The basic knowledge of PCB

I saw some basic knowledge about PCB, today I summarized it and I want to share with you now.

1. Definitions:

For standard PCB, it is the combination of bare board and components on it. While for bare board PCB, it is called printed wiring board. We usually call it PWB.

(1) Traces:

The base material of PCB is comprised of dielectric, heat insulation, uneasy to bend materials. On its surface, we can see thin traces, its material is copper. Originally the copper covered the whole PCB. Parts of it were etched during the process of production. Those parts left become net-like thin traces, which are called conductor pattern, and are used to provide circuit connection for components on PCB.

(2) Name of two side:

On most common single sided PCB, components are on one side, conductor pattern is on the other side. Then we would need to drill holes on the board, so that pins can put through the board to the other side. Therefore, the both sides of PCB are called component side and solder side.

(3) Gold finger:

Gold finger is functioned to connect two PCBs. We usually insert the gold finger of one PCB to the proper slot of the other PCB. 

(4) Solder mask and silk screen:

The green or white oil on PCB is called solder mask, which is functioned as dielectric protective layer, can protects copper traces and also can prevent components to be soldered at incorrect place.

On the dielectric layer, a layer of silk screen will be printed. Some words will be printed to show the position of those components. It is also called legend.

FPC Application Market Analysis

The market of FPCs applied on hard disk still can secure a place, although its growth rate is lower. The speed of NAND replacing the hard disk won’t be so fast. At present in the times of information explosion, the quantity of storage increases rapidly. The hard disk can corresponds very well. However NAND doesn’t work. Although NAND’price reduces quickly too, it can’t match the increasing speed of mainstream storage density. There are only 6 manufacturers of hard disks, and the quantity will reduce; while the number of FPC manufacturers is much larger. They have little space to discuss the price. Meanwhile, the patterns of FPCs needed for hard disk are so unitary. All of those determine that the growth of FPC in hard disk market is strong.

The situation in optical drive industry is similar to that in  hard disk industry, thanks to the increase of Blue Ray. The market of FPC in printer is also relatively stable. The electronics of automobiles is obvious too. On the occasion of using ECU, FPCs are always needed. They are of high reliability and space superiority.

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FPC Application Market Analysis

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flx24       As the development of high technology products, FPC becomes more and more popular. Its application becomes wider and wider. Therefore, let us analyze its market prospects.

 Industry research institute Shuiqingmuhua published “global and China circuit board industry research report of 2009”. The FPC industry emerged in 2002, developed very quickly in 2003 and 2004, and went into the stage of price struggle. In 2006 and 2007 the FPC industry has been in downturn, however in 2008 it began to recover. Even the economy crisis didn’t get in the way of the recovery of FPC industry. On one hand, the price reduction stimulates the demand of FPC; on the other hand, the demand for lighter, thinner, shorter, smaller and high reliability products also creates the room of using FPC. At the same time, the FPC manufacturers which went through price struggle and survived had all faced great pressure. They have already been familiar with the cost control and dealing with stress, and won’t be afraid of the arrival of economy crisis. Cell phone is the critical motive for FPC market enlargement, especially intelligent cell phone and high-end cell phone. The growth of consumer electronics field is also favorable. In consumer electronics such as notebooks, PND, digital camera, DV, LCD TV, plasma TV, media player, frequency of using FPC will increase as its performance and its strive for ultra-thin volume. Notebook and flat panel TV use LED backlights a lot. FPC is the most common used way in which LED connects to the main circuit. Even in 2008 and 2009 it still has great potential to grow. The swing joint between notebook screen and motherboard has great probability of using FPC. (Continued)

How to calculate the thickness of FPCs

As is known to all, the FPCs are divided into single layer, double layers, dual access, multiayer and flex rigid.

Now I want to introduce the basic method of calculating the thickness of FPCs.

(!) single sided FPC

The constructure of single layer FPC is as follows:the board thickness for single sided FPC

25um+20um+35um+20um+25um=125um≈0.12mm

Note: the thickness of adhesive changes according to the thickness of Cu. For 1/2, 1/3 OZ Cu, we usually use 12um adhesive. But for 1OZ Cu, we have to use 20um adhesive.

That’s according to the common material. If we want the board to be thinner, we also can use the adhesiveless material to make the board about 0.8mm thick.

(2) double sided FPC

Double sided FPC means there are two layer of traces in the FPC.

The constructure of double sided FPC is as follows:

the board thickness for double sided FPC25+12+35+12+25+12+35+12+25=193um≈0.20mm

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.