5.1 Bare Board Inspection (IPC-A-600)

The Printed Circuit Board (PCB) is not a component; it is the chassis of the entire system. If the foundation is compromised, the best soldering in the world cannot save the product. We utilize IPC-A-600 (Acceptability of Printed Boards) to define the boundary between a functional substrate and a piece of scrap fiberglass. Incoming Quality Control (IQC) must treat the bare board as a complex assembly, not a raw material.

Class Selection Criteria

Define the rules of engagement before inspection begins.

• Class 2 (Dedicated Service): Business machines, instruments. (Standard for General Electronics).
• Class 3 (High Reliability): Life support, aerospace, automotive. (Failure is not an option).
• Rule: If the drawing does not specify, default to Class 2.

Base Material (Laminate Integrity)

The laminate provides the dielectric isolation. Structural failures here lead to electrical shorts under thermal stress.

Defect Taxonomy:

• Measling: Discrete white spots (resin separation from glass fibers).
  • IF measling bridges two conductors -> THEN Reject. (Arcing risk).
  • IF measling is isolated -> THEN Acceptable (Cosmetic only).
• Delamination/Blistering: Large-scale separation of layers.
  • IF delamination exceeds 25% of the distance between vias/conductors -> THEN Reject.
  • IF blister is adjacent to a plated hole -> THEN Reject. (Heat from soldering will expand the gas and rupture the barrel).
• Bow and Twist: Flatness is critical for SMT placement.
  • Standard: Max 0.75% for SMT boards. (e.g., A 100mm board cannot bow more than 0.75mm).

Plating & Solder Mask

These protect the copper and ensure solderability.

Inspection Logic:

• Hole Breakout: The drill missed the center of the pad.
  • Class 2: Acceptable if 90° breakout occurs but the hole is not "open" to the PCB edge.
  • Class 3: Reject. The hole must be fully contained within the pad (Tangency is the limit).
• Solder Mask Skip:
  • IF mask is missing between two IC pads (webbing) -> THEN Reject. (High risk of solder bridging during reflow).
• Hole Wall Voiding:
  • IF > 1 void per hole or void > 5% of barrel length -> THEN Reject. (Open circuit risk).

Pro-Tip: Perform the "Tape Test" (IPC-TM-650 2.4.1) on one board per lot. Apply 3M 600 tape to the silkscreen and rip it off. If ink comes off, the lot is curing defective and will fail in the field.

Final Checklist