x 7.5 x 6 mm;
latch orientation/access must be confirmed on the real harness. A 10 x 0.15 mm nylon pull ribbon
wraps below the pack, with its free end in a recess in the rear foam pad.

The battery door inserts under an upper tongue and uses an integral cantilever
beam with a positive retaining hook and two parallel compliance slots. Its
perimeter-only inner tongue stays outside the pouch face. The two pack leads,
bonded NTC leads, locking disconnect and device-side harness to J_BAT have CAD
route envelopes. AWG18 factory wires reduce to AWG24 through individually
insulated TE 34130 parallel splices; they must not enter ZOM crimps directly.
Sensor extensions require insulated solder joints, not crimping the pre-tinned
AWG30 sensor leads. See `battery-harness.json` for the prototype harness BOM
and qualification gates. CAD intersection checks do not validate cut length,
bend radius, crimp strength, insulation OD or assembly access.

USB-C provides 5 V input, BQ24072T charging/power-path operation and USB2/FEL
data, with a 450–500 mA USB input limit. The running console may consume all
available USB power and draw from the battery; charge-while-playing is not assured. The
power slider only controls the TPS63020 regulator enable input.

## Audio

The manufacturer-dimensioned Same Sky CMS-2004-18SP 20 x 4.2 mm speaker rests
on a closed-cell gasket below the front-lid grille. Its terminal carrier, two contact
pads, two leads and a two-channel snap strain relief are represented in the
evaluated assembly. A broad screw-eared retainer and two M2 screws provide positive retention and acoustic
compression. A 19-hole, 2 mm grille provides 59.7 mm² of opening (19.0% of the
speaker face) with at least 1.5 mm printed webs. The front outlet is clear of
the normal hand grips and is not blocked when the handheld rests on a table. A modeled two-wire internal harness connects
the driver to J_SPK; no exterior connector opening is required.

## Service order

1. Power down, unplug USB, remove the battery door and disconnect the protected pack.
2. Remove the four recessed rear M3 screws.
3. Support the PCB while lifting the front shell; its attached volume-wall
   sector lifts away from the wheel. Do not pull on the speaker lead.
4. Disconnect J_SPK, set the front shell aside, and disconnect J_BAT if needed.
   Capture the face caps and D-pad: they are only trapped while the PCB is fitted.
5. Withdraw the PCB edge ports carefully before lifting the board clear.
   Shoulder keys are rear-loaded from inside the empty base before PCB fitting;
   their vertical retaining tabs are not inward-squeeze lateral clips.
6. Remove the four M2.5 LCD-carrier screws only when screen service is needed.

UART, I2C, SPI, reset and the boot solder jumper are internal service features.
They deliberately have no enclosure holes.

## What still requires physical validation

CAD and circuit checks cannot validate switch feel, print shrinkage, heat-set
insert pull-out, pouch swelling, charger behavior, acoustic buzz, cable dress or drop
survival. The gates listed in `enclosure-spec.json` must be completed before the
enclosure can be called production-ready.