First, molecular structure and direction of copper foil(i.e. copper foil types);
Folding strength of RA copper is obviously superior to ED copper.
Second, thickness of copper foil;
In terms of the same type, the thinner the copper foil, the better the folding strength.
Third, types of adhesive used by substrate;
As a general rule, the flexibility of epoxy glue is better than that of acrylic adhesive. So, if high flexible material is required, epoxy glue is recommended. Moreover, the adhesive with high tensile modules can improve flexibility.
Fourthly, the thickness of adhesive;
The thinner the adhesive, the softer the material. Thin adhesive can improve FPC flexibility.
Fifth, insulation substrate.
In conclusion, for material, type and thickness are the most important 2 factors influencing FPC flexibility.
First, symmetry of FPC combination;
After substrate is pasted with coverlay, good symmetry of material on both sides of copper foil can improve flexibility. This is because they bear the same stress when bending. PI thickness on both sides of PCB tends to be the same, and thickness of adhesive on both sides of PCB tends to be the same.
Second, control of lamination technology.
During coverlay lamination, the adhesive is required to be completely filled into the middle of the trace, and there can be no delamination. If there is delamination, it is equivalent to bare copper bending, which will reduce the number of bending.