Why does FPC need stiffener and FPC production process

Flexible circuit boards are widely used because they are flexible and thinner than rigid-type circuit boards. So many electronic devices use FPC. It can also satisfy the electronic products’ pursuit of small size, thinness and fashion.

But FPC also has its weaknesses. Because it is thin, it has low mechanical strength and is easy to break or crack. FPC is generally used for linking, plugging, etc. Therefore, these parts are easily damaged. At this time, the processing and production of FPC need to strengthen these positions by stiffener.

Stiffener: generally there are the following types of stiffener; PI stiffener, PED stiffener, FR4 stiffener, steel sheet stiffener, etc.

PI stiffener:

tolerance can be controlled within +/- 0.03mm, high precision, high temperature resistance (130-280 degrees Celsius)As a special engineering material, PI has been widely used in aviation, aerospace, microelectronics, nanometer, liquid crystal, separation membrane, laser and other fields. It has flame retardancy, high temperature and low temperature resistance, and the long-term use temperature can be -200℃~426℃. In FPC production, the PI used is mainly PI protective film and PI reinforcing sheet. The PI protective film is used for circuit insulation, and the PI reinforcing plate is used in the area on the back of the FPC golden finger. The thickness of the PI reinforcement sheet is mainly based on the design drawings and the use environment, and the PI reinforcement of different thicknesses is selected for pressing.

In addition, regarding PI reinforcement, the main material has two colors. Black and reddish brown. Black is the most common and commonly used color, and the function and quality of the two are the same. Once a Japanese customer of our company asked why our PI reinforcement color was different from his previous inventory. Here is also a little explanation.

Steel sheet stiffener: manual assembly is required, the process is more complicated and the cost will be higher.Steel sheet stiffenermainly refers to 303 stainless steel reinforcement. 303 stainless steel is an austenitic free-cutting stainless and wear-resistant acid steel containing sulfur and selenium respectively. It is used for occasions that require easy cutting and high surface finish. FPC reinforcement often has different shapes, and stainless steel 303 is easy to etch. Therefore, in FPC products that require high stability, this kind of steel sheet reinforcement is often used. Because the steel sheet reinforcement cannot be drilled by CNC, nor can it be cut by FPC laser, it is mainly made by the method of etching with liquid medicine. The cost is also relatively high.

FR4 stiffener: If the thickness is less than 0.1mm, the tolerance can be controlled within +/- 0.05mm. If the thickness is greater than 1.0mm, the tolerance is +/- 0.1mm.As a flame-resistant material, FR4 has higher mechanical properties, dimensional stability, impact resistance, and moisture resistance than paper substrates. It has excellent electrical properties, higher working temperature, and its performance is less affected by the environment. In terms of processing technology, it has great advantages over other resin glass fiber cloth substrates. It is mainly used as a reinforcing sheet in the production of FPC, and is mainly used as the back of the FPC welding area to strengthen the hardness of the welding area and protect it. After patching, the electronic components in this area are not bad due to the continuous bending and stretching of FPC, and at the same time, because its wear resistance is not as good as PI, it is generally not necessary to use golden fingers to reinforce it.

FPC stiffener thickness

PI stiffener thickness: 0.075mm, 0.1mm, 0.125mm, 0.15mm,0.175mm, 0.2mm, 0.225mm, 0.25mm

FR4 stiffener thickness: 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 1.6mm

Steel sheet stiffener thickness: 0.1mm, 0.2mmThe thickness is determined according to customer requirements.No matter what kind of stiffener, it needs to be pressed on the FPC circuit board at the end. According to different needs and use environments, choose different stiffenermaterials, adjust the pressing parameters correctly, control the pressing time, and improve the quality of the FPC。
The main process of stiffener: stiffener fit.

1. Pressure stiffener:Without heating, the reinforcement can be attached to the product.Laminate with a cold press.
2. Hot compressibility stiffener

At a certain temperature, the glue on the reinforcement sheet begins to melt, making the reinforcement and the product fit closely.

Mature

The glue on the hot-pressing stiffener may not be able to fully mature after the first melt. So after that, bake for a period of time to melt the glue completely. In this way, the stiffener and the product are completely bonded.

Production process of double-sided FPC

(1) Cutting: The size of the material is specified. The length of the material size is fixed at 250mm on one side, and the length of the other side depends on the needs of the panel. The size of the material is determined according to the size of the Panel.

(2) Drilling: PTH, positioning hole, direction hole

(3) PTH: Through the autocatalytic oxidation-reduction reaction of the plating solution, the barrel ion is plated on the activated hole wall to prepare for the next electroplating, so that the copper thickness on the copper wall reaches a certain thickness, thereby To achieve the role of conduction.

(4) Electroplating: Improve the uniformity of the plating layer in the hole, and ensure that the plating thickness of the entire layout (the entire plating layer in the hole and near the hole) meets certain requirements.

(5) Pre-treatment: clean the board surface, treat the oxide and greasy surface of the board

(6) Paste dry film: It is composed of PE, photoresist and PET. Paste a dry film on the board to achieve the function of image transfer and protect the circuit during the etching process.

(7) Exposure, the agricultural production system polymerization reaction will occur after the dry film is pasted, and the circuit pattern will be formed on the board surface after the exposure.

(8) Development: The dry film part that is not exposed to light will be cleaned by NACO3, and the dry film on the circuit and the copper part will not be washed off.

(9) Etching: The copper in the part without dry film will be etched away by the etching solution (HCL, H2O2, CUCL), thereby removing the excess copper.

(10) Inspection: the line width and line spacing after etching, whether there is an open circuit or short circuit caused by impure etching.

(11) Cleaning (pre-treatment): To prepare for the protective film, it is necessary to ensure that the surface of the board is clean

(12) Paste the protective film: The protective film and the Solser mask of the PCB have the same function, and the board and PI are pasted together through the glue on the protective film.

(13) Pressing: The glue on the PI must pass a certain temperature and pressure to be better glued together.

(14) Brushing the board: clean off the impurities on the board surface

(15) Surface treatment: The surface treatment has gold, electro-gold, spray SN, sink SN, so as to soothe the reaction.

(16) Printed characters, according to customer requirements

(17) ET, check for open circuit and short circuit to ensure quality

(18) Assembling, some need the entire panel to be patched and reinforced, and some need to be patched individually, which is determined according to the CAM design.

(19) Cut a single board on the panel through a punching machine, or it can be cut by a laser.

(20) FQC quality inspection, check out the bad problems that may occur in the production process.

(21)Packaging and shipping