Substrate pretreatment: making the copper foil and substrate "adhere more firmly"

The substrate of a regular rigid PCB is tightly bonded to copper foil, while the surface of the P substrate of FPC is smooth and requires special treatment to ensure that the copper foil adheres firmly. During pretreatment, the P1 film is first etched with a chemical solution (such as sodium hydroxide) to create tiny pits, then coated with a layer of adhesion promoter (similar to "glue"), and finally bonded to the substrate through hot pressing. An experiment conducted by a certain FPC factory showed that the adhesion of pre treated copper foil can reach 0.8N/mm, which is twice that of untreated copper foil, and copper detachment is less likely to occur when bent.

The temperature and time of pre-processing should be precisely controlled. Excessive temperature (over 120 ° C) can cause deformation of the P substrate, while low temperature results in poor treatment effect. A certain production line stabilized the pre-treatment temperature at 100 ° C+2 ° C for 3 minutes, improving the consistency of copper foil adhesion to over 95%.

Circuit production: engraving circuits on "elastic film"

The circuit production of FPC is similar to that of rigid PCB, also through photolithography and etching, but with greater difficulty. Because the P substrate will stretch and contract when heated, vacuum adsorption is needed to fix the substrate flat during exposure to avoid deformation of the circuit. A high-density FPC (with a line spacing of 0.1mm) uses a high-precision exposure machine and a vacuum adsorption platform to control the line deviation within ± 10um, which is 60% higher than the ± 25um of a regular exposure machine.

During etching, an acidic solution (such as ferric chloride) should be used to remove excess copper foil and retain the necessary circuitry. The etching speed of FPC is 30% slower than that of rigid PCB, as too fast etching may cause burrs on the edges of the circuit. The line spacing of a certain FPC is 0.08mm. By reducing the etching speed (from 1m/min to 0.7mmin), the roughness of the line edge is reduced from 5um to 2um, avoiding short circuits between adjacent lines.

Cover film bonding: Wear protective clothing on the circuit

Covering film bonding is a key process unique to FPC. First, apply a layer of thermosetting adhesive on the cover film, then align the cover film with the circuit through the positioning hole, and finally press it together with a hot press machine (temperature 160%, LeLi 05MPa, time 30 seconds). When repelling, avoid generating bubbles, otherwise it may cause the circuit to become damp and oxidized. A certain FPC factory adopts the "step-by-step pressing" method: first, low-temperature pre pressing (100 ℃) exhausts the air, and then high-temperature pressing, reducing the bubble rate from 5% to 0.5%.

The thickness selection of the love film is very important. Thin covering film (25um) has better flexibility, but slightly lower protection; Thick Love Cover Film (50um) has strong protective properties, but it increases stress when bent. The FPC of smart watches usually uses a 35um thick cover film to achieve a balance between flexibility and protection.

Punching and Forming: Cutting out the desired shape

FPC needs to be cut into specific shapes according to the device structure, commonly using mold punching or laser cutting. Mold punching is suitable for large-scale production, with an accuracy of ± 0.1mm. For a certain mobile phone, FPC batch production uses mold punching, with an efficiency of up to 50 pieces per minute; Laser cutting is suitable for small batches or complex shapes, with an accuracy of ± 0.05mm. The irregular FPC of a certain medical equipment is cut with laser, perfectly matching the curved structure of the equipment.

The cutting edge should be smooth and free of burrs, otherwise it will scratch the covering film when bent. The laser cutting parameters of a certain FPC have been optimized (power 10W, speed 50mm/s), and the edge roughness Ras1um has increased by 67% compared to the unoptimized 3um.

Surface treatment: Both weldability and oxidation resistance are required

The solder joints of FPC require surface treatment, and commonly used methods include immersion gold and tin plating. The deposited gold layer is thin (0.05-0.1um), does not affect flexibility, and is suitable for fine pitch solder joints (such as chips with a spacing of 0.3mm); The tin plating layer is slightly thick (1-3um), with low cost, but tin whiskers (small tin crystals) may appear when bent. It is necessary to control the baking temperature after tin plating (125 '℃, 2 hours) to suppress the growth of tin whiskers. The FPC of a certain car sensor is treated with tin plating, and after baking, the length of the tin whiskers is controlled below 5um, which meets the safety standards of automotive electronics.

Reinforcement process: adding a hard board in key areas

Although FPC is soft, the parts where components are soldered require a certain degree of hardness, otherwise it will deform during soldering. At this point, a reinforcing plate (made of P1, steel sheet, or epoxy resin board) with a thickness of 0.1-0.5mm should be attached to the back and bonded to the FPC through an adhesive layer. The position deviation of the reinforcing plate cannot exceed ± 0.1mm, otherwise it will block the solder joints. After attaching a 3mm thick P | reinforcement plate to the battery solder joints of a smart wristband's FPC, the welding yield increased from 90% to 99%.