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
  
• 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
  

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.

Where To Learn More

Books

Braithwaite, Nicholas and Graham Weaver, eds.

Electronic Materials.

Butterworths, 1990.

Koshel, Dal., ed.

Manufacturing Engineer’s Reference Book.

Butterworth-Heinemann, 1993.

Lotter, Bruno.

Manufacturing Assembly Handbook.

Butterworths, 1986.

Periodicals

Alford, William. “Screen Printing PC Boards.”

Electronics Now,

September 1993, pp. 38-41.

Fernando, James R. “Successful Implementation of a CIM Strategy
for a PCB Manufacturing Facility.”

Electronic Manufacturing,

March 1990.

Kirkland, Carl. “What Ever Happened to Molded 3D Circuit
Boards?”

Plastics World,

February 1993, pp. 32-36.

Nishioka, Alan. “Iron-On PC Board Patterns.”

Electronics Now,

September 1993, pp. 42-45.

Yam, Philip. “Plastics Get Wired.”

Scientific American,

July 1995, pp. 82-87.

—

David


N.


Ford

Chris


Cavette