FR-4 (aka FR4), is a widely accepted international grade designation for fiberglass reinforced epoxy laminates that are Flame Retardant (self extinguishing).
Eurocircuits laminate requirements of finished boards (based on definitions out of IPC-A-600 (x) class II)
The boards must be free of dust and dirt.
Measling (1) (separation of the fibers at the weave intersection) is permitted.
Crazing (2) (glass fibers are separated from the resin) is permitted.
Blistering (3) (local swelling and separation in the base material, between individual glass weave layers or between the base material and the Cu-foil). See: delaminating below.
Weave exposure (4) (outermost resin-layer is missing) is permitted.
Weave texture(5) (outermost resin-layer is very thin) is permitted.
Delaminating (extended separation inside the PCB)
Not more than 25 % of the distance between adjacent conductors or plated-through holes.
Not more than 1% of the printed wiring area on each side may be affected.
No propagation as a result of thermal stress testing or representative condition.
(6A) shows separation between two glass weave layers in the base material. The separation can also occur between the base material and the copper foil.
(6B) shows a separation between individual layers.
(6C) and (6D) show separations between laminate and internal or external pads respectively, or copper planes.
Haloing (7) (light area in the machined areas of the board) is permitted if not penetrated more than 2,54 mm or 50 % of the distance to the closest conductor, whichever is less.
Foreign Inclusions (metallic or non-metallic entrapped in an insulating material) is permitted depending on size and position.
Laminate voids – prepreg voids – stress crack – resin crack – fiber bundle cracks : (typical voids and cracks in laminates named after the position and/or cause of occurring) are permitted if not in excess of 76 micron and not violating minimum dielectrically spacing.