To improve the flexural performance and peel strength of FPC, it is necessary to consider not only from the selection of materials, but also from the production process. For flexibility, we hope to choose thinner materials, which are constrained by peel strength and cost. This may be a contradiction that has always existed in the FPC industry. The trend of electronic products is smaller, lighter and more convenient, which makes FPC require more layers, thinner materials and better performance.

The development of flexible circuit boards (FPC) to today is becoming more and more widely used. As a result, the requirements for flexible circuit boards are getting higher and higher. From 3-Lay to 2-Lay are all aimed at its main properties such as flexural properties and peel strength.

A) Flexural properties of flexible circuit boards

a) First of all, from the perspective of the FPC material itself, the following points have an important influence on the flexural performance of the FPC.

The molecular structure and direction of the first p copper foil (ie the type of copper foil)

The bending resistance of rolled copper is significantly better than electrolytic copper foil.

Second p Thickness of copper foil

In terms of the same variety, the thinner the thickness of the copper foil, the better the folding resistance.

Third p Type of base material used

Generally speaking, epoxy resin has better flexibility than acrylic glue. Therefore, when the selection of highly flexible materials is required, the epoxy system is mainly used. And the higher tensile modulus (tensilemodulvs) glue can improve the flexibility.

Fourth p Thickness of glue used

The thinner the thickness of the glue, the better the flexibility of the material. The flexibility of FPC can be improved.

Fifth p insulating substrate

The thinner the thickness of the insulating substrate PI, the better the flexibility of the material and the improvement of the flexibility of the FPC. The PI with low tensile modulus (tensile modolos) is better for the FPC.

Summarizing the main influencing factors of materials on deflection are two main aspects: the type of material selected; the thickness of the material

b) Analyze the influence of FPC's flexibility from the process aspect of FPC.

Symmetry of the first pFPC combination

After the substrate is laminated with the cover film, the better the symmetry of the two sides of the copper foil, the higher the flexibility. Because the stress it receives when flexing is the same.

The thickness of PI on both sides of the circuit board tends to be the same, and the thickness of the glue on both sides of the circuit board tends to be the same

Control of the second p lamination process

When the coverlay is pressed, the glue is required to be completely filled into the middle of the line, and there should be no delamination (slicing observation). If there is a delamination phenomenon during deflection, it is equivalent to bare copper deflection will reduce the number of deflections.

B) Peeling strength of flexible circuit board

Peel strength is mainly to measure the performance of the adhesive. Generally speaking, the thicker the thickness of the glue, the better the peel strength, but this is not absolute, because the formula and structure of the glue of different manufacturers are different. If the molecular structure of the glue is small, the bonding area between the glue and the copper foil will increase. As a result, the adhesive strength is improved, and the peel strength is increased accordingly. Among the current material manufacturers, the materials of Sehan Korea use this method to improve the peel strength and reduce the thickness of the glue.

In addition, the quality of the blackening process of the copper foil itself and the composition of the blackening layer will also affect the adhesion of the adhesive to the copper foil.

The main components of the blackening layer provided here are: In terms of production process, the degree of curing of the glue also directly affects the adhesion. Epoxy adhesive has three stages of ApBpC in FPC application. When the material factory adjusts the glue, the cross-linking degree of the cross-polymer is A stage when the epoxy glue is in liquid state, the cross-linking degree of the cross-polymer is in the semi-cured state after coating and baking, and the cross-linking degree is cured when the FCCL is cured The degree of polymer crosslinking is C stage. Coverlay is C-stage after being pressed by FPC factory. When the glue is in C-stage, it must be fully cured. If the temperature and pressure during curing or pressing fail to reach the curing temperature and complete curing time of the epoxy adhesive, the peel strength will be significantly reduced.