PCB is the fundamental electronic material in PCBA processing, and every PCBA process relies on PCB. How should one accept the PCBA circuit board, and what acceptance standards should be considered? How are these criteria assessed?
Various characteristics can be observed on the PCB surface, with common external and internal traits that are not visible from the surface. These appearance inspections are key reference standards for circuit board acceptance.
1. Sheet edges and surface defects
▪ Burr
▪ Gap
▪ Scratches
▪ Groove
▪ Fiber scratches
▪ Exposed fabrics and voids
▪ Foreign inclusions
▪ White spots/microcracks
▪ Layering
▪ Pink circle
▪ Laminated void
2. Plated through holes
▪ Misalignment of holes
▪ Foreign inclusions
▪ Plating or coating defects
3. Printed contact piece
▪ Pits
▪ Pinhole
▪ Nodulation
▪ Exposed copper
4. Graphic size
▪ Size and thickness
▪ Aperture and graphic accuracy
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▪ Wire width and spacing
▪ Coincidence
▪ Ring width
5. The flatness of the printed circuit board
▪ Bow
▪ Distortion
1. PCB Board Edge
Defects such as burrs, nicks, or halos can occur along the board’s edge, necessitating specific acceptance criteria.
**Burrs**: These appear as small, irregular lumps or bumps protruding from the surface due to machining processes like drilling or cutting. Burrs can be categorized as metallic or non-metallic.
▪ **Ideal**: The edge of the board is smooth and free of burrs.
▪ **Acceptable**: The edge is rough but does not damage the board.
▪ **Rejection**: The edge is severely damaged.
**Gap**:
▪ **Ideal**: Smooth edges without gaps.
▪ **Acceptable**: Rough edges, but no defects; the depth of any notch does not exceed 50% of the distance between the edge and the nearest conductor, or 2.5mm, whichever is smaller.
▪ **Reject**: Fails to meet standards.
**Halo**:
A halo refers to conductive fragmentation or delamination on or beneath the substrate’s surface due to machining, typically manifesting as white areas around holes or other machined sections.
▪ **Ideal**: No halo.
▪ **Acceptable**: The halo reduces the unaffected distance between the board’s edge and the nearest conductive pattern by no more than 50%, or 2.5mm, whichever is less.
▪ **Reject**: Fails to meet standards.
**Laminated Substrate**:
Defects in the laminate may be present upon receipt from the substrate manufacturer or may emerge during PCB fabrication.
**Weave Reveal, Breakage, and Fiber Breakage**:
▪ **Exposed Fabric**: A surface condition where unbroken fabric fibers are not entirely covered by resin. If the remaining distance between conductors meets minimum wire spacing requirements, it is acceptable.
▪ **Significant Cloth Pattern**: Refers to a condition where the unbroken fabric fibers are covered by resin, but the weave pattern remains obvious. While visible patterns are acceptable, distinguishing them from actual exposure can be challenging.
▪ **Exposed Fiber/Fiber Break**: If exposed fibers or breaks do not cause wire bridging and maintain minimum spacing, they are acceptable; otherwise, they should be rejected.
**Pits and Holes**:
▪ **Ideal**: No pits or holes.
▪ **Acceptable**: Pits or voids shall not exceed 0.8mm (0.031in), with affected areas on each side being less than 5%. Pockmarks or voids must not bridge conductors.
▪ **Reject**: Fails to meet standards.
**Leukoplakia**:
White spots appear as discontinuous white squares or “cross” patterns beneath the substrate’s surface, typically related to thermal stress. These spots occur on new laminated substrates and fabric-reinforced boards. According to IPC-A-600G, white spots are acceptable in all products, except high-voltage applications as specified by the user, and should be noted as a process warning for manufacturers.
**Microcrack**:
Microcracks are internal separations in the laminated base material, visible as white dots or “cross patterns” beneath the substrate surface, usually due to mechanical stress.
▪ Microcracks reducing wire spacing below minimum values, while not exceeding 50% of the distance between conductive patterns, may be accepted. Microcracks on the edge of the board should not reduce the minimum distance from the edge (typically 2.5mm), and if defects do not expand after thermal stress testing, 2nd and 3rd grade boards may be accepted.
▪ If microcracks exceed 50% of conductive pattern spacing without causing bridging, level 1 boards may be accepted, provided other conditions remain unchanged.
**Delamination and Blistering**:
Delamination refers to separation between substrate layers, conductive foil, or other PCB layers. Blistering indicates local expansion and separation within the laminated substrate or between layers. Acceptance criteria per IPC-A-600G for Class 2 and 3 boards include:
▪ The area affected by defects must not exceed 1% of each side’s area.
▪ Defects must not reduce spacing below specified minimums.
▪ Blistering or delamination spans must not exceed 25% of the distance between adjacent conductive patterns.
▪ Defects should not expand post-thermal stress testing.
▪ Distance from the board edge must meet minimum distance requirements, generally over 2.5mm.
For Class 1 boards, acceptance conditions are:
▪ The defect area cannot exceed 1% of each side’s area.
▪ Blistering or delamination must exceed 25% of adjacent wire spacing, but not below minimum spacing.
▪ Defects should not expand post-thermal stress testing.
▪ The distance from the board edge must meet or exceed minimum distance requirements, typically greater than 2.5mm.
**Foreign Inclusions**:
Foreign inclusions refer to metal or non-metallic particles embedded in insulating materials. They may be detected in substrate raw materials, prepreg materials, or multilayer PCBs. The nature of inclusions, whether conductive or non-conductive, determines rejection criteria based on size and location. Generally, translucent inclusions are acceptable, while opaque inclusions are permissible under certain conditions:
▪ The distance to the nearest conductor must be no less than 0.125mm.
▪ Must not lower the distance between adjacent wires below minimum requirements, generally 0.125mm unless specified otherwise.
▪ Electrical performance of the board should remain unaffected.
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