PCB Board Fabrication Guide
PCB Design Articles by Industry Experts
The aim of this guide is to give the reader a general overview of how a double sided PTH (Plated Through Hole) PCB is made while highlighting some of the design considerations along the way. There are slight variations in the fabrication process between manufactures so it is highly recommended to discuss their specific process and therefore the resulting design requirements before starting the design process.
The Two Layer Printed Circuit Board
1.1 Introduction
2. Fabrication Process
2.1 Materials
2.2 Material Choice
2.3 Fabrication Process Overview
2.4 Fabrication Process Steps
2.4.1 Clean copper surfaces
2.4.10 Remove remaining photoresist
2.4.11 Etch
2.4.12 Remove tin
2.4.13 Drill non-plated holes
2.4.14 Cover with solder-resist material
2.4.15 Exposed copper finish
2.4.16 Silkscreen
2.4.2 Drill plated-through holes
2.4.3 Desmear/deburr
2.4.4 Plate holes
2.4.5 Cover material with photoresist
2.4.6 Imaging
2.4.7 Develop photoresist
2.4.8 Developing photoresist
2.4.9 Plating copper and Tin
1. The Two Layer Printed Circuit Board
1.1 Introduction
Whether you are creating a two-layer or multi-layer blind and buried via PCB, the basic principles of board fabrication are essentially the same – you construct a sandwich comprising copper and dielectric materials and etch each copper layer to create the electrical paths, or tracks, thus creating the circuit connections. To join the tracks from one layer to another, holes are drilled through the board – and plated with copper. These layer to layer connections are called vias.
2.1 Overview
2. Fabrication Process
2.1 Materials
Specialised companies produce the materials used in the PCB manufacturing process and supply these to the fabricators in large sheets. The fabricators typically have to reduce the size of the sheets to a workable panel that conform to one of the standard sizes – one common panel size used by the fabricator is 457mm x 610mm – 18”x 24”.
The two main materials used in the process of PCB fabrication are copper – conductors – and insulators also known as dielectrics.
The dielectric material can be made from a number of component parts depending upon the performance required. FR4 is a general purpose dielectric material that is widely used across the electronics industry and is usually comprised of a woven glass fibre sheet (to give some strength and rigidity) mixed with an epoxy resin to form a core. Each side of the dielectric is covered with a thin copper layer to form what is known as a core.
2.2 Material Choice
Choosing the materials for the design has to take into account many aspects such as the final environment and the technology of the design, whether it is considered a high-speed or RF design for example. The designer should be aware that although the materials are manufactured in a variety of weights not all the thicknesses may be held in stock by every manufacturer. If the copper weight chosen is not a standard stock item it will have to be ordered and this could add significant time to overall fabrication lead-time.
2.3 Fabrication Process Overview
The board technology, and therefore the stackup i.e. number of layers, materials, via types etc., chosen by the designer directly affects the way in which the PCB is fabricated and what steps will have to be followed. This guide is only concerned with the fabrication process or steps carried out for the generation of a two-layer PCB. The main document highlights both the order and the steps taken in creating our two-layer PCB.
2.4 Fabrication Process Steps
2.4.1 Clean copper surfaces
The copper surfaces are cleaned continually throughout the design process to ensure that the next step is successful and defects are not introduced through surface contamination. The pre-treatment of surfaces includes……….
2.4.2 Drill plated-through holes
The holes that will provide an electrical connection such as via holes and holes for through-hole will have to be plated; their creation is the next step in the fabrication process and they are either drill mechanically, using laser drilling or using chemical methods……………………..
(Illustration)
The designer normally specifies the required finished hole-size on the fabrication drawing.
2.4.3 Desmear/deburr
(Illustration)
2.4.4 Plate holes
(Illustration)
2.4.5 Cover material with photoresist
2.4.6 Imaging
Both sides of the PCB are covered with a thin layer of dry film photoresist material. The photoresist, which is sensitive to UV (ultra-violet) light, is used to transfer the electrical track pattern to the copper.
(Illustration)
2.4.7 Develop photoresist
(Illustration)
2.4.8 Developing photoresist
(Illustration)
2.4.9 Plating copper and Tin
(Illustration)
2.4.10 Remove remaining photoresist
(Illustration)
2.4.11 Etch
The unwanted copper is now chemically etched away. The board is placed horizontally on a conveyor belt that moves the design through the etcher. The etchant is sprayed onto the board using nozzles that are placed above and below the conveyor belt……………..
(Illustration)
2.4.12 Remove tin
The tin coating that covered the track pattern was only there to stop the copper from being removed during the etching process. Once the board has been etched the tin is stripped from the PCB leaving the final copper pattern.
(Illustration)
2.4.13 Drill non-plated holes
(Illustration)
2.4.14 Cover with solder-resist material
The board is generally covered with a solder-resist material. The solder-resist will provide three major benefits………………
The solder-resist apertures are normally 4thou, for wet ink, and 3thou per side bigger than the copper land to allow for misalignment during the fabrication process.
(Illustration)
2.4.15 Exposed copper finish
(Illustration)
2.4.16 Silkscreen
The silkscreen or legend is the final step in the process. (Illustration)
