Common Flex PCB Defects and Causes

In the precision manufacturing process of flex PCB, various defects directly affect product yield and reliability. As a flex PCB factory, accurately identifying and controlling these defects is the core of quality management. The following is a classification analysis of the most common flex PCB defects encountered in production:

Defects related to flex PCB substrates and coverlays

Substrate wrinkles or creases

Phenomenon: Local arching, wavy wrinkles or obvious crease lines appear on the copper foil surface of the cover film or substrate.

Influence: Affects the appearance, causes the circuit to deform or even break, and is difficult to fit flat during assembly.

Common causes:

• Poor material incoming materials (such as PI moisture absorption and deformation)

• Uneven tension control when pasting the cover film

• Uneven heating or too fast cooling during high-temperature pressing

• Artificial bending during operation or turnover.

Coverlay offset or bubbles

Phenomenon: The cover film does not completely cover the circuit area to be protected (offset), or there is a local unbonded area between the cover film and the substrate (bubbles/delamination).

Impact: Exposed circuits are prone to oxidation and short circuits, resulting in protection failure; bubbles are prone to dirt and grime, and stress can easily expand and delaminate.

Common causes:

• Insufficient alignment accuracy (equipment/mold)

• Improper lamination parameters (temperature/pressure/time)

• Uneven lamination film (such as silicone blanket) or impurities

• Surface contamination (grease, dust)

• Poor coverlay adhesion

Substrate scratches or indentations

Phenomenon: Visible scratches, dents or indentations appear on the surface of the PI substrate.

Impact: It may damage the internal circuit (especially fine circuits), reduce mechanical strength, and affect the appearance.

Common causes: 

Contact with sharp hard objects during production or turnover, foreign objects or damage to equipment rollers or treatment, and excessive local force during stacking and storage.

Circuit Pattern Defects in Flex PCB Manufacturing

Open circuit

Phenomenon: The circuit that should be connected in design is physically disconnected.

Impact: The circuit function is completely ineffective.

Common causes: 

Poor photolithography development (such as incomplete development causing the circuit to be cut during etching), excessive etching (severe side etching), mechanical damage (scratches, punching and tearing), poor coating bonding causing circuit detachment.

Short circuit

Phenomenon: Unexpected conductive connection between circuits or pads that should be isolated in design.

Impact: Abnormal circuit function or even burning of components.

Common causes: 

Photoresist falls off or has pinholes causing the copper to be etched away during etching (incomplete etching), metal particles (copper powder/tin beads, etc.) on the surface of copper foil or coating causing bridge connection, residual conductive substances between circuits (crystallization of etching liquid), insufficient protection of covering film causing foreign matter to cause short circuit.

Edge Gaps or Depressions in Circuits

Phenomenon: Irregular local missing or inward depression at the edge of the circuit.

Impact: Reducing the effective cross-sectional area of the wire, increasing resistance, and may become a stress concentration point, leading to the risk of fracture.

Common causes:

Foreign matter on the film or substrate blocks light during exposure, poor development (partial or incomplete development)

Uneven etching solution spraying or abnormal local concentration

Defects in the substrate copper foil itself.

Circuit Pinholes or Trachoma

Phenomenon: Tiny holes appear on the surface or inside of the circuit.

Impact: Reduce the current carrying capacity of the circuit, and easily become the starting point of corrosion in a high-humidity environment.

Common causes:

Quality defects (holes) of the substrate copper foil itself, electroplating process (such as impurities or hydrogen precipitation in the plating), etching process, and mechanical damage in subsequent processing.

Surface treatment and Soldering defects

Pad oxidation or Contamination

Phenomenon: The surface of the pad or plating (such as gold plating, tin plating) loses gloss, darkens, changes color, or has foreign matter attached.

Impact: The solderability is seriously reduced, resulting in cold soldering and false soldering.

Common causes:

Incomplete cleaning (residual medicine, fingerprints, sweat stains), improper storage environment (high temperature and high humidity), failure of anti-oxidation treatment, poor sealing of packaging bags, or excessive ion pollution.

Poor Electroplating (poor gold plating)

Phenomenon: The gold plating layer appears black, red (nickel exposure), white (rough crystal), uneven thickness, missed plating, poor bonding (peeling and shedding), etc.

Influence: Affects appearance, contact impedance, wear resistance, corrosion resistance, and solderability (for solder paste).

Common causes:

Incomplete pre-treatment (poor degreasing/micro-etching/activation), imbalance or contamination of electroplating solution components (metal impurities/organic pollution), improper current density, poor temperature/PH value control, and insufficient water washing leading to cross-contamination of the solution.

ENIG (chemical gold) black pad

Phenomenon: After the chemical nickel-gold process, the surface of the pad is partially or completely abnormally dark gray or black.

Influence: Very poor solderability, usually accompanied by corrosion of the nickel layer, extremely low welding strength, or even impossible to weld.

Common causes: 

The activity of the gold solution is too strong or the reaction time is too long, the phosphorus content of the nickel layer is abnormal, the nickel layer and the gold layer are not thoroughly cleaned, resulting in the displacement reaction being out of control, and the solution is contaminated (such as copper ions).

Appearance and Hole-Related Defects

Poor punching/laser cutting

Phenomenon: Burrs, burrs (edge protrusions), glue overflow (covering film is squeezed and overflowed), crooked cutting/deformation, tearing, size deviation, positioning hole offset or damage.

Influence: Affects the appearance, assembly accuracy, and electrical clearance. Burrs may cause short circuits.

Common causes: 

Mold wear/poor design/insufficient precision, improper setting of laser parameters (power/speed/frequency), reduced precision of the punching machine or laser machine, unstable material fixation, and improper selection of flex PCB pad.

Through-Hole Defects

Voids/cracks in the through-hole plating: The copper plating on the hole wall is discontinuous or cracked, affecting electrical connectivity and reliability.

Blocked holes: The substrate at the edge of the hole is damaged or the hole is blocked by foreign matter (such as resin, glue).

Hole position deviation: The drilling or punching position deviates from the designed position.

Common causes:

Improper drilling parameters (drilling speed/feed), wear of drill bit, poor pre-treatment of hole metallization (removal of glue residue/copper deposition), poor electroplating uniformity, improper control of material expansion and contraction, inaccurate or worn punching die alignment.

Other functional defects

Poor bending resistance

Phenomenon: flex PCB has an open circuit or cover film delamination within the specified number of bends.

Impact: Product failure in a dynamic use environment.

Common causes:

Improper material selection (such as PI is too thick, copper foil is too thick or the type is wrong), unreasonable circuit design (improper change of circuit direction/width/copper thickness in the bending area), excessive process stress (multiple high-temperature processes), poor bonding between cover film and substrate.

Impedance out of control

Phenomenon: The measured impedance value exceeds the tolerance range required by the design.

Impact: The quality of high-speed signal transmission is reduced (reflection, distortion).

Common causes: 

Fluctuation of dielectric layer thickness (substrate + cover film + glue), large deviation of line width/thickness (copper thickness), and environmental temperature and humidity affecting measurement.

Conclusion

Flex PCB manufacturing is a complex and precision-intensive process, where defect prevention and quality control are vital. Understanding the root causes of common flex PCB defects—from substrate handling to circuit processing and surface treatment—is essential for improving yield, performance, and reliability.

At Flex Plus, as a professional custom flex PCB manufacturer, we apply strict quality management systems and continuous improvement strategies to ensure defect-free products for our global clients in medical, automotive, consumer electronics, and industrial applications.