Double-sided PCB layers with plated through holes (PTH)

Double-sided PCBs are boards with printed circuits on both sides. That means we can extend our design/layout and component placement space from one layer (top) to the other one (bottom). Beginner's minds will definitely have questions about how two different layers will be connected to each other if we generate designs on two different layers. That feature is provided by plated through holes (PTH). PCB GLOBAL DESIGN will describe later on this blog how plated through holes allow connections between two layers.

Another question in a beginner's mind should be, "Why do we require a 2-layer PCB if we can make any PCB on a single side? The answer will be that the two- or multi-layer PCB allows us to organise complex, larger designs in less space. That means we can save space by using double-sided or multi-layer PCBs. For example, if our application or product required 50 components on a circuit, we would arrange them on a single-sided PCB, which will take up more space, while 25 components on top and 25 components on bottom on a double-sided PCB can reduce effective space. The same scenario applies for traces required in design.

Let's see how two-layer PCBs are structured in the below video, Vid. 3. In the below video, PCB GLOBAL DESIGN shows how all layers (base material, copper layer, solder mask, surface finish, and silkscreen layer) and PTH look in a cross-sectional view of the PCB and on the actual board. We have tried to explain the layer structure of a double-sided PCB in an easy way.

Vid.3: Double-sided plated through-hole (PTH) copper PCB construction

Plated through hole (PTH): I hope you get a basic idea of what PTH holes are after watching the above video. Plated-through holes are like traditional holes on boards before they go through the plating stage. As we know, the base materials of a PCB are FR-4 and Roger, both of which are non-conductive. The raw materials of these boards are glass fibre/ceramic base with epoxy composite. Hence, a thin copper layer is deposited on the hole walls using the electroless process. In the electroless process, a thin chemical copper layer is deposited on the non-conductive walls of the holes.

While you are thinking of metal core PCB (MCPCB), you will have a question about how PTH happens in metal core PCB, where the base material of the PCB is conductive material. The answer is that while processing PTH on a metal-core PCB, we need to apply non-conductive material to the hole wall where the metal core is present. This means we need to make the hole wider at the location where the metal core is present, and in that wider space we can apply non-conductive material. After applying non-conductive material to the metal core, the PTH process will be the same as for FR-4 or Roger material. We will explain this topic with pictures and videos later on in the other blog. Also, we will explain the different types of holes used in PCBs.

Refer below to Fig. 7, which shows a cross-section of PTH holes that provide connection from the top layer to the bottom layer.

Cross-sectional view of PTH (plated through holes) in a double-sided PCB - PCB GLOBAL DESIGN
Fig. 7: Cross-sectional view of PTH (plated through holes) in a double-sided PCB

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