5.1 Bare Board Inspection (IPC-A-600)

The quality of the final assembly cannot exceed the quality of the raw 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 is(Acceptability of Printed Boards) to define the acceptanceboundary standardbetween fora rigidfunctional substrate and flexiblea barepiece boards.of Thisscrap chapter defines the criteria forfiberglass. 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)Laminate ConditionsIntegrity)

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

Defect Taxonomy:

• Measling: Discrete white spots where (resin has separatedseparation from fibers.glass fibers).
  • Class 2/3 Acceptable:IF Ifmeasling itbridges does not bridgetwo conductors or-> reduceTHEN spacingReject. below(Arcing minimumrisk).
  • IF electricalmeasling clearance.is isolated -> THEN Acceptable (Cosmetic only).
• Delamination/Blistering: SeparationLarge-scale separation of layers.
  • Class 2/3 Defect:IF Any delamination exceedingexceeds 25% of the distance between holesvias/conductors or-> internalTHEN conductorsReject.
  • IF blister is adjacent to a rejection.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:

• Solder Mask Skip: Absence of mask between pads.
  • Defect: If it exposes adjacent conductors, creating a risk of solder bridging.
• Hole Breakout: The drill holemissed isthe notcenter perfectly centered onof the pad.
  • Class 2: Acceptable if 90° breakout allowedoccurs (hole exitsbut the pad) provided minimum annular ringhole is maintainednot on"open" to the wire-side.PCB edge.
  • Class 3: HoleReject. The hole must be fully contained within the pad (Tangency allowed,is nothe breakout)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

Feature	CheckControl MethodPoint	Critical LimitRequirement	Non-Negotiable (Class 2)Rule
Warp/BowFlatness	Surface Plate	< 0.75% ofBow/Twist.	Boards diagonal> 0.75% jam the Pick & Place.
Hole PlatingSolderability	Cross-SectionPads clean & bright.	AverageOxidation/Stains 20µm= Reject entire lot.
Annular Ring	Class 2: 90° Breakout OK.	Class 3: 0° Breakout (ClassTangency 2)only).
Solder Mask	Visual	No encroachment on padspads.	Mask on pad = Solder reject.
SilkscreenGold Fingers	VisualHard Gold, smooth surface.	Legible; no inkScratches/Pits on padsfingers = Reject.