High Performance Etchable RoHS Compliant Thick Film Gold Conductor

Abstract

Abstract The thick film paste manufacturers are expected to produce conductors which are lead and cadmium free, yet have excellent fired film properties and the same performance and properties as the cadmium and lead containing formulations. The fired film surface of these conductors must be defect free (i.e. imperfections, pills, agglomerates) after multiple firing steps and must perform on dielectric as well as substrates from different suppliers. Typically, the thick film gold conductors are used in high reliability applications such as medical devices, military applications, and high frequency circuits, which require robust performance at high and low temperatures, in chemically aggressive environments, or extremely humid conditions. As circuits decrease in size and become more complex, the thick film gold properties become increasingly critical. The challenge is to develop an alternative gold conductor formulation, which can print and resolve fine features (down to 4 mil lines and spaces) as well as have the ability to be etched for higher density circuit designs (down to 1–2 mil lines and spaces). Gold conductors are typically used in conjunction with other high temperature thick films so good performance after multiple firings was also a targeted requirement. Heraeus has been proactive for the past decade in the development of thick film products that are both RoHS (lead and cadmium free) as well as REACH compliant. This paper discusses the experiments that were performed in order to understand the contribution of gold powder, organic and inorganic system to improve the fired film performance. These formulations were compared against existing gold conductors including the high performance gold conductor options as well as other available standard gold conductor options. Thin wire bonding trials including both gold and aluminum wire are used to compare influences of raw materials which includes high volume wire bonding reliability including failure modes and aged wire bond adhesion at elevated temperature exposures (300°C) for extended periods of time. In order to analyze fired film morphology and link this up to wire bond performance, SEM images of the conductor surface and cross sections were conducted. These studies resulted in a newly developed thick film gold conductor paste for use in a wide variety of applications. We present wire-bonding data with gold and aluminum wire and reliability results on both 96% Al2O3 ceramic substrates as well as on top of standard dielectrics.