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How Multilayer Printed Circuit Boards are Made
Posted on 29/05/2015

Multilayer Printed Circuit Boards are made up of two or more PCB’s, which are stacked together to create a reliable mutual connection between them. 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.

 

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.

The next step involves plating, using a chemical deposition of a very thin layer of copper to cover the inside of the holes drilled. The outer layers are imaged to ensure that there are no short circuits in the board. The boards are then electroplated with copper and then they are checked to ensure there is a good electrical connection.

 

The outer layers are then etched, to remove any excess and unwanted copper. To protect the ‘wanted’ copper, the areas required will be coated with Tin as a protective barrier. After this process, the solder mask is printed on to each side of the board to protect the copper surface and prevent the solder from short-circuiting between components during the assembly. A solderable surface finish is applied to the copper for protection until the components have been soldered onto the board.

 

If the board requires edge connectors, gold is electroplated onto the board with protective tape placed above the connectors to protect the rest of the board. A silk screen provides an image of the legend to where the components will be place and this is placed on the board. Every multilayer board is electrically tested with a flying probe tester to ensure that there are no open circuits or any areas of short circuits.

 

The final stage before the final inspection is to profile and cut the PCB’s out of their production panel or as per request of the customer, it can be ‘V-Cut’ on a scoring machine to partially cut the boards out. 

 

PCB Global is the only manufacturer in the world that can supply 4 layer to 20 layer Printed Circuit Boards On-line with the fastest manufacturing lead times with our Express Delivery Service as listed below;

 

LAYERS                     EXPRESS Delivery

4 layer                        2 working days

6 layer                        3 working days

8 layer                        4 working days

10 layer                      4 working days

12 layer                      5 working days

14 layer                      5 working days

16 layer                      5 working days

18 layer                      6 working days

20 layer                      6 working days

 

Our online capabilities cater for the above 4 to 20 layer Printed Circuit Boards, FR4 TG130/140, FR4 High TG=170, Thickness 1.6mm to 4.8 mm, Copper Finish 1oz - 2oz – 3oz & 4oz, Surface Finish - HASL Leaded and ROHS HAL - ENIG Immersion Gold with various solder mask and component legend colours. These qualities are unique to PCB Global as we have found no other online PCB manufacturer can offer these specifications or services in such a prompt lead time. PCB Global also offer a complete line of products and services from Standard, Fast and Express Quick-Turn Prototype manufacturing, low to medium volume pre-production to high volume mass production services, all at the click of a button!

For a fast quotation for all your standard PCB’s up to 20 layers please visit our web site www.pcbglobal.comfor an instant online quotation.