• Double-sided board with plated through-hole connection (PTH)
• Double-sided board without plated through-hole connection (non-PTH)

Double-sided PTH board has circuitry on both sides of an insulating substrate, which is connected by metallizing the wall of a hole in the substrate that intersects the circuitry on both sides. This technology, which is the basis for most printed circuits produced, is becoming popular in cases where the circuit complexity and density is high.

Double-sided non-PTH board is only an extension of a single-sided board. Its cost is considerably lower because plating can be avoided. In this case, through contacts are made by soldering the component leads on both sides of the board, wherever required. In the layout design of such boards, the number of solder joints on the component side should be kept to a minimum to facilitate component removal, if required. It is generally recommended that conductors should be realized as much as possible on the non-component side and only the remaining should be placed on the component side.

These types of boards are mostly used in case of simple circuitry and where the manufacturing costs are to be kept at a minimum. Nevertheless, they represent a large volume of printed boards currently produced for professional and non-professional grades.

The single-sided boards are manufactured mostly by the print and etch method or by the diecut technique by using a die that carries an image of the wiring pattern; and the die is either photoengraved or machine-engraved.

Normally, components are used to jump over conductor tracks, but if this is not possible, jumper wires are used. The number of jumper wires on a board cannot be accepted beyond a small number because of economic reasons, resulting in the requirement for double-sided boards.

A multilayer PCB contains two reference planes and a signal via. The signal via allows a signal to flow through all the planes. A stitching via is connected to one of the planes next to the signal via and serves to reduce the area through which the signal passes through. This is very important as it may assist in reducing noise and cross talk.

The electrical circuit is completed by interconnecting the different layers with plated through-holes, placed transverse to the board at appropriate places. Multi-layer boards have three or more circuit layers, while some boards have even as many as 50 layers.

Advantages of multilayer PCBs include high reliability and uniform wiring. However, the initial costs are higher than that of one-layered PCBs. Also, repairing a multilayer PCB is quite difficult.

Multilayer PCB applications

• Wherever weight and volume savings in interconnections are the overriding considerations
• When the complexity of interconnection in sub-systems requires complicated and expensive wiring or harnessing
• When frequency requirements call for careful control and uniformity of conductor wave impedances with minimum distortions and signal propagation, and where the uniformity of these characteristics from board-to-board is important
• When coupling or shielding of a large number of connections is necessary
• With multilayers, all interconnections can be placed on internal layers, and a heat sink of thick solid copper can be placed on the outer surfaces.

Consumer PCBs were generally used in consumer products such as radio, television, and cheap test and measuring equipment. They used less expensive base material and allowed greater tolerances for manufacture to keep the cost low. Much importance was not given to good and consistent electrical properties.

Professional boards were made of better quality material to achieve tighter electrical and environmental specifications using controlled fabrication techniques. Higher reliability boards, normally used in strategic applications, were meant to provide the best of electrical properties through the use of high quality base material and tightly controlled manufacturing processes.

Modern PCB classification

A more simple and understandable classification is now used, which is based on the number of planes or layers of wiring, which constitute the total wiring assembly or structures, and to the presence or absence of plated-through holes.
The important distinguishing constructions of PCBs are detailed below.

• Single-sided Printed Circuit Boards
• Double-sided Printed Circuit Boards
• Multilayer PCB

Printed Circuit Board made of

Core material is a rigid sheet of fiberglass resin material that has two sheets of copper adhered to either side. Some material may have a copper sheet on only one side. The copper is measured in ounces (oz). PCB manufacturers will refer to the copper thickness in ounces, but during board lay-up, or when the materials are stacked together, the inch/mm thickness is used.

Pre-Preg material is made of similar material as the core material but is in a soft, pliable form and comes in standard-sized thin sheets.

Copper foil is a thin sheet of copper that is placed on or between Pre-Preg materials and bonds to the Pre-Preg with the adhesive that is part of the Pre-Preg.

Copper plating is primarily used only on the finished board, on the external layers, and provides an additional thickness of copper to the board while plating the wall of the holes drilled in the board. Usually the external plating is added after the board is drilled and the external copper on the board is etched, leaving a thicker trace.

Solder flow is a process in which solder is applied to the external surfaces of the board on exposed copper areas. This helps prepare the board for soldering and protect the copper from oxidation. The copper areas on the entire board may be solder flowed, or a process called SMOBC (Solder Mask Over Bare Copper) will be used. SMOBC process is when the board is “masked” and only the exposed area (usually pads or areas that are to be soldered) will be coated with solder flow.

Solder mask is a material that is used to coat the board to:

• Protect from surrounding environment.
• Insulate the board electrically.
• Protect against solder bridges.
• Protect components mounted to the board.
• Protect the board from heat generated from components mounted to the board.

The Trace – a trace on/in a PCB is relative to a wire. It provides the same function of transferring electricity from one point to another while the strength of the PCB provides a rigid material to place components on. The copper layers, as explained later, will each go through an etch process that removes unneeded portions of the copper, leaving only those traces and pads required.

The Pad - a pad may consist of several different shapes and styles. Normally two types of pads are used, commonly referred to as a soldered surface-mount pad or a soldered thru-hole pad. A surface mount pad is nothing more than a square or rectangular copper area that is used for mounting surface-mount components. The size and shape depend on the component that is mounted/soldered to the pad. Most component manufacturers have recommended pad sizes for their components.
A soldered pad may consist of a plated thru-hole pad (PLTH) or a nonplated thru-hole pad (NPTH). Both are nothing more than a round, square, or oblong pad with a hole through the pad. This allows a leaded component to be mounted to the board by placing the lead through the hole and the lead soldered to the pad area.

The plated hole, consists of a pad in almost any shape with a hole through the pad. The walls of the hole are surfaced or plated with copper and, in some cases, solder or some other protective plating. The plating in the hole extends from the external area and flows into the hole, “plating” the hole wall.

Slots and Cutouts are similar to plated/non-plated thru-holes but are defined separately because of their shapes and characteristics. PLTH/NPTH are usually round holes but not always round pads, but the hole is drilled. A slot or cutout is cut into the board by a router bit and is an oval, oblong, or a rounded square shape. The “corner” of a slot or cutout is not square unless a special corner punch is used. Slots and cutouts are cut with a round bit; therefore the corner must be the radius of the bit, or larger. The router bit size depends on board thickness and manufacturer’s capabilities. A small bit may be used on a thick board but must cut slowly to prevent breakage, increasing time and cost.

The board edge deserves its own definition and attributes, due to the values that define the board edge. The board edge is any part of the board that exposes the cross section of the board, including slots, cutouts, and the outer edge of the board.

• the base, which is a thin board of insulating material, rigid or flexible, which supports all conductors and components. It provides mechanical support to all copper areas and all components attached to the
  copper.
• the conductors, normally of high purity copper in the form of thin strips of appropriate shapes firmly attached to the base material. It provide not only the electrical connections between components but also solderable attachment points for the same

The term printed became popular because the conductive areas are usually generated by means of a printing process like screen printing or photo-engraving.

• The circuit board fabrication cost (pcb cost) is lower with mass quantity production
• Electronic circuit characteristics will be maintained without introducing parasite capacitance with a proper circuit board design.
• Component wiring and assembly can be mechanized in a circuit board manufacturing facility.
• PCB’s offer uniformity of electrical characteristics from assembly to assembly.
• The location of electronic parts is fixed and so it simplifies components identification and maintenance of equipment.
• Inspection time is reduced because printed circuitry eliminates the probability of error.
• Chances of miswiring or short-circuited wiring are minimized.

circuit board design