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PCB-Dec2014

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December 2014 • The PCB Magazine 53 GETTING TO THE ROOT CAUSE: SOLDERABILITy DEFECT ANALySIS continues was to achieve a greater gold deposit thickness. As one can see, the outcome was undesirable. Longer dwell times can certainly lead to hyper- corrosion of the nickel, which in turn affects solderability. Clearly, this particular ENIG sys- tem was not designed to deposit a higher thick- ness. This should be the job for a true electro- less gold, not an immersion gold. Certainly it has been shown that for solderability purposes 1–2 µin of gold is sufficient. Wire bonding is a different story and requires a thicker gold de- posit. A phosphorous content analysis of the nickel deposit (prior to immersion gold) pro- vided additional insight to this problem. Phos- phorous concentrations were on the low end (4–6%) of what is normally expected for ENIG plating process. With low phos content in the nickel, the deposit is more susceptible to corro- sion. This situation promotes higher gold thick- ness deposits and leads to hyper-corrosion. It is suggested that in order to promote good sol- derability and reduce or eliminate the chance of hyper-corrosion, a mid- to high-phos nickel (8–10.5%) process be used. Summary In this case, poor solderability of the nickel was caused by the hyper-corrosion of the depos- it. However, the root cause of the corrosion was two-fold. First, the fabricator was operating the ENIG out of specification. Essentially, this was caused by extended immersion dwell times in the gold solution. A second cause was the use of a low phosphorous nickel process. Low-phos nickel has a greater tendency to corrode than the mid- to high-phos nickel process. PCB michael carano is with omg electronic chemicals, a devel- oper and provider of processes and materials for the electron- ics industry supply chain. to read past columns, or to con- tact the author, click here. ucla researchers have developed a perovskite photodetector that could reduce manufacturing costs and improve the quality of medical and com- mercial light sensors. Photodetectors are semiconductor devices that convert incoming light into electrical signals. they are used in a vast array of products, from visible and infrared light detection systems to television remote controls. Perovskite is an organic-inorganic hybrid mate - rial with a crystal structure that is extraordinarily efficient for converting light into electricity. in recent years, the use of perovskite materials has led to rapid advances in the efficiency of solar cells. a research team led by Yang Yang, the carol and lawrence e. tannas Jr. Professor of engineering at the ucla henry Samueli School of engineering and applied Science, has developed a photodetector that uses thin coatings of perovskite—rather than silicon or other common materials. the perovskite coating is roughly 300 nanometers, about the width of a single bacterium, while the silicon layer in com - mon photodetectors is 100 micrometers, or more than 330 times as thick. as a result, the device ef- ficiently and quickly transports signals with mini- mum loss. it also offers improved sensitivity under dim light. "our innovation is using the perovskite ma- terial on a photodetector, and then putting it in the proper structure so that the material can work most efficiently," said Ziruo hong, one of the au- thors of the paper and a research engineer in Yang Yang's lab. the research was published re- cently in nature communications and supported by the national Science Foundation and air Force office of Scientific research. New Semiconductor Device for Better Photodetectors

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