The following points should be considered in FPC mold design:
① Selection of mold steel
② Mold manufacturer's manufacturing process level
③ Mold structure design
④ Number of mold cavities
⑤ Layout of mold positioning pins
⑥ Mold pre-expansion and pre-shrinkage
1. Selection of mold steel
There are many types of steel available on the market, both domestic and imported. However, most manufacturers use imported steel, such as those from Japan (HITACHI), Sweden (ASSAB), Germany (SSE), and Austria (BOHLER), which generally perform well. For FPC molds, high-quality mold steel is essential, requiring high hardness, good hardenability, and low wire EDM stress, such as Japanese SKD61 and SKD11.
2. Mold manufacturer's manufacturing process
Because FPC requires high shape and positional accuracy, the processing technology of the mold itself is highly demanding. Generally, slow-speed wire EDM is required. The quality of wire EDM is one of the important factors affecting the mold's performance, which is closely related to the machine's grade and the worker's skill level. The mold assembly process is a key part of the later stages of the process and is crucial. It should also be considered that during heat treatment, the male and female molds should have a temperature difference of 8-10 degrees to facilitate mold repair.
3. Mold structure design
Mold structure design is relatively simple in metal stamping dies, divided into blanking type and surface-forming type. For stamping cover films, thermosetting adhesive films, plating lines, PI and FR4 reinforcing sheets, the blanking type is used because these materials are not easily deformed and the efficiency is higher; for external shapes, to ensure no deformation, and because there are often structural holes, the surface-forming type is used; in addition, stainless steel reinforcing sheets use the surface-forming type because blanking would cause deformation.
4. Number of Cavities in the Mold
To achieve high production capacity and efficiency, more cavities are generally better. However, this is limited by the size of the press platform, mold stability, and the stability of the FPC substrate itself, as well as the number of parts that can be arranged based on the product shape. Mold cost and structural rationality must also be considered. For products with high tolerance requirements, FPC molds typically have one or two cavities.
5. Layout of Mold Positioning Pins
Positioning pins are used to locate the product to be stamped. Stamping molds require balanced force distribution, and FPC molds are no exception. The mold shape should be centered as much as possible, so the positioning pins should also be distributed around the product. However, the specific requirements of the product itself must also be considered. For example, in areas with particularly strict tolerance requirements for fingers and center distance, the positioning pins should be placed as close as possible.
6. Mold Pre-expansion and Pre-shrinkage
For areas with high product tolerance requirements, especially the gold fingers, if the requirement is +/-0.05MM, and the mold cannot achieve this tolerance, technical treatment of the mold is necessary. Considering that the mold will wear after a certain period of use, a unilateral pre-shrinkage of 0.02 is applied to the finger portion of the mold. The mold manufacturer is required to pre-shrink two lines during processing. After this technical treatment, the defect rate, specifically for the fingers, is only about 1%. Without this technical treatment, the defect rate would be over 10%.
