On the design of FPC multilayer board for mobile phones

On the design of FPC multilayer board for mobile phones

In recent years, FPC has played an increasingly important role in the design of slider phones and foldable phones with its own characteristics.

The number of orders for sliding mobile phone boards and multi-layer boards for folding mobile phones continues to increase. It is important for FPC manufacturers to control the quality and make such products in the process.

I. Introduction

With the miniaturization, high speed, and digitization of electronic products, the rapid development of personal communication terminals, cottage phones and 3G communications and the network of information and information terminals (computers, TVs, telephones, faxes) are suitable for mobile phones on the communication side. The FPC life and impedance requirements of the industry's slider phones and foldable phones are becoming more and more detailed, and it is important for manufacturers of FPC to control how to make such products in the process.

The characteristics of FPC (Flexible Printed Circuit, or Radial Printed Circuit Board, also known as FWPC) mainly include the following aspects:

(1) Good flexibility: it can be bent and deformed arbitrarily, the coil radius is small, and it can move freely in the three directions of X, Y and Z;
(2) Small occupied space: it is light and thin, which makes the narrow space of instruments and meters fully utilized and meets the requirements of miniaturization and lightness 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: adopt low-tension sealing design, can withstand harsh environment;
(5) Stable transmission characteristics: the wire spacing can be freely designed according to electrical parameters, and the general layout is finalized;
(6) The manufacturability of the assembly is good: the product's free termination and overall termination performance are good, suitable for welding, plugging, as well as three-dimensional wiring and three-dimensional space connection;
(7) Good insulation performance: The base materials PI and PET and other polymer materials used in the flexible board have high insulation strength, and the general circuit is covered with a protective film, so the insulation performance is greatly improved.

In recent years, FPC has played an increasingly important role in the design of slider phones and foldable phones with its own characteristics. People's requirements for their longevity are also getting stricter. As a result, the number of orders for sliding mobile phone boards and folding mobile phone multi-layer boards has recently increased. Process maintenance and other aspects are controlled in the production process to reduce the occurrence of defects and increase the percentage of one-time pass rate.

2. Production requirements:

1. Design selection

The first step is important. If the customer does not reflect or specify what substrate to use, it should consider rolling copper, because its bending resistance is better than electrolytic copper. However, the presence or absence of glue on the base material has a relatively large influence on the bending performance. Generally speaking, the bend resistance of the glue-free base material is better than that of the glue-based base material.

Classification of substrates:

1.1 Copper foil:

1.1.1 Rolled copper.

Rolled copper is the smelting of the electrolytic cathode copper deposit into strips, which are formed by rolling. Due to the melting, the composition is relatively single and the crystal distribution is uniform. Since the crystallization direction is parallel to the flexible board, it is suitable for the transmission of high-frequency signals. The characteristics of rolled copper are better than electrolytic copper. Its thickness is 1 / 4OZ, 1 / 3OZ, 1 / 2OZ, 1OZ, 2OZ.

1.1.2 Electrolytic copper:

Electrolytic copper foil uses the electroplating principle to deposit copper ions on the rotating smooth cathode drum, and then separate the copper foil from the cathode roller to obtain a smooth and rough copper foil, 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 will be rough due to the high current density due to the contact with the plating solution. This rough surface can increase the surface contact area after surface treatment, which is conducive to improving the adhesion with the protective film. Its thickness is 1 / 4OZ, 1 / 3OZ, 1 / 2OZ, 1OZ, 2OZ.

PI (polyimide), PET (polyethylene) or GE (glass fiber) are often used. The PI performance is the best and the price is higher. There are several thicknesses of 1 / 2mil, 1mil and 2mil.

1.3 Material selection and collocation during design

Due to the high performance requirements of slider phones, both the base material and CVL should be considered "thin" in the choice of materials.

1.4 Design and typesetting

1.4.1 Bending area line requirements:

a) There should be no through holes in the parts to be bent;
b) Additional protection copper wire is required on the two sides of the line. If there is insufficient space, additional protection copper wire should be selected at the inner R corner of the bent part;
c) The connecting part of the line should be designed as an arc.

1.4.2 Bending area (air gap) requirements: the bending area needs to be layered, and the glue is removed to facilitate the distribution of stress. The bending area is as large as possible without affecting the assembly.

2. Production process

After the material is selected, it is more important to control the sliding cover and multilayer board from the manufacturing process. In order to increase the number of bendings, the production process, especially the sinking copper process, must be specially controlled. Generally, the sliding board and the multi-layer board are all required for life, and the mobile phone industry generally requires a minimum of 80,000 bends.

Because the general process used by FPC is the whole-board electroplating process, unlike the hard board that will go through a picture, the copper thickness is not required to be too thick when plating copper. The surface copper is generally 0.1 to 0.3 mil. (The deposition ratio of hole copper and face copper is about 1: 1 when electroplating copper) But in order to ensure the quality of hole copper and the non-layering of hole copper and substrate at high temperature of SMT, as well as the conductivity and communication of the product, Thickness of copper is required to be 0.8 ~ 1.2mil or above.

In this case, there will be a problem. Some people may ask, how is the requirement for surface copper to be only 0.1 to 0.3 mil, and (without substrate copper) hole copper to be above 0.8 to 1.2 mil? This requires an additional process: the flow chart of the general FPC board (if only 0.4 ~ 0.9mil plating is required) is: cutting → drilling → sinking copper (black hole) → electric copper (0.4 ~ 0.9mil) → graphics → after Process.

The process flow for making bending resistance is as follows: cutting material → drilling → sinking copper (black hole) → one electric (electric 0.1 ~ 0.3mil) → through hole production → second electric (0.4 ~ 0.9mil) → graphics → After process.

3. Process control

A board has to go through many steps in the manufacturing process, from the original copper foil to the finished product. How to control it?

3.1 Cutting

According to the requirements on the product flow sheet, find the required substrate and cut it. Personnel should pay attention to the type and type of materials, and the size of cutting materials. Avoid breaking 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 packaging is closely related to the quality of drilling, especially for multilayer boards.

3.3 Immersion copper

When making multi-layer boards, a plasma hole treatment process should be added before sinking copper, in order to better remove some of the glue formed in the hole due to drilling. If the copper wire is an automatic line, there will be no problem in the production of peripheral equipment and the potion of each tank is normal. If the copper sink wire is a manual wire, especially when making multi-layer boards, the main degreasing cylinder, prepreg cylinder, activation cylinder, acceleration cylinder, and copper sink cylinder should add vibrators and ensure that the equipment swings normally It can be produced so that the potion can penetrate deeper into the hole. The most important thing is that the concentration of medicine in each tank is within the normal control range.

3.4 Copper

The general double-sided board and the multi-layer board can be directly charged to the required thickness. As long as the current output of the rectifier is normal, the conduction of the rack is normal, and the current of the operator is accurate, the problem is not big. And after the copper sinking, the front of the electric copper also needs to be controlled. The board after the copper sinking cannot be slightly etched before the electric copper. The layer of copper that sinks in the micro-etched hole will be slightly eroded, resulting in no holes. Copper reduces the yield.

But for the mobile phone board with sliding requirements and the multilayer board with folding requirements, it can not be directly charged to the required thickness. It should be plated in two times. The first time the whole board is plated, only 0.1 ~ 0.3 is required. The mil is enough, because the electric copper is aimed at the hole, but in the electric copper, the hole and the surface will be electrically copper. Therefore, the base material will only increase the thickness of 0.1 ~ 0.3mil, which has no effect on bending. After the first plating, transfer to the graphics station and expose the product with a well-made through-hole film (through-hole film: only the holes exposed after exposure and development are covered with dry film elsewhere) . Then conduct the second copper plating, the second copper plating is only for the hole, not the surface. That is to add a selective plating (selective plating).

It should be noted that the design of the project should accurately calculate the plating area of ​​the second plating and mark it on the process sheet. Because in the process of copper electroplating, in addition to the influence of the electrical thickness on the bending of the board, the amount of additives (light agent) added in the process of copper electroplating also has an effect on the bending of the plating layer. It will be bright, but the coating will become very brittle and can not withstand bending, so in the production process, the addition of light agent must be added according to the amount given by the supplier.

3.5 Graphics

The graphic station is the process that best reflects the yield of the product. The defects of the FPC, such as Open / short, gap, line width and line spacing are unqualified, etc. are related to this process. Each company may have its own methods for yield control, which will not be described here. Special attention should be paid to the interlayer misalignment of the multilayer board. When producing the inner layer of the multilayer board, the film should be set with PIN or sandwich film. The identification hole used for alignment should also be considered in its design, so as not to cause the inner layer and the outer layer of the multilayer board to be misaligned.

3.6 Suppression (pressing)

The temperature of the pressing process also has a certain effect on the bending resistance of the product. Whether it is CVL or substrate, too high temperature will relatively reduce the bending resistance of the product and affect the expansion and contraction. It can be produced according to the parameters provided by the supplier.

After quick pressing, the temperature of the baking board should not exceed 180 ° for the products with bending requirements. It is reasonable to control at 160 °.

The treatment of the surface film does not affect the bending resistance, after all, the bending resistance area will not be the place where the above treatment has been done. However, the quality of the surface coating is also a major factor that affects the product yield. After the general treatment, the surface coating must not have blistering, shedding and other phenomena, and the thickness of the coating can be released to the thickness required by the customer.

3. Summary

With the increasing demand for the development of the communication market, for FPC, product protection and personal operation quality awareness have a greater impact on it, and this article describes only the product bending resistance and multilayer board should pay attention to Quality requirements for some processes. I also hope that more and more colleagues can discuss together.