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24 PCB007 MAGAZINE I NOVEMBER 2020 results in high surface copper to meet mini- mum hole requirements. This surface copper condition limits the use of any application for HDI-type work. 3. Periodic Pulse Reverse Plating These chemistries are used to metallize high technology, high aspect ratio work. In conjunc- tion with specialized pulse rectification, these systems can throw into holes with aspect ra- tios of >25:1 while minimizing surface cop- per plating. 4. Via-Fill Plating These DC chemistries are used for the com- plete filling of microvias for more advanced HDI technologies to individually connect layer to layer for higher density signal routing. The filling of microvias is generally limited to sizes of approximately 150 µm in diameter x 125 µm deep. Depending upon the application, these systems can be designed to fill microvias in panel plating mode while keeping plated sur- face copper to a minimum. The majority of HDI boards are made by the anylayer process. Conventional HDI involves a multilayer core with one or several layers of HDI on the outer layer(s). Conventional HDI is done by pattern or button plating. Anylay- er starts with a double-sided core and adds HDI layers in panel plate without any drilled through vias. A via stack would be a through via. Advanced HDI starts with a core and adds layers with pattern plated vias and can add drilled though holes or not. Anylayer is a buildup sequence starting with a double-sided core and building layers up to 10L or 12L without drilling any through vias. In the anylayer process, plating is done in pan- el mode, and etching is a print and etch sim- ilar to inner layer processing. The trace has a slightly trapezoidal shape. Recently, the use of ultra-thin foils or ad- vanced tenting processes can achieve line/ space (L/S) down to 30/35 µm using equip- ment common to most PCB fabricators. Anyl- ayer plating is traditionally done with insoluble anodes and direct impingement solution flow. 5. Via-Fill and Through-Hole Plating These DC chemistries are designed for more advanced fabrication techniques such as mSAP to fill microvias and plate through-holes of lower aspect ratio (<4:1) in pattern plate mode. These solutions provide good microvia filling and through-hole plating with strong knees while providing a good trace profile for