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PCB-Jan2016

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68 The PCB Magazine • January 2016 Following is a list of commercial high-perfor- mance laminate constructions/compositions, as provided by the manufacturers: • Woven E-glass/PPO • BT/epoxy • Woven E-glass/high-speed, low-loss epoxy • Woven SI-glass, APPE thermoset resin • Woven E-glass, ceramic filled thermoset hydrocarbon (polybutadiene) • Woven SI-glass/high-speed, low-loss epoxy • Woven glass, ceramic filled PTFE • Woven glass, ceramic filled thermoset Second Level Packaging: Material and Processing Requirements and Comparison to First Level Packaging There are many electrical, mechanical, safe - ty and environmental requirements for PCBs that translate into specific requirements for di- electrics, which is the focus of this column on high-performance dielectrics. Likewise, there are of course specifications for the conductors, typically copper, such as tensile strength, elon- gation, dimensional uniformity (height, width) and peel strength (adhesion to the surrounding dielectric material). Peel strength is of course a function of the nature of the chemical and me- chanical interface between the copper and the dielectric and as such, affects the copper, as well as the dielectric and the preparation of such sur- faces to achieve the desired peel strength. Compared to first-level packaging require- ments, the second-level packaging require- ments typically don't include processability by the so-called semi-additive circuitizing process, and they are less stringent regarding low CTE, high chemical and dimensional stability. Re- quirements derived from PCB manufacture in- clude desmear chemistry compatibility (i.e., the removal of resin smear from innerlayer copper with potassium permanganate chemistry) and the compatibility with conventional mechani- cal drilling as well as laser drilling. It should be noted that there is great reluctance in the indus- try to substantially change a process to accom- modate a new material. Also, price/performance trade-offs are critical. A PCB fabricator will try everything to avoid replacing FR-4 with high- er cost, high-performance laminate, including electrical redesign of the board, and will switch to high-performance laminate only as a last re- sort, especially if there are processing issues. There is no single parameter of goodness for high-performance PCB materials. Performance parameters are driven by end-use requirements, processing needs, and by requirements derived from semiconductor characteristics, as specified by OEMs. Depending on the IC performance and complexity and depending on the end prod - uct, different performance criteria will make the "most critical" list. Acceptance/performance standards and test methods vary for PCBs, and the most widely ac- cepted standards and methods are developed and published by IPC. The following is a list of performance param- eters for dielectrics most often encountered: • Low loss (low Df, loss tangent, or dissipation factor), i.e., a dielectric material that contributes very little to the attenuation of the electrical signal during its transmission • Low Dk (dielectric constant) • Low moisture absorption • Low CTE (coefficient of thermal expansion) • High Tg (glass transition point) • Dimensional stability, stiffness • Good thermal conductivity (heat dissipation) • Flame retardancy • Chemical stability • Conductive anodic filament (CAF) resistance There may be a single driver demanding one of these properties, or one performance property could be dictated by several requirements—re - quirements that might stem from semiconduc- tor needs, assembly needs or end-use needs. Or one single end-use requirement may affect sev- eral performance parameters. The following list tries to explain these base material performance requirements. Low dielectric loss is most important at high frequencies and where there is a long circuit path. The latter is obvious because loss is a func- tion of transmission length and is usually stated HIGH-PERFORMANCE LAMINATES karl's tech talk

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