Technological process

1、 Tooling production:

The thickness of FPC is generally less than 0.5mm and its mounting points are limited, so fixtures are made in the process to facilitate printing and mounting.

1. Material: The tooling material should have a certain mechanical strength, and the density should be relatively low. Aluminum alloy is more appropriate. During double-sided production, some parts must be avoided, so the thickness is generally 2.0mm.

2. Splicing: In order to improve production and actual machine utilization, splicing production is generally adopted. The splicing data depends on the size and material placement accuracy of the single board, and 6-10pcs is generally preferred.

3. Processing: Process the corresponding parts of the fixture on the milling machine (milling, drilling, leveling), and if necessary, make corresponding matching positioning fixtures (buckle plate base).

2、 Stencil production and use:

After positioning the FPC on the fixture, it can be handed over to the outsourcing factory (steel mesh manufacturer) to customize the steel mesh according to the production requirements.

1. Steel mesh thickness: The optimal thickness for steel mesh is generally 0.1-0.2mm, and for IC (0.4pitch) or BGA and CN with micro spacing, 0.12mm is preferred.

2. Steel mesh punching: The punching and manufacturing method of steel mesh is the same as that of ordinary PCB technology.

3. Use of steel mesh: Due to the use of fixtures, the wear of steel mesh scrapers is relatively severe, so it is necessary to regularly measure key parameters such as tension and flatness.

3、 Printing:

After adopting the splicing production, it is necessary to carry out the FPC crimping operation. When crimping, the positioning pin is used as the reference point, and the corresponding parts are fixed with high-temperature adhesive tape. In addition, the FPC can be positioned by applying high-temperature double-sided tape at the bottom to reduce the workload of pasting high-temperature adhesive paper. Due to environmental factors such as nitrate, it is necessary to regularly replace the high-temperature double-sided tape.

2. Printing parameter settings: Production using tooling.

4、 Patch:

1. Splicing production reduces the time for board entry and exit to a certain extent, but in order to ensure the stability of the components, it is necessary to first read the reference mark (Fidual mark) for each single FPC before mounting.

2. In terms of program development, corresponding extensions and optimizations must be made based on the panel data.

3. As FPC products mainly serve as connectors (such as flip phones and laptops), they often have some irregular parts attached, such as sockets with a length of up to 35mm. In this case, special nozzles need to be made to ensure the stability of picking and placing during production.

4. In addition, due to the use of thicker fixtures than FPC for production, the PCB Thickness option in the program must be set based on the fixture thickness when making the SMT program.

5、 Reflux:

At present, most FPCs in SMT are soldered using hot air reflow with lead solder. Based on this, we analyze the soldering process between FPC and PCB:

By comparison, it can be seen that:

Although FPC is produced using tooling, its peak reflow time is relatively lower than that of PCB technology due to the substrate material's inability to withstand high temperatures.

6、 Reflux effect:

The main problems that occur in FPC technology are as follows:

1. Splicing errors (such as buckle plate, steel mesh production, and program production) may cause short circuits, displacement, and insufficient tin.

2. The distance between two PADs is too large, resulting in false welding of the hard parts.

3. Different thermal expansion coefficients of substrates, components, and fixtures, as well as folding during production, can cause tin cracking.

4. The solder mask on the FPC is immersed in oil onto the PAD, resulting in exposed copper.

7、 Testing and Repair:

1. AOI(Automated Optical Inspection)

2. FCT(Functional Testing)

3. ICT(In Circuit Testing)

The testing and repair methods are basically the same as the PCB process.