Mastering Product Design Details: Screw Bosses, Seams, Snap-Fits & More

Designing great products hinges on nailing the smallest details. After all, it’s screw bosses, seams, and snap-fits that determine whether a device rattles, cracks, or withstands daily abuse. Below are battle-tested guidelines from the trenches of TONGDA LINK product design.

Purpose: Fix shells + position PCBs.
Key Rules for Self-Tapping Screws:
Outer diameter = 2× screw diameter (e.g., 2× M1.6 screw Ø).
Inner diameter:
ABS/ABS+PC: Screw Ø – 0.40mm
PC: Screw Ø – 0.30mm (test & adjust mold if needed)
Shell gap: 0.05mm between mating surfaces.

Below table shows the dimensional relationship between the M1.6×0.35 self-screw and the stud.

The design values for screw holes in different materials and screws are presented in Table as below.

Role of the Seam

1. Environmental Barrier: Creates an indirect path between the shell’s interior and the external environment, effectively blocking the ingress of dust and static electricity.

2. Positioning & Limiting: Provides alignment and constrains the relative movement between the upper and lower shells during assembly and use.

Shell Mouth Design Considerations

• Draft Angle: The mating surfaces must incorporate a demoulding draft angle > 3°–5°.

• Edge Treatment: Terminating edges should be chamfered or rounded to facilitate assembly.

• Corner Radii (R角) Matching: For fitting upper and lower shell corners:

  • Ensure the inner corner radius (R角) is larger than the corresponding outer radius.

  • This increases the gap between the mating rounded corners, preventing interference.

• Seam Direction: Position the seam towards the interior at the end with the stronger side wall to better resist external forces.

• Seam Dimensions:

  • Outer seam flange thickness: 0.8mm

  • Inner seam flange thickness: 0.5mm

  • Gap B1: 0.075–0.10mm

  • Gap B2: 0.20mm

• “Art Line” Design: Dimension: 0.50×0.50mm. Inclusion of an “art line” (decorative parting line) is optional and dependent on specific design requirements.

Decorative Part Design

Design Precautions

• Large Surface Adhesion (>400mm²):

  • Adhesive bond width ≥2mm

  • Assembly requires fixtured pressing >3kgf for ≥5 seconds

• Large Exterior Surfaces (>400mm²):

  • Use alternative processes: aluminum, spray-painted plastic, or stainless steel

  • Electroforming prohibited: Limited to small areas (<400mm²) due to:
    • Poor flatness at scale
    • Inadequate wear resistance

• ESD Protection for Electroplated Parts:

  • Eliminate through-holes in assembly grooves when <10mm from internal electronics

  • For snap-fit designs requiring through-holes:
    • Apply shielding film over clips (non-electroplated surfaces)

• Dual-Side Decorative Parts:

  • Ensure direct contact between shell ribs and decorative part interior

  • Critical: Decorative parts must not provide structural support

• Universal ESD Consideration:

  • Evaluate ESD risks during initial design phase

• Small Electroplated Parts (Ø<5.0mm):

  • Use double-sided tape or back-loading

  • Avoid snap-fit designs

Electroplating for Decorative Parts

Electroplated Edge Angles

• For mirror-polished bevels:

  Critical Angle Requirement: a >45° (Refer to below image)

• Rationale: Angles ≤45° appear black with poor reflectivity

• Dimension B follows ID specifications

Electroplated Plastic Part Design

Layer Structure Reference as per the Image.

1. Coating Thickness:

   • Nominal: 0.02mm (20μm)

   • Production tolerance: ≤0.08mm (80μm)

   • Design Note: Specify minimum thickness; add range if critical

2. Blind Hole Specifications:

• Depth ≤ 0.5 × Diameter

• No color matching requirements for hole bottoms

3. Wall Thickness Guidelines:

• Optimal: 1.5–4.0mm

• Thin walls (<1.5mm) require reinforcement to control plating deformation

4. Surface Quality Requirement:

• Injection molding defects must be eliminated

• Electroplating amplifies surface imperfections

5. Material Selection:

• Preferred: ABS substrate

• Advantages: Superior plating adhesion, cost efficiency

Button Size and Spacing Requirements
Based on ergonomic analysis and operational requirements, button spacing must prevent accidental activation during use:

Vertical spacing: Center-to-center distance a≥ 9.0 mm

Horizontal spacing: Center-to-center distance b≥ 13.0 mm

Minimum function key size: 3.0 × 3.0 mm

Button-to-Substrate Clearance Design

• Headphone Port Interface:

  Parameter	Specification
  Host shell opening	Socket width + 0.3 mm per side
  Plug-to-shell clearance	0.05 mm per side

• Plug Retention Design:
  • “+” Rib Configuration (cross-shaped retention features):

  • Rib width: 0.8 mm

  • Socket depth: 2.0 mm

  • Radial interference: 0.05 mm (outer contour to jack perimeter)

  • Edge treatment: R0.3 fillet on rib apex (eases insertion)

Critical Design Requirements

1. Active Area Border Offset:

   • Maintain 1.50–2.00 mm perimeter clearance between plastic cover window edge and touch screen active area

   • Standard practice: 1.50 mm (Reference: Figures 15-1, 15-2)

2. Dimensional Tolerance Management:

   During 3D modeling, define the plastic cover cavity dimensions using the touch screen’s maximum material condition (MMC). Maintain a radial clearance of 0.15 mm per side from the MMC envelope.

3. Deformation Prevention System:

   Component Interface	Shock Absorption Material	Thickness Requirements
   Touch Screen ↔ Plastic Cover	EVA Foam (elasticity: 40-60 Shore 00)	Uncompressed: 0.50–1.00 mm → Compressed: 0.30–0.50 mm
   Touch Screen ↔ TFT Screen	EVA Foam (identical properties)	Same compression range