Embedded Thin Film Resistors on BCB by Low-Cost PWB Compatible Electroless Plating by Direct Foil Lamination Transfer Processes

Abstract

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. Realization of embedded resistors on low loss Benzocyclobutene (dielectric loss ∼0.0008 at > 40 GHz) has been explored in this study. Two approaches, viz, foil transfer and electroless plating have been attempted for deposition of thin film resistors on Benzocyclobutene (BCB). Ni-P alloys were plated using conventional electroless plating, and NiCr and NiCrAlSi foils were used for the foil transfer process. 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. 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. The measured sheet resistance values are in the range of 250-400 ohms/square. 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. This paper reports NiP and NiWP electroless plated embedded resistors on BCB dielectric for the first time in the literature.