Flexible Printed Circuit Board

Common Mistakes Made with Flex Printed Circuit Board

• Placing vias outside the stiffener area or where the flex bend which can cause via cracking.
• Not providing proper stiffener information. It is important that you show the x & y dimensions of the stiffener, show location of the stiffener, and dictate the type of stiffener material - FR-4, Kapton, Ceramic, etc.
• Failing to state a tolerance on the overall thickness of the stiffener area. The tolerance specified should not be less than +/- 10% of the overall thickness in this area.
• Bending a flex circuit tigher than is recommended. See “Minimum Bend Radii for Flex Circuits” chart for guidelines.
• Not showing the material stackup.(Should indicate base material, copper thickness, cover layer thickness, as well as stiffener material and thickness.)
• Traces routed on top of each other on adjacent layers. Traces should be staggered on the adjacent layers to avoid “I-beam” effect which could cause trace fracture.
• Having silkscreen nomenclature less than .007” which can cause illegibility or placing silkscreen too close to exposed pad areas. Silkscreen should be no closer than .010”.
• Designing without correct pad geometries. Teardrops and tie downs are important to add to pad geometries to avoid stress fractures and assembly issues.
• Placing edge of feature closer than .010" to edge of board.
• Making hole size on stiffener too small. Should be +.020” over finished hole size.

Flexible Circuit - Application Drivers

• Reduction in package size required
• Reduction in package weight sought
• Decrease in assembly time sought
• Need to reduce assembly costs
• Requirement for assembly error reduction
• Increased system reliability sought
• More than 25 point to point wire needed
• Dynamic flexure of circuitry required

• Controlled impedance signal transmission required
• Lower inductance cabling needed
• Improved heat dissipation capability
• Airflow improvement inside "box" required
• 3-Dimensional packaging scheme sought
• Compliant substrate for surface mounting
• Desire to improve product appearance