Can you guess what does this picture represent?
It looks like a light bulb, but the picture is actually a cross-section image of a microvia in a printed circuit board (PCB). The “light bulb” is a void above the microvia, forming when the PCB was moulded into epoxy for cross-sectioning.
Microvias are small, plated vias which are used in PCBs to electrically connect different layers. Since electronics is getting smaller and smaller, higher density PCBs are required. One way to increase the density of PCBs is to use microvia layers. Commonly, these layers are built on top of a core board which can be a single or a multilayer PCB. The picture below shows a schematic picture of a sequential built-up (SBU) PCB with microvia layers.
Build-up layers can be formed on both sides of the core board allowing the use of high density components. A flip chip components, for example, can directly be attached to such PCB and the I/Os (inputs and outputs) from very high-density components can be routed using microvias. As the pad size of a microvia is very small, board size and weight can be saved. Moreover, in high-density applications such structures are often cost-effective. The use of microvias can also improve electrical performance. The microvia layer can be made of neat resin or of fibre reinforced resin layer.
In this week’s picture the copper forming the microvia can be seen underneath the light bulb void in grey colour. On top of the copper there is ENIG (Electroless nickel immersion gold) surface finish (slightly different grey colour). Structure of the ENIG can be seen through the void. This PCB used fibre reinforced resin layer. The structure of the FR4 resin with fiber glass cloth can be seen on both sides of the microvia.