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Flex Circuit Design

PCB and Flex Circuit Fabrication  |  Flex Circuit  |  Flex Circuit Design Rules
Coverlay is a material applied to the outside layers of the circuit to insulate the copper conductor.  Below are types of coverlays used.

flex circuit design rules

Polyimide/Polyester Coverlay:

These materials are normally produced with a drilling process. Limitations to the drilling process enable production of round and oval holes, leaving the inability to have square openings, dense features or any unique shape other than round or oval. Both polyester and polyimide coverlay can be punched or blanked in lieu of the drilling process and is normally done for large coverlay openings.

Via Hole Covered with coverlay
Component Hole .010″ (.25mm) minimum larger than the copper pad
Feature Definition .008″ (.20mm) minimum
Note: Coverlay openings can be individual bared pads or group bared pads depending on area available.
Conductor Pad Design:
Pads should have tie downs (also called anchoring spurs or rabbit ears). Tie-downs are captured by the coverlay to anchor the copper to prevent separation between the copper and the base material.Filleting:
All pads, on both through-hole and surface mount pads, should be filleted because they reduce stress points. This helps eliminate breaking during flexing.
flex circuit PCB design rules
Calculating Pad Size:
Recommended pad size is dependent on the component pad requirement that is specific to your application.
Hole size + customer required tolerances + .020″ (.50mm)
Hole size (finished): .030″ (.76mm)
Customer tolerance: ±.003″ (.07mm)
FLEX manufactured factor: .020″ (.5mm)
Calculated pad size required is: .053″ (1.34mm)
flex circuit design rules 3
Radiused Traces help to alleviate breaking during folding and bending. (Refer to diagram above for example).
Bend Raidus of a flex should be approximately 10 times the material thickness and at least .050″ away from the plated through hole. Button plating is a process that allows for the through holes to maintain their connection while the traces are not plated, allowing the circuit to have increased flexibility.
flex circuits and plated through holes

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