Multilayer PCBs are made up of two or more PCB’s, which are stacked together to create a reliable mutual connection between them.
Why Multilayer PCBs?
The way multilayer PCB’s are constructed is to allow for a much higher component density meaning better conduction and connection. There are at least three conductive layers in one multilayer PCB with the two outside layers and the one layer synthesized and encapsulated by the outer insulation. It is due to the complex manufacturing procedures and lower production volumes that the cost for multilayer PCB’s are generally quite high.
Multilayer PCB’s are used in many basic and complex electrical items, depending on the amount of layers required and used. Most of the basic items require 4-8 layers and the more complex electronic equipment with high speed circuits require above 8 layers, usually 8-40 layers. These boards are typically utilised for professional electronic equipment such as computers and military equipment. The number of layers on a multilayer PCB is identified as the number of separate conductor patterns, which are usually an even number and includes the two outer layers.
Multilayer PCB Manufacturing Process
The process of manufacturing multilayer PCB’s starts with the Front end tool data Preparation, whereby the engineer or designer prepares his/her layout on a Computer Aided Design (CAD system) which uses their own internal data format, later extended into a Gerber File. The Gerber Files provide information for the copper tracking layers required as well as the solder mask and all other various options for the board.
Once the Factory has established a final confirmation from the designer, Photo tools are created by a Photo plotter, which provides an image of the PCB’s. This image provides information for the machines to print the inner layers. The inner layers are created with an epoxy resin, Laminate, and comprised of a core of glass fiber which includes copper foil pre-bonded to each side of the resin.
The next step involves using a powerful alkaline solution to dissolve or ‘etch’ the inner layers to remove the unwanted copper as per requirement. The inner core areas of the multilayer PCB is now complete. Next the required holes are punched in to meet and align the inner layers to the outer layers. This process is known as the Register Punch and Automatic Optical Inspection (AOI).
After the required holes have been created, it is now time to bond all the layers together and then move into drilling the PCB. The holes for leaded components are drilled as well as the ‘via’ holes that connect the copper layers together. An x-ray drill is used to locate these previously created ‘target holes’. The via holes described here are holes that penetrate the whole board. In the case of many multilayer PCB’s, when the design does not penetrate the whole board, to utilise the space of certain layers for other circuit patterns, ‘Blind’ and ‘Buried’ Vias are used. Blind Vias connect at least one of the inner layers with one of the surface layers without penetrating the whole board, while Buried Vias only connect to inner layers so they are impossible to see.