2.2 Surface Finishes and Panelization Strategy

The physical interface between the component and the PCB is defined by two factors: the planarity of the surface finish and the mechanical stability of the panel. A poor finish selection leads to wetting failures and "Black Pad" defects, while a weak panel design causes sagging in the reflow oven and component cracking during separation. This chapter transforms these choices from aesthetic preferences into throughput and reliability mandates.

Surface Finish Logic

Select the surface finish based on component density, shelf-life requirements, and cost constraints. Do not default to HASL simply because it is cheap.

Decision Matrix:

• If the design includes Fine Pitch components (≤ 0.5 mm) or BGAs, Then specify ENIG (Electroless Nickel Immersion Gold) or Immersion Silver.
  • Reasoning: HASL (Hot Air Solder Leveling) creates a domed topology that causes BGA balls to slide off pads or fine-pitch leads to bridge. ENIG provides the necessary flat surface.
• If the product is low-density, primarily Thru-Hole, and cost-sensitive, Then specify Lead-Free HASL.
  • Reasoning: It provides the longest shelf life (> 12 months) and robust solderability for larger joints.
• If using OSP (Organic Solderability Preservative), Then enforce strict humidity control and limited reflow cycles.
  • Reasoning: OSP degrades rapidly after the first reflow pass. It is not recommended for double-sided SMT with subsequent wave soldering.
• If the application is high-frequency RF or Aluminum wire bonding, Then specify ENEPIG.
  • Reasoning: Eliminates the nickel corrosion ("Black Pad") risk associated with standard ENIG and provides a wire-bondable surface.

Panelization & Array Design

A single PCB is rarely efficient to manufacture. Panelize boards to maximize SMT machine utilization and provide mechanical rigidity.

1. Waste Rails (Break-away Strips)

• Width: Mandate ≥ 5.0 mm (preferably 7.0 mm) on the two parallel transport edges.
• Copper Clearance: Keep copper ≥ 0.5 mm away from the V-score or tab line to prevent exposure or shorting during separation.
• Tooling Holes: Place 3.0 – 4.0 mm non-plated holes in the corners of the rails for test fixtures and stencil alignment.

2. Optical Fiducials

Machine vision requires high-contrast reference points to align components.

• Global Fiducials: Place 3 fiducials on the panel rails in an "L" pattern to determine panel orientation.
• Local Fiducials: Required for all Fine Pitch (≤ 0.5 mm) and BGA components.
• Dimensions: Solid copper circle 1.0 mm diameter with a 2.0 mm solder mask opening (clearance). Do not cover with silk screen.

3. Mechanical Stability

• If the panel contains heavy components or is thin (≤ 1.0 mm), Then add "Webbing" or spacing between boards to increase stiffness.
• If boards are rotated 180˚ within the panel (A-B flip), Then verify thermal mass distribution. Uneven heating can cause warping.

Depanelization Method

How the boards are separated determines the stress applied to ceramic capacitors (MLCCs) and solder joints.

V-Score (V-Cut)

Best for rectangular boards and reducing waste material.

• Constraint: Requires straight lines across the entire panel.
• Residual Thickness: Specify 1/3 of board thickness (e.g., 0.5 mm web for 1.6 mm board).
• Keep-out: No components within ≥ 1.0 mm of the score line. MLCCs placed parallel to the V-cut are at high risk of flex cracking.

Tab-Route (Mouse Bites)

Required for non-rectangular shapes or overhanging components.

• Perforation: Use 5 – 7 holes of 0.5 mm diameter.
• Placement: Locate tabs away from sensitive components. Breaking the tab releases significant mechanical energy.

Pro-Tip: When using Tab-Routing, ensure the router path width is at least 2.0 mm to prevent the milling bit from clogging or breaking.

Final Checklist