The electronic products are getting smaller, slimmer and more digital, the personal communication terminals and the 4G communications information and the information terminals (computer, television, telephone, fax) have being developed rapidly. In this condition, the life period and resistance of the FPC flex printed circuit should be more refined to meet the communications and folding products requirements. And it is pretty important for the flexible circuits manufacturer to control the manufacturing process the best to ensure the reliable quality.

Here is how Treefpc did in multilayer FPC design and manufacturing

A. design and base material selection
The first step is important. If the customer does not reflect or specify what kind of substrate, then we will consider the rolled annealed copper foil (RA), because it is better bending resistance than the electro-deposited copper foil (ED). But the adhesive and adhesiveness substrate will affect the bending resistance, in general, the adhesive-less base material is betty bending resistance than adhesive type.

Substrate classification:
A.1 Copper foil:
A.1.1 rolled annealed copper foil (RA).
Rolling copper is the electrolytic cathode copper starch melting into strips, formed by the pressure, because of melting, it is single composition and the crystal is distributed evenly. Since the crystal direction is parallel to the flexible board, it is suitable for the high-frequency signals transmission. Rolled copper is better than electro-deposited copper. There are several thicknesses of it: 1 / 4OZ, 1 / 3OZ, 1 / 2OZ, 1OZ, 2OZ.

Multilayer FPC

A.1.2 Electro-deposited copper:
Electro-deposited copper foil is using the electroplating principle of copper ions deposited in the rotation of the smooth cathode drum, and then the copper foil from the cathode roller separation to get the smooth and matte side of the copper foil, can be used after surface treatment. Electro-deposited copper foil and cathode drum contact surface is very smooth, but the other side because it is contacting with the plating liquid, it will be getting rough under the high-density current. The rough surface after treatment can increase the surface contact area and good to improve the adhesion with the protective film. The thickness can be 1 / 4OZ, 1 / 3OZ, 1 / 2OZ, 1OZ, 2OZ.

A.2 Substrate

Normally we use PI (polyimide), PET (polyester) or GE (glass fiber). PI performs the best, and the price is higher. The thickness can be: 1 / 2mil, 1mil, 2mil.

A.3 design with material selection
Due to the high-performance requirements of the slider phone, we should consider “as thin as possible” no matter selecting the substrate or coverlay.

A.4 design layout
A.4.1 Circuit requirements in the bending area:
a) There should be no through hole in the bending part;
b) Add the protecting copper wire at the two sides of the circuits, if there is limited space, should add the protecting copper wire at the R angle of the bending part;
c) The connection part of the circuit must be designed as an arc.

A4.2 bending area (air gap) requirements: layout the bending are, remove the plastic to disperse the stress easily. The bend area should the bigger the better if does not affect the assembly.

A.5 the production process
When the material is selected, it is more important to control the slide and flexible multilayer circuit board from the manufacturing process. To increase the number of bending, we should control well the production process, especially the copper plating process. Normally there the life period for the sliding board and the layered board of the multi-layer board, generally it requires a minimum of 80,000 times for the bending in the mobile phone industry.
Usually it is immersion gold surface treatment for the Flex circuit PCB whole board, unlike the rigid board there will be the pattern process, so copper plating is not required to be too thick, generally 0.1 ~ 0.3 mil will be the most suitable. (In the copper plating the hole copper and surface copper deposition ratio is about 1: 1) but in order to ensure the quality hole copper and make sure the hole copper and the substrate is not layered during SMT high temperature, and the conductivity and communication of the products after installed, the copper thickness should be 0.8 ~ 1.2mil or more.

We may have a question, perhaps someone will ask, the surface copper requires only 0.1 ~ 0.3mil, and (without the substrate copper) the hole copper requires 0.8 ~ 1.2mil or more, how to do it? This requires the additional process, the general FPC board manufacturing process should be (if only require plating 0.4 ~ 0.9mil): cutting base material → drilling → copper (black hole) → plating copper (0.4 ~ 0.9mil) → graphics → post-Process.
While for the bending requirement, manufacturing process should be: cutting base material → drilling → copper (black hole)→plating(electric 0.1 ~ 0.3mil) → through-hole drilling → second plating (0.4 ~ 0.9mil) → graphics → post- process.

A.6, Copper plating requirements
To produce the multi-layer board, we should add the same-hole treatment process before copper plating, in order to remove the glue in the hole which formed during the drilling. If it is automatic copper plating line, there won’t be any problem if the equipment and chemical water are normal. If it is manual copper plating line, especially fore the multi-layer board manufacturing, we should add the vibrator to the cylinder, preheat cylinder, activated cylinder, accelerating cylinder, and copper plating cylinder, and only produce when ensuring that the equipment swing normally, so that the potion can penetrate deeper into the via hole and go inside. The most important is the concentration of each pot are in the normal control range.