Trelic’s picture of the week – 03/2017

Can you guess what does this picture represent?

This week’s picture shows a very close view of a printed circuit board (PCB) cross-section and its through-hole structure, representing an interface between copper and PCB material. The round gray areas are the glass fibres and the dark gray area is the epoxy resin. Copper can be seen on the right side as a light gray area. Due to drilling, the glass fibres on the edges have broken and copper has filled these cracks.

What are PCBs and why are through-hole structures used?

A PCB has many purposes. It provides mechanical support to the components, connects them electrically and helps to conduct away excess heat. Rigid substrates are made of reinforced copper clad laminates and prepregs. From these elements different PCB types ranging from simple single sided to very complex multilayer substrates can be manufactured. A prepreg is reinforcing fabric which has been pre-impregnated with a resin system. Commonly woven glass fibre fabric is used. A laminate forms when copper films are attached on both sides of a prepreg using pressure and heat. These structures are shown in the picture below. A popular choice for resin is epoxy. The most widely used rigid PCB is FR-4, which is epoxy-based, and has woven glass fibre reinforcement.

When a PCB is formed, copper on top of the laminates is patterned and several laminates and prepregs may be laminated together to form multilayer structures. In order to connect different layers to each other, holes are drilled through the laminate. When these holes are plated with copper, plated-through-hole structure forms. In the picture below a schematic cross-section of a multilayer PCB is shown.