Printed circuit boards (PCBs)

Once it has been established that the circuit functions as intended, the next step is usually to make a printed circuit board.

PCB showing copper connections running beneath the board

Making a PCB

The first step is to draw the circuit on to a mask. This is a kind of filter which protects the 'positive' parts of the diagram (the connections between components) and exposes the 'negative' parts (all the rest). The mask is then placed on the board which has a thin coating of copper, and the circuit diagram transferred onto the board using an etch-resist pen. (An alternative technique uses a photosensitive board which is covered by the mask and then exposed to light.) The circuit is then etched onto the board using iron (III) chloride in an etching tank; this removes all the unwanted copper, leaving the req uired tracks and pads in place on the board. NB - this should only be done under a teacher's supervision.

Next, a small PCB drill is used to drill out the center of the pads. Finally the components are pushed through the board and soldered in place.

The graphic below shows a mask [mask: a filter or partial covering which protects some parts of a plate, while allowing the remaining parts to be exposed to acid or light. ] for a 555 monostable timer circuit, ready for transferring to a board. Note that masks are normally shown from the component side - that is, as though the board was transparent and viewed from the side from which the components are pushed through.

PCB mask of 555 timer circuit

With the components installed, the board would look like this:

PCB with 555 timer circuit components

Soldering

In electronics the main method of joining circuits is soldering. Electrical solder is made from an alloy of 65 percent tin and 35 percent lead. It also has a flux running through its core. The flux is needed to ensure that the solder makes a good joint between the components and the tracks and wires on the PCB.