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76 PCB007 MAGAZINE I AUGUST 2019 SAP Utilizing Very Uniform Ultrathin Copper Article by Steve Iketani and Mike Vinson AVERATEK CORPORATION Abstract The demand for miniaturization and high- er density electronic products has continued steadily for years, and this trend is expected to continue, according to various semiconduc- tor technology and applications roadmaps. The printed circuit board (PCB) must support this trend as the central interconnection of the sys- tem. There are several options for fine line cir- cuitry. A typical fine line circuit PCB product using copper foil technology, such as the modi- fied semi-additive process (mSAP), uses a thin base copper layer made by pre-etching. The ul- trathin copper foil process (SAP with ultrathin copper foil) is facing a technology limit for the miniaturization due to copper roughness and thickness control. The SAP process using sput - tered copper is a solution, but the sputtering process is expensive and has issues with via plating. SAP using electroless copper deposition is another solution, but the process involved is challenged to achieve adequate adhesion and insulation between fine-pitch circuitries. A novel catalyst system—liquid metal ink (LMI)—has been developed that avoids these concerns and promotes a very controlled cop- per thickness over the substrate, targeting next- generation high density interconnect (HDI) to wafer-level packaging substrates and enabling 5-micron level feature sizes. This novel cata- lyst has a unique feature, high density, and atomic-level deposition. Whereas conven- tional tin-palladium catalyst systems provide sporadic coverage over the substrate surface, the deposited catalyst covers the entire sub- strate surface. As a result, the catalyst enables improved uniformity of the copper deposition starting from the initial stage while provid- ing higher adhesion and higher insulation re- sistance compared to the traditional catalysts used in SAP processes. This article discusses this new catalyst pro- cess, which both proposes a typical SAP pro- cess using the new catalyst and demonstrates the reliability improvements through a com-

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