PCB Engineering

writer: G May 20, 2019

1. Basic requirements for PCB engineering

The level of PCB engineering production can reflect the design level of the designer, and can also reflect the production process capability and technical level of the printed board manufacturer. At the same time, due to the integration of PCB engineering and computer-aided design and auxiliary manufacturing, it requires extremely high precision and accuracy, otherwise it will affect the electrical performance of the final onboard electronic products. In serious cases, it may cause errors, which may lead to the entire batch of printed boards. The product is scrapped and delays the manufacturer's contract delivery time and suffers economic losses. When processing PCB design files, you should double check:

Does the receiving document meet the rules set by the designer? Can it meet the PCB manufacturing process requirements? Is there a positioning mark?

Is the line layout reasonable? Line and line, line and component pad, wire and through hole, component pad and through hole, the distance between the through hole and the through hole is reasonable, can meet the production requirements. Are there any conflicts between components in 2D and 3D space?

Does the printed board size match the processing drawing? Whether the graphics (such as icons, and markers) added to the PCB graphics will cause a signal short circuit.

Edit and modify some unsatisfactory lines.

Is there a process line on the PCB? Whether the solder mask meets the requirements of the production process, whether the solder mask size is appropriate, and whether the character mark is pressed on the device pad, so as not to affect the quality of the electrical equipment. and many more

2. Generation of light mapping data.

2.1. Panel

PCB design is completed because the PCB shape is too small to meet the production process requirements, or a product consists of several PCBs, so it is necessary to combine several small boards into one large area that meets the production requirements, or use more than one product. The PCBs are put together to facilitate the production of electrical equipment. The former is similar to the stamp plate. It can meet the PCB production process conditions and facilitate the component electrical installation. It is very convenient to separate in use. The latter is to assemble several sets of PCB boards of one product, which is convenient for production. It is easy to be clear about a product.

2.2 Generation of light drawing data.

The basis for PCB production is the film bottom plate. The accuracy of the basemap must be the same as that required for the printed board, and compensation for variations in the production process should be considered. The base map can be provided by the customer or by the manufacturer, but the two parties should cooperate and negotiate closely so that they can meet the requirements of the user and adapt to the production conditions. In the case where the user provides the base map, the manufacturer shall inspect and approve the base map, and the user may evaluate and approve the original or the first printed board product. Basemap production methods include hand drawing, texture, and CAD drawing. With the development of computer technology, the CAD technology of printed boards has been greatly improved, and the production level of printed boards has been continuously increased to multiple layers. The fine wires, small apertures, and high-density directions have been rapidly improved. The original film-making process has been unable to To meet the design needs of the printed board, the light painting technology appeared. Using the photoplotter, the PCB design data file of the CAD design can be directly sent to the computer system of the photoplotter, and the photoplotter can be used to directly draw the image on the film by using the light. Then, after development and fixing, a film bottom plate is obtained. Printed board film base made by photo-painting technology, fast speed, high precision, good quality, and avoids human error that may occur when manually mapping or drawing a base map, greatly improving work efficiency and shortening the printed board. Production cycle. According to the structure of the laser photoplotter, it can be divided into a flat type, an internal drum type (Internal Drum) and an external drum type (External Drum).

The standard data format used by photoplotters is the Gerber-RS274 format, which is also the standard data format for the printed board design and production industry. The naming of the Gerber format is quoted from the pioneer of the design and production of photoplotters - Gerber Corporation of the United States.

The generation of light drawing data is to convert the design data generated by the CAD software into light drawing data (mostly Gerber data), and then modify and edit it through the CAM system to complete the photo-painting pre-processing (imposition, mirroring, etc.) to achieve Printed board production process requirements. Then, the processed data is sent to a photoplotter, and converted into raster data by a rasterizer (Raster) image data processor of the photoplotter. The raster data is sent to the laser photoplotter through a high-speed fast compression reduction algorithm to complete the photoplotting. 

2.3 Photographic data format.

The light drawing data format is developed based on the data format Gerber data of the vector photoplotter, and the data format of the vector photoplotter is expanded, and is compatible with the HPGL HP plotter format, Autocad DXF, TIFF, etc. Dedicated and generic graphics data formats. Some CAD and CAM developers have also extended Gerber data.

The following is a brief introduction to Gerber data.

The official name of Gerber data is the Gerber RS-274 format. Each symbol on the vector ray plotter has a corresponding D code (D-CODE) in the Gerber data. In this way, the photoplotter can control and select the code disc through the D code to draw the corresponding graphic. The shape and size of the symbols corresponding to the D code and the D code are listed, that is, a D code table is obtained. This D-code table has become a bridge from CAD design to photoplotter using this data for light painting. The user must provide the corresponding D code table while providing Gerber light mapping data. In this way, the photoplotter can determine which symbol disc should be used for exposure based on the D code table to draw the correct pattern.

In a D code table, it should generally include the D code, the shape and size of the code wheel corresponding to each D code, and the exposure mode of the code wheel. Take a D code table of Protel, the most commonly used electronic CAD software in China, as an example. Its extension is .APT, which is an ACSII file, which can be edited with any non-text editing software.

Each line defines a D code that contains six parameters.

The first column is the D code serial number, which consists of the letter 'D' plus one digit.

The second column is a shape description of the symbol represented by the D code. For example, CIRCULAR indicates that the shape of the symbol is a circle, and SQUARE indicates that the shape of the symbol is a square shape.

The third and fourth columns define the dimensions of the symbol pattern in the X and Y directions, respectively, in mil; 1 mil = 1/1000 inch, approximately equal to 0.0254 mm.

The fifth column is the size of the center hole of the symbol graphic, and the unit is also mil.

The sixth column explains how the symbol disk is used. For example, LINE indicates that the symbol is used for scribe lines, FLASH indicates for pad exposure, and MULTI indicates that it can be used for both scribe lines and exposure pads.

In the Gerber RS-274 format, in addition to using the D code to define the symbol disc, the D code is also used for exposure control of the photoplotter; in addition, some other commands are used for the control and operation of the photoplotter. There may be some minor differences in the Gerber data format generated by different CAD software, but the overall framework is unchanged for the Gerber-RS0274 format.