[cms_heading title=”SHORT CHECKLIST FOR DESIGNING RELIABLE FLEX CIRCUITS:” font_size_title=”28″]
BEND AREA CONSIDERATION:
- Route traces perpendicular to bend line
- Traces must be evenly spaced across the bend area, wide & uniform, and staggered on adjacent layers
- Sharp angles should be avoided
- Traces should always come out perpendicular to the rigid edge, when transitioning to the flexible region
- Cross-hatch ground and plane layers, especially in the bend area
- Number of layers at the bend area should be kept at a minimum
- Flex layers can be separated (loose-leaf) for multilayer construction to increase flexibility
- Mechanical slots, slits or ICT custom hinge design can be used to increase flexibility
- No vias or PTH should be placed in the flex region, unless used for shielding or away from bend region
- Use RA copper grain direction for dynamic flexing or tight bend radius
- Conductors should be close to neutral axis, especially for dynamic application
- Minimize the dielectric thickness to reduce stress on the conductors
- Follow minimum bend radius rules
PADS, HOLES & MASK CONSIDERATION:
- Tear-drop all pads and tie-down all unsupported pads
- Pad size should be large enough for selective plating the holes
- All vias should be in the stiffened/rigid area
- Stiffener holes should be at least 20 mils larger than finished hole size
- Gang open the coverlay wherever possible
- Use LPI/Coverlay combo for fine pitch components such as BGA, CSP, etc.
- Allow for possible squeeze out of the adhesive onto the pads
STRAIN RELIEF CONSIDERATION:
- Radius all inside corners. Additionally copper trace can be used as tear stop
- Use holes at the end of slits or slots as tear stop
- Use epoxy bead in the rigid to flex transition area
- Use appropriate stiffener thickness and material to support solder joints
- Keep vias, holes and SMT features at least 25 mils away from the board edge
- Vias on rigid-flex should be minimum 50 mils away from the rigid-to-flex transition edge
- Never have edge of coverlay opening and edge of stiffener right on top of each other
MATERIAL SELECTION CONSIDERATION:
- Take into account stress due to environment such as Temperature, Vacuum, Vibration, Bends, etc.