Printed circuit board

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Printed circuit board with components attached

A circuit board is a card made specially for attaching electronic components. The board is made of a material that does not conduct electricity, like fiberglass or plastic. Parts are then attached to this base using a conductive bonding material. This allows electricity to travel from one part to another, so the parts can work together. Inside the board, between the layers of insulating material, is copper for connecting the electricity between the electronic components.[1]

Boards carry many different parts which are connected together in electrical circuits to do their different tasks. The most common circuit boards are called PCBs (printed circuit boards) and are most commonly mass produced ready for a specific job. Some circuit boards are made plain so a person can build their own for a particular electrical task. Most things that use electricity have at least one circuit board inside of them that makes them run.

Flexible circuit boards are those that are made thin enough to flex.

History[change | edit source]

Printed circuit boards came from electrical connection systems that were used in the 1850s. Originally metal strips or rods were used to connect large electric components mounted on wooden bases. The metal strips were replaced by wires connected to screw terminals, and wooden bases were replaced by metal frames. Smaller designs allowed spacing components more closely, for increasingly complex circuits. In 1925, Charles Ducas of the United States submitted a patent application for a method of creating an electrical path directly on an insulated surface by printing through a stencil with electrically conductive inks. This method gave birth to the name "printed wiring" or "printed circuit."

In 1943, Paul Eisler of the United Kingdom patented a method of etching the conductive pattern, or circuits, on a layer of copper foil bonded to a glass-reinforced, non-conductive base. Eisler's technique was noticed by the US military and they started to use it in their new weapons in World War II. His idea did not come in to commercial use until the 1950s when the transistor was introduced. Up to that point, vacuum tubes and other components were so large that the traditional mounting and wiring methods were all that was needed. With the introduction of transistors, however, the components became very small, and manufacturers turned to printed circuit boards so the connections could also be small.

Plated through-hole technology and its use in multi-layer PCBs were patented by the U.S. firm Hazeltine in 1961. The resulting increase in the complexity and detailed paths started a new era in PCB design. Integrated circuit chips were introduced in the 1970s, and these components were quickly incorporated into printed circuit board design and manufacturing techniques. Today the Printed Circuit Board can have up to 50 layers in some applications.

Surface-mount technology was developed in the 1960s and became widely used in the late 1980s.

Design[change | edit source]

The main task in designing a PCB is figuring out where all the components are going to go. Normally there is a design or schematic that will be turned into a PCB. There is no such thing as a standard printed circuit board. Each board is designed for its own use and must be the right size to fit the required space. Board designers use computer-aided design software to layout the circuit designs on the board. The spaces between electrical paths are often 0.04 inches (1.0 mm) or smaller. The location of the holes for component leads or contact points are also laid out. Once the circuit pattern is laid out, a negative image is printed out at exact size on a clear plastic sheet. With a negative image, the areas that are not part of the circuit pattern are shown in black and the circuit pattern is shown as clear. This design is made into instructions for a computer-controlled drilling machine or for the automatic solder paster used in the manufacturing process. [2]

Manufacturing[change | edit source]

Photo-resist[change | edit source]

Most circuit boards are made with a light sensitive covering, for photolithography. This covering is sometimes called photo-resist. The covering reacts with light. Then the circuit board and covering are put in a developer. Developer is a liquid that dissolves the covering that was exposed to light. Then the board is put in an etchant. An etchant dissolves the copper that is not covered by the covering. Then the board is put into another liquid that removes the covering.[3] Some companies also use this method. It is expensive per board, but very cheap to set up in the beginning.

Silkscreen[change | edit source]

Some professionally made circuit boards use a different method to remove extra copper from the circuit board. A process called silk-screening is used. Silk-screening is when a cloth is pulled tight over a frame. Then an image is printed onto the cloth. Then ink is pressed through the cloth. The ink does not go where the image has been printed on the cloth. It is called silk-screening because the cloth is usually silk. The cloth is usually silk because it has very small holes. silk-screening is used to print an ink called resist onto the board. Resist is an ink that resists the etchant used to make the circuit board. Etchant dissolves the copper on the board. This is cheaper for each board than photo-resist, but is more expensive in the beginning.

Milling[change | edit source]

Another way to make a circuit board is to use a mill. A mill is a drill that moves in many directions. The drill removes a small amount of copper each time it moves. The mill removes the copper around the wires on the board. This leaves extra copper on the board. Other methods do not leave the extra copper on the board. This method is cheaper per board, but is expensive in the beginning. It is also not commonly used in professionally made boards, because the first two ways are better.

Notes[change | edit source]

  1. Rozenblat, L. (2008). PCB Printed Circuit Board Design – Guidelines, Layout Tutorials, Software. Retrieved April 26, 2009, from [1]
  2. Ford, D. N. (n.d.). How printed circuit board is made – Background, History, Design, Raw materials, The manufacturing process of printed circuit board, Quality control. (C. Cavette, Editor) Retrieved April 26, 2009, from [2]
  3. (2009, March 9). In-house PCB Manufacture. Retrieved April 26, 2009, from [3]