How to design PCBs?
PCB designs begins when a electronic engineer chooses the components required to perform the functions of the end product and then determines the best way to connect those components electrically.
The design gives the manufacturer a lot of information including the PCB dimension, hole sizes and positions, and overall mechanical definition; it may also incorporate notes referring type of material, specifications, UL requirements, solder mask, and test requirements.
CAD: Creating Hardware with Software
While PCB design were once played out manually on mylar film, they are now created using Computer Aided Design (CAD) software. The CAD software automatically determines the routes for the conductors, reducing the manual labor required. Most CAD providers provide a library of shapes and sizes of available components. These shapes and sizes are known as outlines. The area where these components come into contact with the board is call the footprint.
Because a variety of copper thickness, epoxy properties, and types of glass weave are available, the designer must define the desired combination. One important consideration is the makeup of the prepreg – the agent that bonds the layers of a multilayer PCB. Prepreg (or B-stage resin) is a glass cloth that has been impregnated with epoxy and then partially cured. There are 3 basic properties: thermal, electrical and chemical.
Specifying Metallic Finishes
Copper is used as the conductor in most PCBs in production, and if left unprotected, will tarnish and prevent the proper solderability of components to the bare board. A metal that doesn’t tarnish or is slow to tarnish is applied to protect the exposed copper surfaces of the PCB. Metallic surface finishes vary in price, shelf life, reliability, and assembly processing. Some of the popular surface finishes are listed as below:
- Hot Air Solder Leveling (HASL): the PCB is dipped into a bath of molten solder(tin-lead).
- Electroless Immersion Gold (ENIG): This is one of the best and most popular RoHS finishing methods. ENIG offers excellent wetting, coplanarity, oxidation, and long shelf life.
- Immersion Silver
- Organic Solderability Preservative (OSP): OSP is a RoHS compatible , water based organic compound that selectively bonds to copper and provides an organic/metallic layer that protects the copper during soldering.
- Lead Free HASL: The molten bath is free of lead and RoHS. It uses a nickel modified alloy to give similar results.
Creating the Stack-up Sheet
As the final step, the designer creates a stack-up sheet that provides an overall picture of the board.
Generating Manufacturing Data
- Internal or Industry specifications
- Underwriters Laboratory (UL) requirements
- Test requirements
- Custom specifications
Transferring the Design to the Manufacturer
The most used data transfer formats are used below:
- Gerber and its derivative, Gerber RS-274X.
- ODB++, GenCAM, and PIC-D-350.
Converting the Design for Tooling
The manufacturing process begins with the transfer of the electronic data received from the designer, using special programs known as Computer Aided Manufacturing (CAM) software. CAM permits the manufacturer to perform:
- Performing Panelization and Photo-tooling
Panelization process is to multiple PCB images to fill the entire surface of the panel of laminate most economically. Auxiliary features: layer numbers, UL symbols, borders, and test coupons, are added to the panel. Then it is transferred to the photo plotter: a machine that draws the panelized image with laser light through the imaging process. The final image is drawn on photographic film.
Laser direct imaging (LDI) is another method of applying or exposing the PCB design to the manufacturing panel by directly imaging the circuitry to the manufacturing panel using a powerful laser.
- Generating Drill and Routing Data
Generating the numerically controlled (NC) drill and the routing data for transmission to the drilling section.
- Programming Inspection and Test Data
Manufacturers use automatic optical inspection (AOI) to find breaks or shorts in the PCB routing. Manufacturer program the AOI memory by using a golden board or CAD data as a reference. The AOI machine scans the board under inspection and compares it with the golden board. In the case of a mismatch, the machine identified the location of the defects.
The bare board is electrically tested to ensure the original design is followed. There are two methods for testing.
- Golden Board Testing: This method is primarily used when no Gerber or electronic data is available to generate artwork used in the manufacturer of the PCB. Legacy product that is usually through hole, simple 2-, 4-, or even 6- layer boards are still tested in this way.
- Netlist Testing: This method can be used when Gerber data is available. It is performed when all the nets (a string of points along a circuit) can be extracted from the information supplied. Where golden board testing is a comparison test, the netlist tests all points, ensuring a high confidence of electrical integrity. This true electrical test of point to point is just a step away from the actual schematic drawing of the PCB.
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