Computer-aided manufacturing takes the output of the design system and applies it to the manufacturing process. Broadly, the hardware and its operation are very similar to the CAD system.
While the CAD system has placement and routing programs, the CAM station has programs suitable for the manufacturing process. CAD/CAM systems can be integrated through a common database (Ginsberg, 1992b) as shown in Figure 5.33 .
All the necessary data to operate numerically controlled (N/C) Printed Circuit Board fabrication machinery can be derived entirely from the common database after converting the same to the equipment’s required data format. The data can also be optimized by the computer to take advantage of the actual machine characteristics, such as drill speed, routing capabilities, tool selection speeds etc. In addition, the data obtained from the CAD/CAM database can be used for the control of component assembly equipment in sequence that provides the maximum use of automation. CAD/
CAM systems can also provide the necessary inputs for test equipment. With the appropriate software,
the test data can be provided in the correct format for bare-board testers, in-circuit testers; etc. This has led to an increasing interest in computer-aided testing (CAT) technology by printed circuit manufacturers.
Business planning and support Business forecasting Customer order servicing Finished goods inventory management
Manufacturing process monitoring
Manufacturing process control Machine performance monitoring
Machine load monitoring
Labor monitoring Material monitoring Preventive maintenance In-process quality testing Stores monitoring
Common data
Purchasing/receiving Shop routing
Methods and standards In-process inventory management Short-term scheduling Short order generation Process automation
Numeric control Direct numeric control Computer numeric control
Automatic inspection Automatic assembly Computerized testing Manufacturing
process planning Engineering design
Process planning Part programming N/C graphics Tooling and materials catalog
Materials requirement planning
Production line planning Bill of materials processing
Computerized cutter, die selection Group technology Materials/parts inventory management Computer-aided design Computer-assisted drafting/plotting Computer-assisted tool design
Group technology Bill of materials processing
Fig. 5.33 Common CAD/CAM database: CAD system with a common database can be accessed to satisfy CAM requirements (adapted from Ginsberg, 1992 c)
A typical sequence of activities and operation, which are followed to take a printed circuit board from concept to final delivery is shown in Figure 5.34. The diagram illustrates as to how the computer-
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aided design, manufacturing and testing functions can be interfaced with each other during the process flow.
Product specification Develop design concept
Analyze critical features Create detailed design Analyze detailed design Refined design elements
(components, etc.) Generate detailed drawings
Build prototype Test prototype Program N/C equipment
and automate manufacturing and test
Manufacture product Test finished product(Q/C)
Ship product Monitor field performance
Historical and present test and performance data
Modify design Computer-
aided design
Computer aided manufacturing
Design database
Fig. 5.34 Typical CAD/CAM System showing sequence of activities and operations required to take a printed circuit board from concept to final delivery (adapted from Ginsberg, 1992 b)
Integrated computer-aided engineering, design and manufacturing (CAE/CAD/CAM) with a common database and networking are now becoming popular as the users can link front-end CAE and back-end CAM. This is shown in Figure5.35. For example, test patterns created with the aid of CAE’s simulation capability can offer the possibility of production testing, especially as the increase
of fine pitch surface mounting may drive electronic manufacturers from in-circuit to functional testing.
ATE Pick-and-place
machine Photoplotter
Personal computers and
workstations
Hardware Test pattern generation Pick-and-place drive Photo plotter drive CAD software CAD software
SMTautorouting Component placement Gate allocation
Common database
Fault simulation Logic verification Schematic design
CAE software
Local area network Integrated CAE/CAD/CAM Relationships
Fig. 5.35 Integrated CAE/AD/CAM relationships, based on a common database and networking, link front-end CAE with back-end CAM (redrawn after Ginsberg, 1992 a)
The use of CAD/CAM systems also helps to optimize printed circuit board manufacturability and can also enhance the electrical characteristic of the product. In addition to minimizing the use of vias, CAD/CAM software can be used to achieve enhancement features such as:
a Converting sharp 90 degree bends in conductors to 45 degree bends or curves;
a Increasing spacing between conductors and vias;
a Increasing the clearance between conductors;
a Increasing land and hole sizes.
a Centering conductors routed between lands.
a Generally increase conductor feature size, (e.g., more copper).
In a CAD system, the designer selects the basic routes that determine the board’s complexity.
The rules include the number of layers, minimal track and via sizes, and minimal spacing between objects. In the placement stage, the designer selects the physical location of the components on the board. When the placement process is complete, the physical netlist can be derived. A netlist is composed of groups of pins, each of which is expressed by a reference designator, a side, and an X-
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Y location. Each group of pins represents a physical net. In addition, the placement of the components determines the location of the footprints, which are composed of all the pads in all the layers (toeprints) needed to connect the component to other parts of the design. Most footprints contain information about auxiliary features to be added to support layers and drawing. A typical example is the silk screen and soldermask layers derived from the placement process. It is an unwelcome reality that these layers are rarely viewed by the designer, who is primarily interested in the copper layers.
The algorithmically challenging routing process can take several hours or days to complete. Its intent is to implement the physical netlist using physical features on the board. These features are typically composed of three types of entities: traces, which are chains of lines carrying the signal between various locations on the same layer; vias, composed of pads in all or some of the layers and a hole that creates the physical connection, and which can be viewed as vertical lines carrying the signal between layers; and planes, the solid or hatched copper areas carrying power or ground signals between multiple toeprints.
The last stage of the design process is the manufacturing output. Through the previous stages, the designer worked mostly with logical entities. Components were added as needed, and as a result, all kinds of pads were automatically attached to multiple layers. Routing was initiated with a set of technology rules, which automatically implied the usage of certain line widths and via hole sizes.
Through the manufacturing output stage, the designer has to generate files in various formats including:
a Gerber files representing the plotted layers;
a Drill files, typically in Excellon format, representing toeprint holes, via holes, and mechanical holes;
a HPGL/DXF files representing the mechanical drawings;
a A netlist file (IPC-356D or various CAD formats) to represent the physical connectivity;
and
a A bill of material with a list of components and packages.
For an efficient CAD/CAM system, standardization of the output data is desirable. Therefore, PCB manufacturers increasingly use CAD data to assist in generating production ready photo-tooling.
For this purpose, the following drawings are, therefore, normally plotted:
i. photomaster for component side;
ii. photomaster for solder side (mirror image);
iii. negative for component-side ground plane;
iv. negative for circuit-side ground plane (mirror image);
v. board assembly drawing including board outline, component outlines, and reference designators;
vi. board assembly reference designator tables (optional — none or one drawing per table);
vii. dot pattern; and viii. drill edits.
Management data
Automatic Optical Inspection
Automatic Test Equipment Drill
data Cad data
Input scanner
Drill Router
Front end system
Photo plotter
The master drawing is usually prepared from a component side view. However, to avoid misunderstanding, it is always advisable
to print ‘component side view’ clearly on each drawing.
As the conductor tracks become finer and clearances smaller, front-end automation appears to be the only real solution to the problems of PCB manu- facture.Figure 5.36 shows a scheme using a front-end system which helps to produce accurate PCB tooling. The input to the manufacturer who uses front-end automation is the output from the CAD systems in the form of photoplotter steering data. The most commonly used data structure is the Gerber format.
There are also companies with libraries of artworks for which Gerber plot files are not available. In this case, each art- work layer can be scanned and converted into a Gerber plot file, which can be loaded into the front-end system.
The data for each layer is loaded into
the workstation and electronically registered in X,Y and theta. Once loaded, the operator has the opportunity to execute automatic design rule checks to verify clearances between tracks, pads, track and pad, hole to copper edge; etc. The workstation operator can activate the system to step and repeat and rotate and/or mirror the design to obtain the maximum raw material utilization by appropriate planning of the panel. The operator can then add the test coupons which conform to the relative approval or to the customer’s own coupon, the reference numbers, the resin venting pattern, test pattern, plating border and tooling holes. The operator can also compensate for mash stretch in a silk screen process.
Data is the output to a laser photoplotter and it is these first generation silver halide masters that are used directly in production. These masters are more accurate than contacts and reduce the amount of visual inspection required. In fact, tooling pins can be inserted into the film platen or flatbed of the laser photoplotter, enabling the imaging of the panel outwork directly onto pre-punched film.
The CNC drill data is produced with 100 per cent accuracy. The routing data can also be the output as a post-processing exercise. Similarly, data can be the output to the bare-board test equipment and automatic optical inspection machine. This procedure enables the manufacturers to set up the inspection tolerances for the process against the database feature dimensions, generating the whole range of tooling in a short time.The high degree of accuracy and the quality of outputs enable the
Fig. 5.36 Scheme using front-end automation system which helps to produce accurate PCB tooling (redrawn after Williamson, 1990)
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PCB manufacturers to produce the board with a higher yield. Williamson (1990) explains the concept of front-end automation system for the bare board manufacturing process.
Murray (1996), while discussing the issue of CAD to CAM data transfer, brings out the importance of Design for Manufacturability (DFM) and states that it will now break the wall between the designer and the fabricator of the printed circuit boards. Cost-effective manufacturing can and should be the normal output from design, even when the fabrication time is compressed (Baumgartner, 1996).