4.4 Mechanical Assembly WI: Torque, Sealants, TIM and ESD Controls

The Box Build process is where "tribal knowledge" destroys consistency. If the assembly sequence relies on an operator remembering which screw goes where, or how much glue to apply, the product will vary with every shift change. Create a single, unified Work Instruction (WI) that governs the mechanical integration as a precision process, not a craft project.

The Master Work Instruction Structure

Do not fragment instructions into separate emails or loose PDFs. The WI must be a sequential document (Digital or Hardcopy) at the workstation.

Required Elements per Step:

1. Step ID & Action: "Step 10: Install Main Board."
2. Part Numbers: List specific PNs required for this step.
3. Tooling: Specific driver bit (e.g., Torx T10), fixture ID, or dispensing tip.
4. Critical Parameter: Torque setting, cure time, or alignment tolerance.
5. Visual Standard: A photo or diagram showing the correct result and common errors.

Torque Control: The Torque Map

A screw is a clamp, not just a fastener. Inconsistent torque leads to loose rattles or stripped threads.

1. The Torque Table

Create a central look-up table in the GDP and reference it in the WI.

• Define Units: Standardize on Nm or kgf-cm. Do not mix units.
• Define Tolerance: e.g., 4.0 kgf-cm ± 10%.
• Differentiate Materials:
  • Metal-to-Metal: Higher torque, abrupt stop.
  • Plastic (Self-Tapping): Lower torque, requires specific RPM to prevent melting the boss.
  • Pro-Tip: For plastic bosses, specify "Reverse-to-Thread" logic. The driver must rotate CCW until it clicks into the existing thread, then CW. This prevents cutting a new thread and destroying the boss during rework.

2. Verification

• Golden Tool: Calibrate electric drivers daily.
• Witness Marks: For critical safety screws (e.g., AC input), apply a tamper-evident lacquer (Torque Seal) across the screw head and chassis after torque verification.

Chemical Processes: Sealants and Adhesives

"Glue" is an engineering process. Uncontrolled application causes leaks (too little) or internal contamination (too much).

• Dispensing Spec: Define the bead path and volume.
  • Bad: "Apply glue to rim."
  • Good: "Apply continuous 1.5 mm bead along groove center. Total wet weight: 0.2g ± 0.05g."
• Cure Time Constraints:
  • Open Time: How long can the glue sit before parts must be mated? (e.g., < 5 mins).
  • Handling Time: How long before the unit can be moved to the next station?
  • Full Cure: How long before the unit can be pressure tested?

Thermal Interface Material (TIM) Hygiene

Thermal paste (Grease) and Pads are functional components. Poor application is a leading cause of field reliability failure (overheating).

1. TIM Pads

• Liner Removal: Explicitly show the removal of protective liners from both sides (if applicable). Blue/Pink liners are often left on by accident.
• Placement: Define alignment references. A pad hanging off the heatsink edge does not cool the chip.

2. Thermal Grease

• Pattern: Define the dispense pattern (e.g., "5-dot die pattern" or "X-pattern"). Do not allow manual spreading unless using a stencil.
• Volume Control:
  • Method A (Stencil): Use a predefined stainless stencil to print grease on the heatsink. This guarantees volume.
  • Method B (Weight): Weigh cartridge before and after (or weigh the part).
• Inspection: Define "Squeeze-out." A small visible line of grease around the chip edge confirms coverage. No grease visible = voids likely.

ESD Controls in Mixed Environments

Mechanical assembly areas are often ESD "dead zones" because operators handle plastic housings and metal screws. However, they are also handling the PCBA. This is where latent ESD damage occurs—the board isn't powered, so the failure isn't found until System Test.

Mandatory Rules:

• The "Touch Rule": Operators must be grounded (wrist strap or heel strap + conductive floor) before touching the PCBA bag/tray.
• Ionization: If peeling protective films (e.g., from LCDs or glossy plastics), huge static charges (up to 20kV) are generated.
  • Requirement: An ionizing fan must be aimed at the peeling operation to neutralize the charge immediately.
• Tools: All electric drivers and hand tools must be ESD-safe (dissipative plastics).

Rework and Recovery Rules

Define failure paths so operators don't improvise.

• Stripped Screw:
  • Action: Stop. Do not drill out. Tag unit as "Mechanical Reject." Replace chassis if boss is damaged.
• Smeared TIM:
  • Action: Clean with Isopropyl Alcohol ( > 99% ) and lint-free cloth. Re-apply fresh TIM. Do not "scoop" old grease back onto the chip.
• Dropped PCBA:
  • Action: Scrapped immediately. Micro-fractures in MLCC capacitors are invisible and will fail later.

Final Checklist

Check	Criterion	Critical Threshold / Requirement
Torque	Values Defined?	Torque Map exists for every screw location + Tolerance ( ± % ).
Drivers	Calibration?	Daily verification log for electric screwdrivers.
Chemicals	Quantity/Time?	Dispense weight/volume and Open Time limits specified.
TIM	Verification?	Stencil used or weight check required. Liner removal verification step.
ESD	Mixed Area Control?	Ionizers present at peeling stations; Wrist straps mandatory.
Safety	Witness Marks?	Torque seal required on AC/High-Voltage fasteners.
Rework	"Dropped Board" Rule?	Explicit "Scrap if Dropped" policy in place.