Printed Circuit Design & Fab - April 2008 - (Page 25)

FIGURE 2. Thieving process applied to PCB. FIGURE 1. Fabrication drawing. Ensuring that automated optical inspection (AOI) is used is another critical step in board fabrication. AOI checks inner layers to ensure there are no opens or shorts on the board, and that layer-to-layer registration is properly aligned. For example, a note to a fabricator can simply state: “Please make sure Layers 2, 3, 5, and 6 are AOI verified before laminating the layers.” Solder mask information must also be covered, including if a solder mask is required on one or both sides of the board, halogen content considerations and also designating the color of the solder mask. In addition, it is always a good idea for the designer to mention a few preferred solder mask manufacturers. Fabrication notes must also detail maximum warpage per square inch that the designer will allow, and this should be in accordance with IPC guidelines. The designer also should include a note on the thieving process that allows for even copper distribution on the board. Thieving adds non-conductive copper material to the board that balances copper weight on the board’s entire surface, so that when etching is performed, it is uniform over the board’s surface. This minimizes the creation of either opens or shorts. FIGURE 2 shows a PCB design with copper thieving added. APRIL 2008 Designing For Odd-Shaped Boards When it comes to fabrication notes, mechanical dimensions and drawings require equal attention from the designer. This is especially true for odd sized or unusually shaped boards that use radial angles to define curvatures or slot dimensions. When creating a fabrication drawing for such boards, the designer must specify even the smallest detail in lengths, widths, and angles. The designer must carefully designate any holes that reference each other, especially if they are at a specific angle or run concurrent to each other. Gold fingers, like cables and chassis wires, are used to connect daughter cards to a motherboard or other subsystems, and when providing details on the beveling of the gold fingers at the edge of the connectors, the designer should specify the angle and at what levels the gold fingers should be bev- FIGURE 3. PCB designer should specify gold finger beveling angle. PRINTED CIRCUIT DESIGN & FAB 25

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Printed Circuit Design & Fab - April 2008 Contents Our Line Market Watch Around the World Happenings ROI Tip Jar Interconnect Strategies Positive Plating Improving Fabrication Yields by Design Solve Design Problems with Signal Integrity Optimization Laser Direct Imaging Made Easy Troubleshooting the Innerlayer Process SIPS Give More to Moore Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - April 2008

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