An Overview of Types of Printed Circuit Boards (PCBs)

The evolution of printed circuit board (PCB) technology since its inception by Dr. Paul Eisner in 1936 has been remarkable. Over the years, several methods and processes have been developed to manufacture different types of PCBs. However, with the ever-increasing demand for sophisticated electronic devices, certain trends have emerged that significantly impact the type of PCBs required and the manufacturing processes used.

Firstly, with the proliferation of computers and portable telecommunications equipment, there is a growing demand for high-frequency circuits, boards, and materials. Moreover, these devices now use more functional components that generate considerable amounts of heat, necessitating effective heat extraction mechanisms.

Secondly, consumer products have evolved significantly, incorporating digital products into their design. This trend has resulted in the need for more functionality at ever-lower total costs.

Finally, products for all applications continue to shrink in size while becoming more functional. As a result, the total circuit package has become more dense, requiring PCBs to evolve to meet these needs. These trends have driven the development of new materials and manufacturing processes to create PCBs that meet the demands of modern electronic devices.

Classification of PCBs

The rapid evolution of electronics technology has brought about changes in the design and manufacturing of Printed Wiring Boards (PCBs). These changes have resulted in the increased use of nonorganic base substrates such as aluminum and soft iron, and the development of alternate ways to create boards. In this article, we will explore the classification of PCBs, including traditional board structures and processes. Throughout the article, the terms printed wiring board, PCB, and board will be used interchangeably, while the words laminate, substrate, and panel will be used synonymously. The main objective of any PCB is to provide electrical conductor paths that interconnect components to be mounted on them. There are many different ways to classify PCBs, based on their attributes, but they all share this common fundamental structure.

Basic PCB Classifications

Different methods and processes are used to create electrical conductor paths on Printed Wiring Boards (PCBs). Two basic methods of forming these conductors are the subtractive and additive processes.

• 1. Subtractive: In the subtractive process, the copper foil on the base substrate is etched away to remove unwanted portions, leaving the desired conductor pattern in place.
• 2. Additive: In the additive process, copper is added to a bare substrate in the pattern and places desired. This can be done by plating copper, screening conductive paste, or laying down insulating wire onto the substrate on the predetermined conductor paths.

When classifying PCBs, it is important to consider these factors, including the fabrication processes and substrate materials used. Figure 2.1 shows various PCB classifications that take into account these factors. This figure can be used to identify the appropriate PCB classification for a particular application based on its fabrication process and substrate material.

FIGURE 1 Classification of printed wiring boards.

• Column 1 shows the classification of PCBs by the nature of their substrate.
• Column 2 shows the classification of PCBs by the way the conductor pattern is imaged.
• Column 3 shows the classification of PCBs by their physical nature.
• Column 4 shows the classification of PCBs by the method of actual conductor formation.
• Column 5 shows the classification of PCBs by the number of conductor layers.
• Column 6 shows the classification of PCBs by the existence or absence of plated-through-holes (PTHs).
• Column 7 shows the classification of PCBs by production method.

Organic and Nonorganic Substrates

The continuing demand for faster and more functional components in computers and telecommunications has resulted in a need for compatible materials for PCB substrates. These materials must withstand the stresses caused by longer and more frequent exposure to soldering temperatures during the assembly process, and also have a coefficient of thermal expansion that matches that of the components and substrate. In response to this demand, a search for new materials has led to the development of both organic and nonorganic substrates. Chapters 6 through 11 contain detailed explanations of these materials, but for now, it is important to understand the basic characteristics of these two substrate types.

Organic Substrates

Organic substrates are composed of multiple layers of paper that have been infused with phenolic resin, or multiple layers of woven or non-woven glass cloth that have been infused with materials such as epoxy resin, polyimide, cyanate ester, BT resin, among others. The choice of substrate material is dependent on the specific physical properties needed for the application of the printed wiring board, such as operating temperature, frequency, or mechanical strength.

Nonorganic Substrates

Nonorganic substrates are a popular choice for printed wiring boards (PCBs) due to their excellent thermal conductivity and stability under high temperatures. These materials include ceramics and metals like aluminum, copper-invar-copper, and soft iron. The selection of a particular substrate is determined by the specific requirements of the PCB application, such as the need for efficient heat dissipation or a particular magnetic characteristic. Soft iron, for instance, is used in PCBs for flexible disk motor drives to provide a magnetic flux path.

GRAPHICAL AND DISCRETE-WIRE BOARDS

Printed wiring boards may be classified into two basic categories, based on the way they are manufactured:

• 1. Graphical
• 2. Discrete-wire

Graphical Interconnection Board

A graphical PCB is the standard PCB and the type that is usually thought of when PCBs are discussed. In this case, the image of the master circuit pattern is formed photographically on a photosensitive material, such as treated glass plate or plastic film. The image is then transferred to the circuit board by screening or photoprinting the artwork generated from the master. Due to the speed and economy of making master artwork by laser plotters, this master can also be the working artwork.

Direct laser imaging of the resist on the PCB can also be used. In this case, the conductor image is made by the laser plotter, on the photoresistive material, which is laminated to the board, without going through the intermediate step of creating a phototool. This tends to be somewhat slower than using working artwork as the tool and is not generally applied to mass production. Work continues on faster resists, as well as exposure systems, and this method will undoubtedly continue to emerge.

Discrete-Wire Boards

One type of interconnection technology used in printed wiring boards is the discrete-wire board. This method involves forming signal conductors directly onto the board using insulated copper wire instead of an imaging process. Wire-wrap® and Multiwire® are two well-known technologies for this type of board. Because wire crossings are allowed, a single layer of wiring can match multiple conductor layers in the graphically produced boards, resulting in very high wiring density. However, the sequential nature of the wiring process means that the productivity of discrete-wiring technology is not suitable for mass production. Despite this limitation, discrete-wiring boards are used in some high-density packaging applications. For an example of a discrete-wiring board, refer to Fig. 2.2.

RIGID AND FLEXIBLE BOARDS

Another class of boards is made up of the rigid and flexible PCBs. Whereas boards are made of a variety of materials, flexible boards generally are made of polyester and polyimide bases. Rigi-flex boards, a combination of rigid and flexible boards usually bonded together, have gained wide use in electronic packaging (see Fig. 2.3). Most rigi-flex boards are three dimensional structures that have flexible parts connecting the rigid boards, which usually support components; this packaging is thus volumetrically efficient.

GRAPHICALLY PRODUCED BOARDS

The majority of boards produced in the world are graphically produced. There are three alternative types:

• 1. Single-sided boards
• 2. Double-sided boards
• 3. Multilayer boards

Single-Sided Boards

Single-sided boards (SSBs) are printed wiring boards that have circuitry on only one side of the board. These boards are also known as print-and-etch boards because the etch resist is typically printed onto the board using screen-printing techniques. The conductor pattern is then formed by chemically etching the exposed and unwanted copper foil.

Typical Single-Sided Board Materials.

Single-sided boards (SSBs) are commonly used for low-cost and low-functionality boards in high-volume production. The materials used for SSBs vary depending on the region. In the Far East, paper-based substrates are the most popular due to their low cost, with XPC-FR being the most commonly used flame-retardant phenolic material. In Europe, FR-2 grade paper laminate is preferred as it emits less odor in high-temperature environments. In the United States, CEM-1 material, which is a composite of paper and glass impregnated with epoxy resin, is the most popular due to its mechanical strength and availability. While XPC-FR and FR-2 are the cheapest options, CEM-1 has become more popular due to its strength and durability.

Single-Sided Board Fabrication Process

Given the emphasis on cost and low complexity, SSBs are generally produced in highly automated, conveyorized print-and-etch lines, using the following basic process flow.

• Step 1: Cut substrate into appropriate panel size by either sawing or shearing.
• Step 2: Place panel in loader which feeds them into the line.
• Step 3: Clean panels.
• Step 4: Screen panel with ultraviolet curable etch-resist ink.
• Step 5: Cure the etch-resist ink.
• Step 6: Etch exposed copper.
• Step 7: Strip the resist.
• Step 8: Apply solder resist.
• Step 9: Screen legend.
• Step 10: Form holes by drilling or punching.
• Step 11: Test for shorts and opens.

Automated print-and-etch lines are machines used to generate conductor patterns on printed wiring boards (PCBs). The speed of the conveyor on these machines typically ranges from 30 to 45 feet per minute. Some lines are also equipped with online optical inspection capabilities, allowing for the elimination of a final electrical open/short test.

When creating PCBs, holes for component insertion must be formed on the panel. If the panel is made of a paper-based substrate, holes are formed by punching. However, if the panel is made of a glass-based substrate, holes must be formed by drilling.

There are some process variations that involve insulating the conductor surface of the PCB and exposing only pads. Conductive paste is then screened onto the same side of the board to form additional conductors, creating double conductive layers on a single side.

The aluminum substrate is a common material for metal-core PCB consumer applications, and it comes as a copper-clad material. PCBs made of this material do not have through-holes, and components are typically surface-mount types. These circuits can also be formed into three-dimensional shapes.

Double-Sided Boards

By definition, double-sided boards (DBs) have circuitry on both sides of the boards. They can be classified into two categories:

• 1. Without through-hole metallization
• 2. With through-hole metallization

The category of through-hole metallization can be further broken into two types:

• 1. Plated-through-hole (PTH)
• 2. Silver-through-hole (STH)

Plated-Through-Hole Technology

Metallization is a process used in printed wiring board (PCB) manufacturing where holes in the board are filled with a conductive material, usually copper, to allow for electrical connections between the different layers of the board. Since the substrate material of a PCB is insulating, the holes must be made conductive before the copper can be plated onto them. The traditional method is to catalyze the holes with a palladium catalyst and then plate them with electroless copper before thicker plating is done by galvanic plating. Alternatively, additive plating can be used to plate all the way to the desired thickness.

However, direct metallization technologies have become popular for double-sided PTH boards and multilayer boards. This process eliminates the use of electroless copper plating by making the hole walls conductive using a palladium catalyst, carbon, or polymer conductive film, and then depositing copper by galvanic plating. This process eliminates the need for environmentally hazardous chemicals like formaldehyde and EDTA that are used in electroless copper-plating solutions. Direct metallization technology is more environmentally friendly and efficient compared to traditional metallization methods.

Silver-Through-Hole Technology.

Silver-Through-Hole Technology (STH) is a type of printed wiring board (PCB) that is made of paper phenolic or composite epoxy paper and glass materials. The manufacturing process involves forming conductor patterns on both sides of the panel by etching double-sided copper-clad materials, followed by drilling holes in the panel. The holes are then filled with silver-filled conductive paste or copper paste.

STH boards have a higher electrical resistance compared to PTH boards, limiting their are a cost-effective option, with the cost of STH boards being typically half to two-thirds that of functionally equivalent PTH boards. Due to this economic advantage, STH boards are often used in high-volume, low-cost products such as audio equipment, floppy disk controllers, car radios, and remote controls.

Multilayer Boards

Multilayer boards are circuit boards that have three or more layers of conductive pathways. They were originally used in advanced electronic products but are now commonly used in all kinds of electronic devices, including consumer products such as cameras, phones, and audio devices.

Layer Count. 

As personal computers and workstations become more advanced, they are being used more often than mainframe computers and supercomputers. This means that the need for very complex multi-layer boards with over 70 layers is decreasing, but we still have the technology to produce them. On the other hand, thin and high-density multi-layer boards with only 4 to 8 layers are becoming more common. We are constantly trying to make these boards even thinner, and thanks to improvements in materials and equipment, we are able to handle these thinner materials.

Via and Via Production Technologies. 

As technology advances and the need for more compact and efficient devices increases, the demand for multilayer printed wiring boards (PCBs) has also risen. Communication between layers in these boards has become essential, which means that the use of vias has also increased. However, the space for vias has decreased, leading to a trend toward smaller and more numerous holes. To accommodate this, buried and blind vias have become standard for multilayer boards.

The increased use of vias has created a problem for fabricators as drilling individual holes for each board has become time-consuming and expensive. This has led to the development of alternate methods for creating vias that do not require drilling. These methods are being developed to mass-produce vias and reduce the cost of fabrication. As technology continues to advance, these processes will become increasingly important for PCB production.

Heading 5: Surface Laminar Circuits (SLCs)

The most notable MLB technology developed to form vias is the sequential fabrication of multilayers without press operations. This is particularly important for surface blind via holes.

The process for fabricating a board using surface laminar circuits is as follows:

• 1. Innerlayer ground and power distribution patterns are formed.
• 2. Panel receives an oxide treatment.
• 3. Insulating photosensitive resin is coated over the panel by curtain or screen-coating methods.
• 4. Holes are formed by photoexposure and development.
• 5. Panel is metallized by usual copper reduction process (consisting of catalyzing and electroless copper plating or by direct metallization process).
• 6. Thicker deposition of copper is made by continuation of electroless copper plating or galvanic plating.
• 7. Circuit patterns are formed by dry film tenting process.

Heading 5: DYCOstrate®

Dyconex AG, a Swiss company, has developed a new method for creating small vias (tiny holes in printed circuit boards). First, the ground and power patterns are formed on the board and the board is treated with an oxide layer. Then, a layer of copper foil backed with polyimide is laminated onto the board. Holes are created in the copper layer using a chemical etching process, and the insulating polyimide material underneath the holes is removed by plasma etching. The resulting printed circuit boards are called DYCOstrate.

In other similar technologies, different materials are used to insulate the holes, which are then removed using alkaline solutions. The remaining process is similar to that used for standard printed circuit boards, where the holes are metalized, a thick copper layer is deposited using electroless or galvanic plating, and the circuit pattern is formed using a tent-and-etch process.

Heading 5: Drilled Vias

In both SLC and DYCOstrate cases, through-holes can also be made by conventional drilling and plating processes, in addition to the surface blind via holes.

Heading 5: Cost Impact.

The manufacturing cost of these sequential technologies is not necessarily directly cheaper than conventional MLB technology, which depends on a laminating press operation. However, since the cost of making standard holes in a board can be as high as 30 percent of the total manufacturing cost, and the creation of holes in these processes is comparatively inexpensive, the overall cost for equivalent functionality can be less. In addition, the fine pattern capability for this process is excellent. For example, an eight-layer conventional structure can often be reduced to a four-layer structure, reducing the total cost for the same packaging density.