5.1 Bare board inspection: IPC-a-600

The bare Printed Circuit Board (PCB) is not merely a passive component; it serves as the critical mechanical and electrical foundation for the entire system. If this foundation is compromised, even the highest-quality reflow soldering later cannot ensure a reliable product. IPC-A-600 (Acceptability of Printed Boards) is adhered to, clearly defining the boundary between a functional, safe substrate and an unacceptable piece of fiberglass. Incoming Quality Control (IQC) must rigorously evaluate the bare board as a complex, engineered assembly rather than treating it like simple raw material.

Class selection criteria

It is crucial to define the rules of engagement before any inspection begins.

• Class 2 (Dedicated Service): Typical for standard business machines and instruments. This is the default standard expectation for General Electronics.
• Class 3 (High Reliability): Required for critical systems like life support, aerospace, or advanced automotive applications where failure is simply not an option.
• Operating Rule: If the engineering drawing does not explicitly specify a class in the notes, use Class 2 criteria by default.

Base material (laminate integrity)

The FR4 (or equivalent) laminate provides essential dielectric isolation and critical structural support. Failures here reliably lead to electrical shorts under thermal stress during assembly, or worse, out in the field.

Defect Taxonomy Guidelines:

• Measling: These are discrete white spots indicating resin separation from the woven glass fibers inside the board.
  • Class Decision: If the measling bridges two adjacent active conductors, Reject the board (High risk of electrical arcing).
  • Class Decision: If the measling is completely isolated, it is generally Acceptable (Considered a cosmetic defect only).
• Delamination/Blistering: A large-scale mechanical separation between the internal copper layers of the board.
  • Class Decision: If the delamination exceeds 25% of the distance between adjacent vias or conductors, Reject.
  • Class Decision: If a blister abuts a plated through-hole, Reject. (The intense, sudden heat from wave soldering will rapidly expand the trapped gas and structurally rupture the copper barrel).
• Bow and Twist: Absolute flatness is a critical physical requirement for successful Surface Mount Technology (SMT) placement.
  • Standard: Maximum 0.75% bow/twist permitted for SMT boards. (For example, a 100mm board cannot deviate from perfectly flat by more than 0.75mm).

Plating & Solder Mask

These protective layers shield the copper traces from oxidation and ensure robust, controlled solderability on the line.

Inspection Logic:

• Hole Breakout: Occurs when the mechanical drill slightly misses the exact center of the copper pad.
  • Class 2: Acceptable if up to a 90° breakout occurs, provided the conductive hole itself is not “open” to the outer edge of the PCB.
  • Class 3: Reject. The drilled hole must securely remain fully contained within the copper pad (“Tangency” is the maximum allowable limit).
• Solder Mask Skip:
  • Class Decision: If the green solder mask is completely missing between two fine-pitch IC pads (missing webbing), Reject. (This presents an unacceptably high risk of solder bridging during the SMT reflow process).
• Hole Wall Voiding:
  • Class Decision: If analyzing a cross-section reveals > 1 void per hole, or a void exceeding 5% of the total barrel length, Reject. (This indicates poor supplier plating and a high risk of open circuits under thermal stress).

Critical Factor

Performing the “Tape Test” (IPC-TM-650 2.4.1) on at least one sample board per incoming lot is highly recommended. 3M 600 tape should be firmly applied directly to the silkscreen ink and removed quickly. If the ink lifts off onto the tape, the supplier’s lot has a curing defect and will likely cause frustrating issues downstream.

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Recap: Bare Board Inspection per IPC-A-600

Parameter	Class 2 Requirement	Class 3 Requirement	Reject Condition
Bow/Twist (Warpage)	≤ 0.75%	≤ 0.75%	Exceeds 0.75% of board length.
Measling	Acceptable if isolated.	Acceptable if isolated.	Bridges two adjacent conductors.
Delamination/Blistering	Reject if >25% of inter-feature distance or abuts a PTH.	Reject if >25% of inter-feature distance or abuts a PTH.	Exceeds size limit or contacts plated through-hole.
Hole Breakout	Acceptable up to 90° (hole not open to edge).	Not acceptable.	Any breakout (Class 3). Hole open to board edge (Class 2).
Solder Mask Skip	Acceptable if not between fine-pitch pads.	Acceptable if not between fine-pitch pads.	Missing mask between fine-pitch IC pads (risk of bridging).
Hole Wall Voiding	Reject if >1 void/hole or void >5% of barrel length.	Reject if >1 void/hole or void >5% of barrel length.	Exceeds void count or size limit.