1. The structure of FPC:
According to the layer of conductive copper, FPC is divided into single layer, double layer, multilayer, dual access, etc.
The structure of single layer: the most simple. Base material + transparent adhesive + copper is one set of the raw material; coverlay + adhesive is another. Firstly, the copper needs to be etched to get the traces, while the coverlay needs to be drilled to expose corresponding soldering pad. After cleansing treatment two set of raw material are bonding together by rolling. Immersion gold or tin is done on the exposed soldering pads to protect from oxidization. At last, FPC is punched into smaller ones of corresponding shape.
Some FPC doesn’t use coverlay but prints solder mask on the copper directly, which lowers the cost, but worsens the mechanical strength of FPC. Using coverlay is the best, unless at the occasion of low mechanical strength and the lower price the better,
The structure of double layer FPC: when the circuits are too complicated, the single layer FPC can’t or needs copper to do ground shield, double layer even multilayer FPC is needed.
The most typical difference between multilayer and single layer FPC is that the multilayer FPC is added with through-hole structure to connect the copper on every layer. The first workmanship of base material + transparent adhesive + copper is to make through holes. Holes are drilled o the base material and copper, after cleansing treatment they are plated with a certain thickness of Cu, then the through hole is done. We call it PTH (plated through hole). The technique after is almost the same with single layer FPC.
The structure of dual access FPC: dual access FPC has soldering pads on its two sides, mainly used to connect with other circuit boards. Although it has similar structure with single layer FPC, its technique has great difference. Its raw material is base material + adhesive + copper, coverlay + adhesive. Holes are drilled on the coverlay according to soldering pads location, then stuck on the copper. After etching the soldering pads and traces another coverlay with drilled holes is stuck on that.
2. The performance and selecting method of the materials
(1) Base material:
PI (Polyimide) is the most common used, which is a kind of macromolecule material of high temperature proof and high strength, invented by Dupont. The polyimide made by Dupont is called Kapton. Besides some polyimide made in Japan can also be found. Its price is much lower.
Polyimide can bear 10 seconds of high temperature of 400 degree centigrade. Its tensile strength is 15,000~30,000 PSI.
The base material with 25um thickness is the cheapest and the most popular one. If we need the circuit board to be more rigid, we should choose 50um. Otherwise, if we need the circuit boards to be more flexible, then choose 13um.
It is divided into epoxy resin and polyethylene, they are all thermosetting adhesive. The polyethylene’s strength is lower. If we hope the circuit boards to be more flexible, then choose polyethylene. The thicker the base material and the adhesive on it is , the more rigid the circuit boards are. If the circuit boards have to be bent in a large area, we should choose thinner base material and adhesive so as to reduce the stress on the surface of the copper, then lower appearance of micro crack.
It is divided into rolled annealed copper foil and electrodeposited cooper foil. Rolled annealed copper foil is of high strength and can stand bending, but its price is higher. Electrodeposited copper foil is much cheaper, but of low strength, easier to be bent.
When we choose to use rolled annealed copper foil, we should pay attention to the direction of the copper. The direction of the copper has to be the same with the bending direction of the circuit boards.
(4) Coverlay and its adhesive:
25um coverlay would make the circuit board more rigid, but it is cheaper. Therefore to the circuit board needing to be bent greatly, we’d better choose 13um coverlay.
The adhesive is also divided into epoxy resin and polyethylene. The FPC using epoxy is more rigid. After thermosetting, there are some adhesive squeezed out at the edge of the coverlay. If the soldering pads are larger than the hole on the coverlay, the adhesive squeeze out would reduce the size of the soldering pads and lead to irregular edge. Therefore, we’d better choose 13um adhesive.