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Circuit Board Equipment

Two types of copper-clad laminates are commonly used for circuit boards, synthetic resin bonded paper phenolic and fibreglass-epoxy. The fibreglass-epoxy type laminate has superior electrical and mechanical properties compared to the other type.

The types of circuit boards used for prototyping are tag-board, protoboard and solderless breadboard. The protoboard you will be using for this lab. is 1.6mm thick, single sided paper phenolic type. This board has horizontal busbars for connecting power supply rails, ground rail and other common rails. The hole pitch is 0.1 inch suitable for IC (integrated circuit). The vertical busbars are used as circuit nodes. The components and wires are inserted into appropriate holes from component side (opposite to copper track side) and soldered onto respective copper pads.

The solderless breadboard you will be using in subsequent labs also has similar busbars and hole pitch. The components (DIP ICs, resistors and capacitors) and single-strand wires (dia 0.3-0.8mm) are inserted into appropriate holes and held by spring loaded nickel-silver contacts. Two or more breadboards can be snapped together to provide vertical or horizontal expansion for larger circuits. This type of board is a convenient tool used in quick prototyping and testing, however has limitations in testing low voltage DC/high frequency AC circuits due to DC drift/AC interference respectively. These limitations are overcome in a protoboard due to compact layout and short wire lengths.

The common Printed Circuit Board (PCB) is 1.6mm thick with copper tracks on one or both sides and holes drilled through the laminate for component leads.

A number of PCB fabrication methods are available to suit the application.

• Draw tracks and pads using etch resist pen/pads/tracks or oil paint/brush and etch off unwanted copper in a ferric chloride bath. Used for one-off single sided small prototype boards.

• Design PCB artwork (using Protel/PSpice or similar S/W), take a laser print, melt the print toner onto copper-clad laminate with smoothing iron, etch off uncoated copper. Used for small quantities of single/double sided small PCBs for prototypes. The plated through holes (PTH) is not possible. The min. track size is limited to approx. 0.02 inch and accurate drilling of a large no. of holes is difficult and time consuming.

• DE has NC controlled circuit board cutting machine which can produce circuit patterns by cutting and grinding away copper around each track/pad. The holes are drilled by machine but PTH is not possible, also min. track size is limited. This is a convenient method for digital logic circuits. The connection from one side to the other is possible with a small wire connection through the hole soldered on both sides. However, the large areas of copper left around tracks increase the risk of shorting adjacent tracks while soldering and also cause leakages/stray impedance's and can affect low voltage DC/high frequency AC circuit performance.

• Screen Printing Method- The screen with transparent circuit pattern is produced using photographic technique and etching. The circuit pattern is transferred onto PCB laminate using etch resist ink and a squeeze. Subsequent etching of board produces circuit board. This method is economic in producing single sided boards of medium complexity in small to medium volume.

• Commercial circuit board manufacturers use a process called photolithography and routinely produce multilayer, PTH boards with tracks as thin as 0.005 inch. PTH is accomplished by electroplating the inside of the hole and enable circuit connection from one side to the other or between two or more layers without the use of jumper wire. Complex and large circuits can be reliably produced in large quantities using this method. The boards edges are milled using NC machines to produce accurate sizes. The cost of producing small quantities is high due to one development and tooling costs, however, large quantities can be produced economically.

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