Introduction:
As the wiring density of flexible substrate becomes higher and higher, the circuit made by subtractive process can no longer meet the requirements of circuit elaboration. Therefore, the industry gradually turns to the semi-additive manufacturing process. At the same time, some thin flexible substrates and embedded flexible substrates manufacturing technology is also gradually applied in the production of mass production.
Electrodeposition Polyimide Process:
The protective layer of flexible substrate circuit not only can use the "binder + polyimide" to form a whole covering film or special flexible substrate ink, in recent years also began to use the electrodeposition polyimide process which only forms a protective layer on the circuit of flexible substrate, and the specific comparison of the three processes can be referred to figure 4.
The Advantages of Electrodeposition Polyimide Process:
1) Meet the requirement of high density of circuit. Using the film of binder and polyimide to protect the circuit, which usually will have a problem that the film does not adhere tightly to the circuit on substrate, even if the use of flexible substrate ink, there will be gaps between the circuits filled with dissatisfaction. However, electrodeposition process can form independent electrochemical film around the circuit to protect the elaborate circuit.
2) Adapt to the trend of high frequency. The coating film or ink forms a complete protective layer on the circuit surface of the flexible substrate. The actual dielectric constant is subject to the material properties, while the electrodeposited polyimide only deposits an independent protective layer around the circuit, and finally forms a protective layer -- air layer -- protective layer.
Etching Process of Electrodeposition Polyimide Process:
The traditional hole processing method of polyimide is realized by mechanical or laser punching and drilling technology, while the hole processing on polyimide substrate by electroless etching technology is a new flexible substrate technology. This process mainly uses alkaline solution to erode the imide bond, causing hydrolysis reaction, and then promote the imide ring cracking, and the low molecular compound in the cracking can be dissolved in the solution, and finally complete the oriented etching of polyimide, forming the required hole.
The main process flow is shown below.
1) Adopt graphic transfer technology, remove copper foil of hole position, which forms hole sites.
2) Special polyimide etching solution was used for substrate etching.
3) Electroless plating or PTH is carried out on hole sites formed by etching.
Compared with traditional technology, etching process can process all holes once, and the processing time and cost no longer depend on the number of holes. When thickness of PI film is 25um, its minimum machining aperture can be up to 35um, and the micropore capacity is equivalent to YAG laser. Since all the hole sites can be formed once by optical imaging technology, the pore position accuracy can be improved to meet the requirements of high-density flexible substrate. In addition, local etching of polyimide matrix to obtain air-displaced polyimide space can effectively reduce capacitance between circuits, thereby alleviating RC delay, crosscutting and resonance frequency drop, and better meeting the requirements of high-speed signal transmission.
