Prototype PCBs: Testing Electrical Circuits
Electrical and electronic industries use printed circuit boards or PCBs for various activities and equipment. These boards may be single-, double-, or multi-layered, with copper traces on each layer confirming to certain schematics. PCBs hold the necessary electrical and electronic components in place with solder, allowing easy wiring and assembly. However, before committing to manufacturing PCBs in large numbers, Rush PCB UK recommends designers test their circuits using prototype PCBs.
What are Prototype PCBs
Prototype PCBs are general-purpose boards available off the shelf. Designers typically use these boards to assemble their circuits and test them for their functioning. Once the designer is satisfied the assembly is functioning according to their requirements, they can proceed to the next step towards designing and layout of the actual board.
The structure of the prototype PCBs is such that it is easy to desolder any component and replace it with another. This feature is helpful to designers since it allows them to change components if the circuit does not function as desired.
Types of Prototype PCBs
The structure of prototype PCBs varies depending on the type of components the designer is planning to use in their application. In general, there are two major types, depending on whether the designer is using Through-Hole Components (THCs) or Surface Mount Components (SMCs).
THCs have long leads protruding from the body of the component. The PCB requires holes to allow the component leads to pass through for anchoring and soldering. It is possible to solder the components manually using a soldering iron, or using a wave soldering machine.
SMCs have very short leads that do not require holes for mounting. It is possible to place the SMC on the board and solder the leads directly to the pads on the board. For medium-SMCs, it is possible to manually solder them using a fine-tip soldering iron. However, it is possible to use a table-top reflow machine to solder SMCs on prototype boards.
Double-sided prototype PCBs for THCs
Two popular types of double-sided prototype PCBs are available for use with THCs:
Perf Boards: These are double-sided FR-4 boards with identical patterns. Each perf, or perforated board, has 1 mm holes drilled at 0.1-inch pitch all over it. Each hole has an unconnected square or round copper pad surrounding it. The holes are not plated through. In general, the boards are available in various sizes such as 4 in x 3 in, 6 in x 6 in, 4 in x 8 in, 6 in x 8 in, and so on.
Strip Boards: Similar to perf boards, strip boards are double-sided FR-4, with 1 mm holes drilled at 0.1-inch pitch. While all the holes have square or round copper pads surrounding them, each pad connects to its horizontally adjacent neighboring pad with a copper trace, with the trace running from the left edge of the board to the right edge. The reverse side has a similar circuit, only the trace connections are vertical rather than horizontal, with each trace starting from the top of the board and running to its bottom. Therefore, the board has many parallel strips of copper trace on each side, with the strips running at right angles on the two sides.
Double-Sided Prototype PCBs for SMCs
Two types of double-sided prototype PCBs are available for SMCs:
General Purpose Customizable Boards: These are double-sided FR-4 boards with patterns on both sides. The patterns have pads capable of accepting various SMC sizes for resistors, capacitors, inductors, edge mount connectors, and SMA connectors. These boards are generally available for medium-sized SMCs like 1210, 1206, and 0805.
Several plated through holes (PTH) are available very close to the component pads but not connected to them. These PTHs help the designer to easily connect to the circuit on the other side.
Specific Purpose Boards: These are double-sided FR-4 boards with specific patterns on both sides. The circuit on each side has pads for SMCs arranged in clusters of 4, 5, or 6 components. Designers can use SMCs of 1210, 1206, and 0805 sizes on these boards.
The boards have numerous PTHs placed close to the clusters of SMC pads. These help the designer to allow connection to the circuit on the other side of the board.
Specific Boards for SMC ICs: These are double-sided FR-4 boards with a specific SMC IC pattern on one side of the board, along with pads for discrete SMCs. The IC pattern has only pads for a quad package or dual-in-line package IC. The pads are usually long to allow mounting different-sized packages. All the pads have a neighboring unconnected PTH to allow connection to the reverse side.
How to Use Prototype PCBs
The basic idea of using prototype PCBs is to build a circuit using electronic components and test its functionality. The designer selects the type of prototype board depending on whether the circuit uses THCs or SMCs.
Using Prototype PCBs for THCs
For THCs, designers can use either perf or strip boards. In general, beginners find perf boards or perforated boards easier to use because they can interconnect components using short pieces of wire or even with solder bridges. The leads passing through the holes also double as connections to the other side of the board.
Experienced designers find the strip board more useful, as they can use the strips as interconnections between components. For a short interconnection, they can interrupt the excess strip on either side with a sharp cut.
As most through-hole ICs have a pin pitch of 0.1 inches, placing ICs on the perf, or stripboard is easy. However, the designer must ensure the strips do not short the pins of the IC.
Using Prototype PCBs for SMCs
Using prototype PCBs for SMCs is somewhat different from those for THCs. The complexity arises because of various factors. SMCs are available in various package sizes. SMC ICs can have quad packages or dual-in-line packages and their pins can be in gull-wing, J-wing, or flat shape. Fortunately, most manufacturers offer the same IC in different packages, so the designers can choose accordingly.
Rush PCB UK recommends designers use prototype boards for testing the functionality of their circuits before finalizing their designs. Having to redesign a board results in unnecessary delays and cost overruns, making the design late to the market. Using prototype boards is an easy way for the designer to maintain the schedule according to the plan.