The advantages and disadvantages of FPC

A: Disadvantages:

1. Original cost is high

Because the FPC is designed and made for special application, the cost of circuit design, wiring and photographic plate is high. Except special need, we’d better not use them.

2. The modification and correction is difficult

Once FPCs are made, if we want to modify, we have to begin from the base drawing or gerber program. Therefore it’s difficult to change. On its surface there is a coverlay. Before the modification, we have to remove it, after the modification, we have to recover it, which is difficult.

3. Dimension limitation

Since FPC is not popular, we usually use intermittent method to make them. Then we can’t make too long or wide FPC with the limit of manufacture instruments dimensions.

4. Easier to be damaged if improper operation

It is easier to cause the damage of FPC circuits because of improper operation. Special trained workers are needed in tin plating and rework

 B: Advantages

FPC is the printed circuit board which is made of flexible insulating base material. It has many advantages many rigid PCBs don’t have.

1. It can be bent, rolled, folded freely, can be arranged randomly according to the space, can be moved and stretched in 3-D space, then integrate the components assembly and wiring connection.

2. It can satisfy the need of electronic products to develop in the direction of high density, smaller size, higher reliability, since FPC can greatly reduce the volume and weight of electronic products. Therefore FPC is widely applied in fields or products of aerospace, military, mobile communication, laptop, computer exterior design, PDA, and digital camera.

3. FPC also has the advantages of good thermal diffusivity and solderability, easy to assembly, low combined cost. The design of rigid-flex boards also make up the disadvantage of components carrying ability of flexible base material.

FPC Application

At present, the most important consumer electronic product promoting the development of FPC is mobile phone. There is a large quantity of FPCs used in cell phones, such as the cell phone keyboard, battery, display screen, display components, and motherboards.

 The second most important is LCD. LCD is divided into categories such as TN, STN and TFT. TN type LCD is applied in black and white display products, such as calculators and electronic watches; STN is mainly applied in colorful display products; TFT is mainly applied in medium-big flat panel display, such as notebooks, LCD Monitor, LCD TV. Due to the stable growth of those consumer products in recent years, FPC also grows stably. Its market and manufacture is mainly distributed in Japan. The combined growth rate of the IC substrate market in 2001~2006 promotes the corresponding growth of FPC.

The PCB market in Germany is recovering gradually

A piece of good news is coming from the PCB market in Germany: the sales of PCB manufacturer in July is increased by 7.2% than last month, which is the third time constant increase recently.

 This index is the highest since November in 2008. However, compared with the same time last year, it is reduced by 1/8. The orders in July, 2009 are unexpected good, which is increased by 22% than in June, even 2.5% higher than in July 2008. Regarding the first seven months this year, this number only reaches the half of the level at the same time last year.

The packing method of FPC

The most common packing method of FPC is to pile up 10~20pcs flexible printed circuit boards (FPC) together, fix every part on the rigid board by using paper strips. We should avoid using tape, because if the chemical substances in the tape adhesives oozes, the terminal would be easy to be oxidized and change color. The base material is polyimide which is easy to absorb moisture. The flexible printed circuit boards together with silica gel, etc. the drying agents have to be put into the polythene bag and sealed. Then put it into the cartons together with cushion material. Due to special shapes of FPC, we should adopt different packing methods according to different shapes.

 Before punching the outline of flexible printed circuit boards, firstly stick the FPC on the polyester supporting spring printed with adhesives, using knife mold to cut the outline, and then send to our clients. The clients can get the flexible printed circuit boards and do the assembly, or they can do the assembly at first, take them from the polyester supporting spring after finishing the assembly. This method can be only applied on the products with small dimensions. Which can greatly improve the operation efficiency  whatever to flexible printed circuit boards manufacturers or to clients.

 The safest way is to use special tray, at first we prepare the trays according to the FPC types. Although the management is complex, the quality is guaranteed. They are convenient to use and helpful for client assembly, low cost and can be thrown away thereafter.

The structure, technique and design of FPC (2)

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.

 (2) Adhesive:

 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.

 (3) Copper:

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.

The structure, technique and design of FPC (1)

As more and more clamshell phones come into market, the FPCs are more and more used.

 According to the combination ways of the base material and the copper, the FPCs can be divided into two kinds: adhesive FPC and adhesiveless FPC.

 The price of adhesiveless FPC is much higher than the adhesive FPC. However it is superior to FPC in the aspects of flexibility, boning force of copper and the base material, and the flatness of soldering pad. Therefore generally it only is used in the occasion of high demand, such as COF (chip on flex, to mount exposed chips on FPC has high requirement of the soldering pad flatness).

 Due to adhesiveless FPC’s high price, at present most of the FPCs applied at the market are adhesive.

 Because the FPC is mainly used at the occasion of bending, if the design or technique is not proper, defaults such as micro-crack, snap of the welding point will be easy to arise. I will introduce the structure of FPC and its special requirements on design and technique.

The advantage of using FPC

The flexible printed circuits are designed to save room and improve product design flexibility, which can satisfy the design need of smaller and higher density installation, in favor of decreasing mounting steps and improving reliability, and become the only solution for satisfying the miniaturization and flexibility of electronic products.

 The flexible circuit boards are to etch the copper traces on the polymer base material, or to print polymer heavy layer circuit. In regard to thin, light, tightly and complicated structured components, their design scheme comprises from single sided conductive circuit to complex multilayer 3-D packing. The overall quality and volume of flexible packing decreases by 70% than using the traditional mega wire thread method. The flexible circuit can also increase its strength by using supporting material or lining board, so as to obtain extra mechanical stability.

 The flexible circuits can be moved, bent, twisted, without ruining the traces. They are with different shapes and special packing dimensions. The only limit is the problem of room and volume. Due to being able to bear millions of dynamic bending, the flexible circuits can be well applied in the inner connection system of continuous movement or regular movement, becoming one part of the final product function. Some products demanding for smaller shape modulus/packing dimensions because of the movement of electrical signal/power all benefit from the flexible circuits.

 The flexible circuit provides good electrical performance. Lower dielectric constant allows electrical signals to transmit quickly. Good thermal performance makes the assembly easy to cool. Higher glass transfer temperature or melting point makes the assembly work well at higher temperature.

 Because the quantity of the hardware needed for inner connection is reduced, such as the welding pots、trunk lines, base board traces and cables usually used in traditional electric packing, the flexible circuits can provide with higher installation reliability and yield. When the traditional inner connection hardware comprised of many complicated systems is mounted, higher rate of assembly out of alignment happens easier. As the emergence of quality engineering, a very thin flexible system is designed to be mounted in only one way, so as to eliminate the human error related to independent wiring installation。

 At early stage the flexible circuits are mainly applied in the fields such as the connection between the small or thin electronic products and rigid printed boards. In the end of 1970s, they were used in the electrical products such as computers, digital cameras, ink jet printers, car audio, CD-ROM and hard disk driver. Opening one 35mm camera, there are 9~14 different flexible circuits. The only way to decrease volume is smaller assembly, more precise lines, tighter spacing, and bendable objects. Heart maker, medical instruments, video camera, hearing aid, laptops, all most all the products we use comprise of flexible circuits today.

NB board and FPC: the only hope in 2009 PCB industry

Look into the prospect of 2009 PCB industry, due to the fact that the Notebook (NB) and intelligent mobile phone still have the potentiality to grow, therefore, people in the industry generally believe that there are still hope for the growth of NB board and FPC.

Because the prices of NB fall year after year, and people’s demand for PC mobility increases constantly, the tendency that NB will replace the desktop computer becomes more and more obvious. Some brokers in Taiwan expect that the worldwide turnout of NB in 2009 will be 131 million sets, increasing by 3.7% than that in 2008. Considering the additional demand for low cost NB, they estimate that the turnout of whole NB will reach 155 million sets. Annual growth rate will reach 12.58%. This growth prediction is rarely seen recently. Therefore it’s unnecessary for the NB board manufacturers to worry about. The NB turnout of Hann Star board and Gold Circuit Electronics in 2008 will increase 7% and word wide market share will rise from 63% (in 2007) to 70% which will make NB market a duopoly and there is nearly impossible for a new company entering into.

Besides, the market generally believes that the overall mobile phone demand in 2009 will decline. For intelligent mobile phones which use lot of FPC, however, the turnout in 2009 is estimated to be 186 million sets. The annual growth rate is still as high as 18.1%, which becomes the largest growth power for Taiwan FPC industry. As for the aspect of supply, because of the fierce competition, many second-tier FPC factories became bankrupt, which will be helpful to keep the  stability of FPC industry.