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JULY 2019 I SMT007 MAGAZINE 125 cases, underprinting other pads—may help compensate for the warpage impact. For exam- ple, where the BGA is bent inwards toward the PCB, there may be evidence of shorting. In those areas, the print volume may need to be minimized. Conversely, in areas where the BGA bends "away" from the board, a larger print volume may be the best solution. If the board is warping (IPC-TM-650 2.4.22 test method for measuring bow and twist), then you may need to use other approaches to reduce the impact of device or board warp- age (Figure 3). Besides the previously-men- tioned approaches, you may need to re-design the board to make sure the copper in the board is more evenly distributed. Low thermal mass (i.e., lower copper content) areas of the board will heat at different rates than higher ther- mal mass areas. In addition, very thin boards (0.020 inches) may need to laid out using a thicker board. Finally, different materials with closely matched CTEs may need to be used in the board or device construction. SMT007 Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. For more information, contact info@solder. net. To read past columns or contact Wettermann, click here. In a paper published in Nature, researchers unveil a new process for producing oxide perovskite crystals in exquisitely flexible, free-standing layers. A two-dimensional rendition of this substance is intriguing to scientists and engineers because 2D mate- rials have been shown to possess remarkable electronic properties, including high-temperature superconductiv- ity. Such compounds are potential building blocks in mul- tifunctional high-tech devices for energy and quantum computing. "Through our successful fabrication of ultrathin perovskite oxides down to the monolayer limit, we've cre- ated a new class of two-dimensional materials," said co- author Xiaoqing Pan, professor of materials science and engineering and Henry Samueli Endowed Chair in Engineering at the University of California, Irvine (UCI). "Since these crystals have strongly correlated effects, we anticipate they will exhibit qualities similar to graphene that will be founda- tional to next-generation energy and information technologies." Pan's cross-disciplinary group of research- ers applied a technique called molecular beam epitaxy to grow the thin oxide films layer by layer on a template with a water-dissolvable buffer followed by etching and transfer. His research team was able to review its work at atomic resolution using aberration-corrected transmission electron micros- copy (TEM), which allowed the team to directly observe novel phenomena, including the crystal symmetry break- ing and unexpected polarization enhancement under the reduced dimension. "Given the outstanding physical and chemical proper- ties of oxide perovskites and novel phenomena emergent at the monolayer limit, this work opens new possibilities in the exploration of quantum behaviors in strongly corre- lated two-dimensional materials," Pan said. Pan and his team were joined by collaborators at China's Nanjing University and the University of Nebraska. (Source: University of California, Irvine) Scientists Create New Class of Two-dimensional Materials

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