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84 The PCB Magazine • May 2016 electrics. Likewise, there are specifications for the conductors, typically copper, such as tensile strength, elongation, 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 mechanical interface between the copper and the dielectric and as such affects both—the copper, as well as the dielectric—and the preparation of such surfaces to achieve the desired peel strength. Compared to first-level packaging requirements, the second level pack- aging requirements typically don't include pro- cessability by the semi-additive circuitizing pro- cess, and they are less stringent regarding low CTE, high chemical and dimensional stability. Requirements derived from PCB manufac- ture include desmear chemistry compatibility (i.e., the removal of resin smear from innerlayer copper with potassium permanganate chemis- try), and the compatibility with conventional mechanical drilling as well as laser drilling. It should be noted that there is great reluctance in the industry to substantially change a process to accommodate a new material. Thus, processabil- ity and process fit are important considerations. Also, price/performance trade-offs are critical. There is no single parameter of "good- ness" for high-performance PCB materials. Performance parameters are driven by end-use requirements, processing needs, and by require- ments derived from semiconductor characteris- tics, as specified by OEMs. Depending on the IC performance and complexity, and depending on the end product, different performance cri- teria will make the "most critical" list. There are acceptance/performance standards and test methods for PCBs. The most widely ac- cepted standards and methods are developed and published by IPC. Examples are: • IPC-A-600 Acceptability of Printed Boards • IPC-4101D (Specification for Base Materials for Rigid and Multilayer Printed Boards) • IPC-TM 650 Test Methods Manual An example from the Test Methods Manual is Method 2.6.8 Thermal Stress (Solder Float). It requires the exposure of a sample to 288°C, for 10 seconds, 3–6 times. The sample is then in- spected for breakages in circuits, through-hole metal, and dielectric, as well as for delamina- tions. PCB Karl Dietz is president of Karl Dietz Consulting llC. he offers consulting services and tutorials in the field of circuit board and substrate fabrication technology. To view past columns or to reach Dietz, click here. Dietz may also be reached by phone at (001) 919-870-6230. Two-dimensional phos- phane, a material known as phosphorene, has po- tential application as a ma- terial for semiconducting transistors in ever faster and more powerful com- puters. But there's a hitch. many of the useful prop- erties of this material, like its ability to conduct elec- trons, are anisotropic, meaning they vary depend- ing on the orientation of the crystal. Now, a team including researchers at Rensselaer Polytechnic In- stitute (RPI) has developed a new method to quickly and accurately determine that orientation using the interactions between light and electrons within phos- phorene and other atoms- thick crystals of black phosphorus. Exploring Phosphorene, A Promising New Material eleCtroniC paCkaGinG levels

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