Printed Circuit Design & Fab - May 2008 - (Page 40)

QUALITY CONTROL FIGURE 1. Five islands of circuitry are shielded from one another to reduce SNR in RF circuitry. Eliminating BOARD DEFECTS The assembly drawing should specify process-related issues. by ZULKI KHAN The processes involved in eliminating board defects cut a wide swath through design, fabrication, and even assembly. Along the way, experience and know-how play pivotal roles because solutions to many defect-prone PCB areas are not covered in textbooks. PCB defects run the gamut, for example, from deteriorated RF signals resulting from inappropriate shielding to incorrectly structured boards due to sketchy or ambiguous fabrication notes, and on to miscalculated thermal profiles, creating BGA, CSP or QFN-based solder defects or flaws. Eliminating board defects starts with the designer. Their experience level and design-for-assembly (DfA) knowledge has a direct bearing on the number of defects and flaws detected at PCB assembly. Certainly, the designer may have a keen sense for design, but if they don’t know how assembly and manufacturing work, have a firm grip on its processes and a good understanding of assembly equipment, the PCB design is likely to experience assembly issues. For example, on paper, an RF design looks exactly the same as any other PCB design. However, in an RF design, a multiplicity of small differences, intricacies and issues demand careful attention. If not properly addressed at the outset, they subsequently become defects, which can keep design engineers 40 busy debugging and trying to eliminate those defects. Defects can be created in an RF design in such areas as an improperly designed antenna, signal deterioration at an unacceptable rate, exceptionally high noise resulting from poor shielding, improper dissipation of EMI from RF signals, and lack of attention to line-of-sight aspects. FIGURE 1 shows a board with five islands of circuitry shielded from one another to reduce signal-to-noise ratio (SNR) in the RF circuitry, and thus avoid defects. The designer must also concentrate on segregating signals to make sure signals of one particular frequency are properly separated from those of a different frequency. Otherwise, crosstalk may be created. A savvy PCB designer incorporates these and other RF design considerations, thus avoiding RF board defects at assembly and manufacturing. Board fabrication and assembly undergo various processes and machines. Particular attention is crucial in these areas to avoid defects. Lack of PCB layout experience, for example, can lead to placing BGAs on a board’s bottom side, although there is plenty of space topside. In a situation like this, the assembly process is duplicated with two setups for pick-and-place machines and another two processes for reflow, plus using ancillary fixtures to ensure those BGAs are MAY 2008 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - May 2008 Contents Our Line Market Watch Around the World Happenings ROI EMC For the Real World PCB East Conference Brochure Positive Plating Don't Let your Signals Stub Their Toes Improve PCB Layout With Skill Utility Programs The Next Generation Design Tool Challenge Thermally Conductive Microwave Materials PCB Dielectric Degradation in Lead-Free Assembly Applications A Tale of Two Trade Shows Eliminating Board Defects Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - May 2008

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