SOP Embedded Thin Film Resistors on High and Low Loss Thin Film Dielectrics

This paper describes low-cost materials and processes for embedded thin film resistors that can be integrated into printed wiring board using sequential build-up processes. Realization of embedded resistors on conventional board level high loss epoxy (∼0.015 at 1 GHz) and proposed low loss BCB dielectric (∼0.0008 at > 40 GHz) has been explored in this study. Foil transfer and electroless plating have been attempted for embedding thin film resistors. Ni-P alloys were plated using conventional electroless plating, and NiCr and NiCrAlSi foils were used for the foil transfer process. A test vehicle consisting of various geometry and values of resistors was designed and fabricated to evaluate electrical and mechanical reliability of embedded thin film resistors. For the first time, Benzocyclobutene (BCB) has been proposed as a board level dielectric for advanced System-on-Package (SOP) module primarily due to its attractive low-loss (for RF application) and thin film (for high density wiring) properties. To realize embedded resistors on multi-layer BCB, a low-cost large format electroless process for deposition of NiP and NiWP thin film resistors using both low- (25°C) and high-temperature (90°C) baths has been developed. The electroless process exhibits uniform resistor thickness in the sub-micron range and offers low profile and excellent adhesion to BCB dielectric. These films also act as a seed layer for the subsequent direct electroplating of copper traces. NiP alloys can also be tailored to variable temperature coefficient of resistance (TCR) with different alloy compositions. The electroless process can be adopted in the PCB manufacturing industries with no additional investment.