Talking about the design of FPC multi-layer board for mobile phone
In recent years, FPC has played an increasingly important role in the design of slider phones and folding phones by virtue of its own characteristics.
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The number of orders for slider mobile phone boards and folding mobile phone multi-layer boards is increasing. How do FPC manufacturers control the quality? It is important to make such products in the process.
I. Introduction
With the miniaturization, high-speed and digitization of electronic products, in the rapid development of personal communication terminals, mobile phones and 3G communication, and the networked information terminals (computer, TV, telephone, fax), it is suitable for communication mobile phones. The FPC life and impedance requirements required in the industry's slide-type mobile phones and foldable mobile phones are becoming more and more detailed, and in the manufacturers of FPC, how to control such products in the process is crucial.
FPC (Flexible Printed Circuit, also known as FWPC) features the following aspects:
(1) Good flexibility: It can be bent and deformed arbitrarily, and the coiling radius is small, and it can move freely in three directions of X, Y and Z;
(2) Small footprint: light and thin, making the narrow space of instrumentation fully utilized, meeting the micro-light requirements of electronic products;
(3) Light weight: The soft board is designed according to the current carrying capacity rather than the mechanical strength, so the weight is lighter;
(4) Good sealing: low tension sealing design can withstand harsh environments;
(5) Stable transmission characteristics: wire spacing can be freely designed according to electrical parameters, and the general layout is finalized;
(6) The assembly process is good: the product has good free termination and overall termination performance, suitable for welding, plugging, and three-dimensional wiring and three-dimensional space connection;
(7) Good insulation performance: the base material PI and PET materials used in the soft board have high insulation strength, and the general circuit has cover film protection, so the insulation performance is greatly improved.
In recent years, FPC has played an increasingly important role in the design of slider phones and folding phones by virtue of its own characteristics. People's life expectancy is becoming more and more strict. As a result, the number of orders for our slide phone boards and folding mobile phone multi-layer boards has increased recently. In order to strengthen the quality and training of personnel, the design and layout concept and material selection of the multi-layer boards for the slider mobile phone board and the folding mobile phone are Control of process maintenance in the production process to reduce the occurrence of defects and increase the percentage of the pass rate.
Second, the production requirements:
1. Design material selection
The first step is very important. If the customer does not reflect or specify what substrate to use, then rolling copper should be considered because its bending resistance is better than electrolytic copper. However, the adhesive and non-glue of the substrate have a relatively large influence on the bending property. Generally, the bending resistance of the non-adhesive substrate is better than that of the adhesive substrate.
Classification of substrates:
1.1 Copper foil:
1.1.1 Calendered copper.
The calendered copper is obtained by melting the electrolytic copper cathode into a strip, which is formed by rolling, and the composition is relatively simple and the crystal distribution is uniform due to smelting. Since the crystal direction is parallel to the soft plate, it is suitable for the transmission of signals with high frequency. The characteristics of rolled copper are better than those of electrolytic copper. Its thickness is 1/4OZ, 1/3OZ, 1/2OZ, 1OZ, 2OZ.
1.1.2 Electrolytic copper:
The electrolytic copper foil is formed by depositing copper ions on a rotating smooth cathode drum by electroplating principle, and then separating the copper foil from the cathode roller to obtain a copper foil having a glossy surface and a matte surface, which can be used after surface treatment. The contact surface of the electrolytic copper foil and the cathode drum is very smooth, but the other side is rough under the action of high current density due to contact with the plating solution. The rough surface is surface-treated to increase the surface contact area and is advantageous for improving adhesion to the protective film. Its thickness is 1/4OZ, 1/3OZ, 1/2OZ, 1OZ, 2OZ.
PI (polyimide), PET (polyethylene) or GE (glass fiber) is often used. Among them, PI performance is the best and the price is high. The thickness is 1/2 mil, 1 mil, 2 mil.
1.3 Design material selection
Due to the high performance requirements of the slider phone, the choice of materials should be considered in the "thin" direction regardless of the substrate or CVL.
1.4 Design layout
1.4.1 Bending area line requirements:
a) There must be no through holes in the bent part;
b) The outer side of the line needs to be additionally protected with copper wire. If the space is insufficient, the copper wire should be additionally protected at the inner R angle of the bent part;
c) The connecting part of the line needs to be designed as an arc.
1.4.2 Air gap requirements: The bending area needs to be layered to remove the glue to facilitate the dispersion of stress. The bent area does not affect the assembly, the bigger the better.
2, the production process
When the material is selected, it is more important to control the sliding cover and the multi-layer board from the manufacturing process. In order to increase the number of bends, special control should be made during the production process, especially the copper electroplating process. The general sliding cover and the laminated board of the multi-layer board have life requirements, and the mobile phone industry generally requires a minimum of 80,000 bends.
Because the general process used by FPC is the whole plate electroplating process, unlike the hard plate, it will pass through a graph. Therefore, the copper thickness is not required to be too thick when electroplating copper, and the surface copper is generally 0.1~0.3 mil. (The deposition ratio of copper to copper on the copper plating is about 1:1), but in order to ensure the quality of the copper and the sinter of the copper and the substrate at the high temperature of the SMT, and the conductivity and communication on the product, The thickness of copper is required to be 0.8 to 1.2 mil or more.
In this case, there will be a problem. Some people may ask, the requirement for copper is only 0.1~0.3mil, and how to make the hole copper (without base copper) 0.8~1.2mil or more? This requires an additional process: the general FPC board process flow chart (if only 0.4 ~ 0.9mil plating is required): material opening → drilling → copper (black hole) → electric copper (0.4 ~ 0.9mil) → graphics → after Process.
The process to make the bending resistance requirements is as follows: cutting→drilling→sinking copper (black hole)→ one electricity (electricity 0.1~0.3mil)→through hole making→second power (0.4~0.9mil)→graphic→ After the process.
3. Process control
A board undergoes a lot of processes in the production process, from the original copper foil to the finished product. How to control it?
3.1 Cutting
Find the required substrate and cut it according to the requirements on the product flow sheet. Personnel should pay attention to the type and type of materials during operation, and the size of the material can not be mistaken. Avoid turning the required rolled copper into electrolytic copper or PET copper foil.
3.2 Drilling
In order to ensure quality during drilling, the number of packages before drilling is very important. The amount of packing has a lot to do with the quality of the hole, especially the multi-layer board.
3.3 copper sink
When making a multi-layer board, an equal-area treatment process should be added before the copper is deposited, in order to better remove some of the glue formed in the hole due to drilling. If the copper wire is an automatic wire, it will not be problematic if the peripherals and the cylinders are normal. If the sinking copper wire is a manual wire, especially when the multi-layer plate is used, the main degreasing cylinder, pre-dip cylinder, activation cylinder, acceleration cylinder, and copper sink cylinder should be added with vibrators, and the equipment should be kept under normal swing. It can be produced so that the potion can penetrate deeper into the hole. The most important thing is that the concentration of each cylinder is within the normal control range.
3.4 Electric copper
The general double-panel, and multi-layer board can be directly electrically connected to the required thickness, as long as the rectifier current output is normal, the hanger is electrically conductive, and the operator's current is accurate, the problem is not large. After the copper is deposited, the copper is also required to be controlled in front of the copper. The plate after the copper sink cannot be slightly etched before the copper. The layer of copper that sinks inside the micro-etched hole will be slightly etched, resulting in no holes. Copper reduces yield.
For the slider phone board with bending requirements and the multi-layer board for folding, it is not possible to directly supply the required thickness. It should be divided into two times. The first time the whole board is plated, only 0.1~0.3 is required. Mil is enough, because the copper is for the hole, but the hole and the surface will be copper when the copper is copper. Therefore, the substrate will only increase the thickness of 0.1~0.3mil, which has no effect on the bending. After the first plating, transfer to the graphic station, use the well-passing film (through hole film: only the hole exposed after exposure and development, other places are covered with dry film) to expose the product . Then a second copper plating is carried out, and the second copper plating is only for the holes and will not be directed to the surface. That is to increase the selective plating (selective plating).
It should be noted that the engineering should accurately calculate the plated area of ​​the second plating and design it on the process sheet. Because in the process of electroplating copper, in addition to the influence of the thickness of the electric plate on the bending of the plate, the additive (light agent) added in the process of electroplating copper also has an influence on the bending of the plating layer, and the coating agent obtained by adding more light agent It will be bright, but the coating will become very brittle and cannot withstand bending. Therefore, the addition of the photo-agent during the production process must be added according to the amount of addition given by the supplier.
3.5 graphics
The graphic station is the process that best reflects the yield of the product. The failure of the FPC, such as Open/short, gap, and line width and line spacing are all related to this process. For yield control, each company may have its own methods, which are not described here. Special attention should be paid to the interlayer misalignment of multilayer boards. Films that are aligned when the inner layer is produced in a multi-layer board should be produced by a set of PIN or sandwich film. The identification hole used for alignment should also be designed with its comprehensiveness in mind, so as not to cause the inner and outer layers of the multilayer board to be misaligned.
3.6 pressing (pressing)
The temperature of the pressing process also has a certain influence on the bending resistance of the product. Whether it is pressing CVL or pressing the substrate, the excessive temperature will relatively reduce the bending resistance of the product, and will affect the expansion and contraction. It can be produced according to the parameters provided by the supplier.
Fast-pressing and baking plates should not exceed 180° for products with bending requirements. Control at 160 ° is more reasonable.
The treatment of the surface film does not affect the bending resistance. After all, the bending-resistant area is not the place where the above treatment is done. However, the quality of the surface film is also a major factor affecting the yield of the product. After the treatment, the surface coating may not have foaming or falling off, and the thickness of the coating can be released to the thickness required by the customer.
Third, summary
As the development of the communication market is growing, for FPC, product protection and personal sense of operational quality have a greater impact on it. This article is only for the bending resistance of the product and the attention of the multi-layer board. The quality requirements of some processes. I also hope that more and more peers can discuss together.
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