Printed Circuit Board

Background

A printed circuit board, or PCB, is a self-contained module of interconnected electronic components found in devices ranging from common beepers, or pagers, and radios to sophisticated radar and computer systems. The circuits are formed by a thin layer of conducting material deposited, or "printed," on the surface of an insulating board known as the substrate. Individual electronic components are placed on the surface of the substrate and soldered to the interconnecting circuits. Contact fingers along one or more edges of the substrate act as connectors to other PCBs or to external electrical devices such as on-off switches. A printed circuit board may have circuits that perform a single function, such as a signal amplifier, or multiple functions.

There are three major types of printed circuit board construction: single-sided, double-sided, and multi-layered. Single-sided boards have the components on one side of the substrate. When the number of components becomes too much for a single-sided board, a double-sided board may be used. Electrical connections between the circuits on each side are made by drilling holes through the substrate in appropriate locations and plating the inside of the holes with a conducting material. The third type, a multi-layered board, has a substrate made up of layers of printed circuits separated by layers of insulation. The components on the surface connect through plated holes drilled down to the appropriate circuit layer. This greatly simplifies the circuit pattern.

Components on a printed circuit board are electrically connected to the circuits by two different methods: the older "through hole technology" and the newer "surface mount technology." With through hole technology, each component has thin wires, or leads, which are pushed through small holes in the substrate and soldered to connection pads in the circuits on the opposite side. Gravity and friction between the leads and the sides of the holes keeps the components in place until they are soldered. With surface mount technology, stubby J-shaped or L-shaped legs on each component contact the printed circuits directly. A solder paste consisting of glue, flux, and solder are applied at the point of contact to hold the components in place until the solder is melted, or "reflowed," in an oven to make the final connection. Although surface mount technology requires greater care in the placement of the components, it eliminates the time-consuming drilling process and the space-consuming connection pads inherent with through hole technology. Both technologies are used today.

Two other types of circuit assemblies are related to the printed circuit board. An integrated circuit, sometimes called an IC or microchip, performs similar functions to a printed circuit board except the IC contains many more circuits and components that are electrochemically "grown" in place on the surface of a very small chip of silicon. A hybrid circuit, as the name implies, looks like a printed circuit board, but contains some components that are grown onto the surface of the substrate rather than being placed on the surface and soldered.

History

Printed circuit boards evolved from electrical connection systems that were developed in the 1850s. Metal strips or rods were originally used to connect large electric components mounted on wooden bases. In time the metal strips were replaced by wires connected to screw terminals, and wooden bases were replaced by metal chassis. But smaller and more compact designs were needed due to the increased operating needs of the products that used circuit boards. 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 the 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. Widespread use of Eisler's technique did not come until the 1950s when the transistor was introduced for commercial use. Up to that point, the size of vacuum tubes and other components were so large that the traditional mounting and wiring methods were all that was needed. With the advent of transistors, however, the components became very small, and manufacturers turned to printed circuit boards to reduce the overall size of the electronic package.

Through hole technology and its use in multi-layer PCBs was patented by the U.S. firm Hazeltyne in 1961. The resulting increase in component density and closely spaced electrical 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.

Design

There is no such thing as a standard printed circuit board. Each board has a unique function for a particular product and must be designed to perform that function in the space allotted. Board designers use computer-aided design systems with special software to layout the circuit pattern on the board. The spaces between electrical conducting 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, and this information is translated into instructions for a computer numerical controlled drilling machine or for the automatic solder paster used in the manufacturing process.

Once the circuit pattern is laid out, a negative image, or mask, 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.

Raw Materials

The substrate most commonly used in printed circuit boards is a glass fiber reinforced (fiberglass) epoxy resin with a copper foil bonded on to one or both sides. PCBs made from paper reinforced phenolic resin with a bonded copper foil are less expensive and are often used in household electrical devices.

The printed circuits are made of copper, which is either plated or etched away on the surface of the substrate to leave the pattern desired. (See "additive" and "subtractive" processes described in step 3 under The Manufacturing Process). The copper circuits are coated with a layer of tin-lead to prevent oxidation. Contact fingers are plated with tin-lead, then nickel, and finally gold for excellent conductivity.

Purchased components include resistors, capacitors, transistors, diodes, integrated circuit chips, and others.

The Manufacturing
Process

Printed circuit board processing and assembly are done in an extremely clean environment where the air and components can be kept free of contamination. Most electronic manufacturers have their own proprietary processes, but the following steps might typically be used to make a two-sided printed circuit board.

Making the substrate

• 1 Woven glass fiber is unwound from a roll and fed through a process station
  

  The above illustrations show an enlarged section of a PCB.
  where it is impregnated with epoxy resin either by dipping or spraying. The impregnated glass fiber then passes through rollers which roll the material to the desired thick-ness for the finished substrate and also remove any excess resin.
• 2 The substrate material passes through an oven where it is semicured. After the oven, the material is cut into large panels.
• 3 The panels are stacked in layers, alternating with layers of adhesive-backed copper foil. The stacks are placed in a press where they are subjected to temperatures of about 340°F (170°C) and pressures of 1500 psi for an hour or more. This fully cures the resin and tightly bonds the copper foil to the surface of the substrate material.

Drilling and plating the holes

• 4 Several panels of substrate, each large enough to make several printed circuit boards, are stacked on top of each other and pinned together to keep them from moving. The stacked panels are placed in a CNC machine, and the holes are drilled according to the pattern determined when the boards were laid out. The holes are deburred to remove any excess material clinging to the edges of the holes.
• 5 The inside surfaces of the holes designed to provide a conductive circuit from one side of the board to the other are plated with copper. Non-conducting holes are plugged to keep them from being plated
  
  or are drilled after the individual boards are cut from the larger panel.

Creating the printed circuit pattern on the substrate

The printed circuit pattern may be created by an "additive" process or a "subtractive" process. In the additive process, copper is plated, or added, onto the surface of the substrate in the desired pattern, leaving the rest of the surface unplated. In the subtractive process, the entire surface of the substrate is first plated, and then the areas that are not part of the desired pattern are etched away, or subtracted. We shall describe the additive process.

• 6 The foil surface of the substrate is degreased. The panels pass through a vacuum chamber where a layer of positive photoresist material is pressed firmly onto the entire surface of the foil. A positive photoresist material is a polymer that has the property of becoming more soluble when exposed to ultraviolet light. The vacuum ensures that no air bubbles are trapped between the foil and the photoresist. The printed circuit pattern mask is laid on top of the photoresist and the panels are exposed to an intense ultraviolet light. Because the mask is clear in the areas of the printed circuit pattern, the photoresist in those areas is irradiated and becomes very soluble.
• 7 The mask is removed, and the surface of the panels is sprayed with an alkaline developer that dissolves the irradiated photoresist in the areas of the printed circuit pattern, leaving the copper foil exposed on the surface of the substrate.
• 8 The panels are then electroplated with copper. The foil on the surface of the substrate acts as the cathode in this process, and the copper is plated in the exposed foil areas to a thickness of about 0.001-0.002 inches (0.025-0.050 mm). The areas still covered with photoresist cannot act as a cathode and are not plated. Tin-lead or another protective coating is plated on top of the copper plating to prevent the copper from oxidizing and as a resist for the next manufacturing step.
• 9 The photoresist is stripped from the boards with a solvent to expose the substrate's copper foil between the plated printed circuit pattern. The boards are sprayed with an acid solution which eats away the copper foil. The copper plating on the printed circuit pattern is protected by the tin-lead coating and is unaffected by the acid.

Attaching the contact fingers

• 10 The contact fingers are attached to the edge of the substrate to connect with the printed circuit. The contact fingers are masked off from the rest of the board and then plated. Plating is done with three metals: first tin-lead, next nickel, then gold.

Fusing the tin-lead coating

• 11 The tin-lead coating on the surface of the copper printed circuit pattern is very porous and is easily oxidized. To protect it, the panels are passed through a "reflow" oven or hot oil bath which causes the tin-lead to melt, or reflow, into a shiny surface.

Sealing, stenciling, and cutting the panels

• 12 Each panel is sealed with epoxy to protect the circuits from being damaged while components are being attached. Instructions and other markings are stenciled onto the boards.
• 13 The panels are then cut into individual boards and the edges are smoothed.

Mounting the components

• 14 Individual boards pass through several machines which place the electronic components in their proper location in the circuit. If surface mount technology is going to be used to mount the components, the boards first pass through an automatic solder paster, which places a dab of solder paste at each component contact point. Very small components may be placed by a "chip shooter" which rapidly places, or shoots, the components onto the board. Larger components may be robotically placed. Some components may be too large or odd-sized for robotic placement and must be manually placed and soldered later.
• 15 The components are then soldered to the circuits. With surface mount technology, the soldering is done by passing the boards through another reflow process, which causes the solder paste to melt and make the connection.
• 16 The flux residue from the solder is cleaned with water or solvents depending on the type of solder used.

Packaging

• 17 Unless the printed circuit boards are going to be used immediately, they are individually packaged in protective plastic bags for storage or shipping.

Quality Control

Visual and electrical inspections are made throughout the manufacturing process to detect flaws. Some of these flaws are generated by the automated machines. For example, components are sometimes misplaced on the board or shifted before final soldering. Other flaws are caused by the application of too much solder paste, which can cause excess solder to flow, or bridge, across two adjacent printed circuit paths. Heating the solder too quickly in the final reflow process can cause a "tombstone" effect where one end of a component lifts up off the board and doesn't make contact.

Completed boards are also tested for functional performance to ensure their output is within the desired limits. Some boards are subjected to environmental tests to determine their performance under extremes of heat, humidity, vibration, and impact.

Toxic Materials and
Safety Considerations

The solder used to make electrical connections on a PCB contains lead, which is considered a toxic material. The fumes from the solder are considered a health hazard, and the soldering operations must be carried out in a closed environment. The fumes must be given appropriate extraction and cleaning before being discharged to the atmosphere.

Many electronic products containing PCBs are becoming obsolete within 12-18 months. The potential for these obsolete products entering the wastestream and ending up in landfills has many environmentalists concerned. Recycling efforts for electronic products include refurbishing older products and reselling them to customers that don't need, or have access to, newer, state-of-the-art electronics. Other electronics are disassembled and the computer parts are salvaged for resale and reuse in other products.

In many countries in Europe, legislation requires manufacturers to buy back their used products and render them safe for the environment before disposal. For manufacturers of electronics, this means they must remove and reclaim the toxic solder from their PCBs. This is an expensive process and has spurred research into the development of non-toxic means of making electrical connections. One promising approach involves the use of water-soluble, electrically conductive molded plastics to replace the wires and solder.

The Future

The miniaturization of electronic products continues to drive printed circuit board manufacturing towards smaller and more densely packed boards with increased electronic capabilities. Advancements beyond the boards described here include three-dimensional molded plastic boards and the increased use of integrated circuit chips. These and other advancements will keep the manufacture of printed circuit boards a dynamic field for many years.