Lamination Defect
Localized separation of the protective polyimide film from the underlying flexible circuit board copper or dielectric layer occurs when trapped volatile gases expand during high-temperature processing. The presence of coverlay blistering develops during lamination or reflow soldering when adhesion is compromised. Visual inspection easily detects these raised domes of unbonded film.
The defect is limited to boards with polyimide coverlays and does not occur in boards utilizing liquid photoimageable solder mask.
Moisture Influence
Polyimide films absorb atmospheric moisture rapidly because of their chemical structure. When the board undergoes rapid heating during assembly reflow, any absorbed moisture vaporizes instantly and generates high internal pressure. If the adhesive layer has not fully cured or contains microscopic voids, this pressure overcomes the bond strength and forces the film away from the substrate.
Baking the boards prior to reflow prevents this vapor expansion by driving out residual water.
Process Control
Controlling the lamination cycle parameters remains the primary method for preventing adhesive voids. Fabricators use vacuum presses to extract air and volatile solvents from the stackup before applying heat and pressure. The temperature ramp rate must allow the acrylic or epoxy adhesive to flow and wet the copper traces before the resin begins to cross-link.
Ensuring a uniform pressure distribution across the entire surface prevents low-pressure zones where voids can coalesce into visible blisters. If the press pads are worn or uneven, the localized reduction in force allows volatile gases to escape and expand, forming defects that fail IPC quality standards. Post-lamination inspection with automated optical systems confirms the absence of these voids.
