Many hardware engineers or layout engineers are very interested in what a PCB board (especially a multilayer board) looks like inside when they first start working with PCBs. Today, let's take a look together.
High-density interconnect (HDI) board --- vias
The circuit fabrication of multilayer PCBs is not much different from that of single-layer and double-layer PCBs; the biggest difference lies in the via process.
The circuits are all etched, and the vias are drilled and then plated with copper.
Multilayer circuit boards typically come in several types: through-hole boards, single-layer boards, double-layer boards, and double-layer stacked-hole boards. Higher-level boards, such as triple-layer boards and arbitrary-layer interconnect boards, are rarely used and are extremely expensive, so we won't discuss them further here.
Generally, 8-bit microcontroller products use 2-layer through-hole boards; 32-bit microcontroller-level smart hardware uses 4-6 layer through-hole boards; Linux and Android-level smart hardware uses 6-layer through-hole to 8-layer first-order HDI boards; and compact products such as smartphones generally use 8-layer first-order to 10-layer second-order circuit boards.
The most common through hole
There is only one type of via, drilled from the first layer to the last. Whether it's external or internal wiring, the via is drilled through the entire surface. This is called a through-hole board.
The number of layers in a through-hole board is irrelevant. The two-layer boards that people usually use are through-hole boards, while many switches and military-grade circuit boards are made with 20 layers and still have through holes.
Drill through the circuit board with a drill bit, then plate copper into the hole to form a circuit.
It's important to note that through-hole inner diameters are typically 0.2mm , 0.25mm , and 0.3mm , but 0.2mm diameter holes are generally significantly more expensive than 0.3mm diameter holes. This is because finer drill bits are more prone to breakage and drilling is slower. The extra time and drill bit cost are reflected in the higher price of the circuit board.
Laser holes in high-density fiberboard (HDI) boards
This image shows the stack-up structure of a 6-layer, 1st-order HDI board. The two surface layers have laser-drilled vias with an inner diameter of 0.1 mm . The inner layers have mechanically drilled vias.
It's equivalent to a 4-layer perforated plate, with 2 more layers on the outside.
Lasers can only penetrate fiberglass sheets, not copper. Therefore, drilling holes on the outer surface will not affect other internal circuitry.
After laser drilling, copper plating is then performed to create a laser via.
Ultra-expensive all-layer interconnect board with multi-layer laser-laid vias
Each layer has laser-drilled holes, and all layers can be connected together. You can route the wires however you want, and drill holes however you want.
Layout engineers must be thrilled! They'll never have to worry about not being able to draw it again!
The procurement process is so complicated it makes me want to cry; it's more than 10 times more expensive than a regular perforated plate!
Therefore, only products like iPhones are willing to use it. I haven't heard of any other mobile phone brands using any layer interconnect board.
