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AUGUST 2019 I SMT007 MAGAZINE 87 very thin, consistent coatings of pure metal or metal alloys to package surfaces in the 1–7-μm thickness range. With the nature of the sput- tering process capable of depositing metals at the angstrom level, the electrical performance of its coating layer has been effective thus far for typical shielding application. However, as the need for shielding grows, sputtering has significant inherent drawbacks for it to be established as a scalable method for manufacturers and designers. Initial capital equipment costs for sputtering equipment are very high—in the million-dollar range. Sputter equipment lines require significant floor space due to its multi-chamber process, and further increases the need for additional real estate with full in-line conveyorized systems. With the inciting of plasma to sputter material from a sputter target to the substrate, typical sput- ter chamber environments could reach into the 400°C range; therefore, cooling of a substrate with a "cold plate" mounting fixture to reduce the experienced temperatures is needed. The sputtering process deposits metal on a given substrate but typically only produces up to 60% thickness coverage on the vertical side- walls of a three-dimensional package com- pared to the thickness of the top surface layer. Lastly, due to the nature of sputtering being a line-of-sight deposition process, metal parti- cles cannot be selectively applied or necessar- ily applied under overhanging structures and topology and can result in significant mate- rial wastage in addition to material accumu- lation inside of the chamber walls; hence, this requires intensive maintenance. It is also nec- essary to pre-apply masking to substrates if there are specific areas of a given substrate that must remain exposed or do not require EMI shielding. Spray Coating: An Alternative Process to Sputtering Spray coating is an established conformal coating method commonly used in the automo- tive and printed circuit board assembly (PCBA) markets to protect substrates from harsh envi- ronmental factors, including moisture and dust. Using similar spray technologies, thin layers of flux material have also been applied to PCBs before ball grid array (BGA) compo- nent attachment and reflow. Military applica- tions use spray technology to apply extremely thin coatings to substrates, at times involving highly customized and expensive fluid formu- lations. Today's EMI spray coating equipment has evolved from these markets and applica- tions, leveraging successful designs to achieve high productivity, long service life, and low capital costs. Spray coating for EMI shielding started to gain attention in the 2000s as a result of exten- sive use in industrial and automotive markets and military applications. By 2012, spray coat- ing EMI shielding fluids to coat the top sur- faces of semiconductor packages was in mass production. However, at that time, sidewall coverage and fluid adhesion to vertical sur- faces were significant challenges toward wider adoption of the process. Furthermore, initial formulations of EMI shielding materials based on conductive silver inks required moderate to high thicknesses to provide adequate shielding effectiveness and had limited adhesion prop- erty than recent formulations. As discussed in a previous publication [3] , the application of tilt spraying with a spray applicator has led to notable improvements in overcoming the prior sidewall coverage limita- tions from earlier implementations (Figure 2). Recent spray applicator design enhancements have also delivered greater selectivity and refined edge definition in the spray pattern. Figure 2: Tilted spray coating applies fluid at an angle to the top and sidewalls of component surfaces.

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