Autocatalytic gold plating process for electronic packaging applications

An autocatalytic electroless gold process suitable for electronic packaging applications, particularly wirebonding needs, is described. This process yields high purity, non porous soft gold of any thickness that meets or exceeds all military specifications. The process can be used for a wide variety of organic laminates (FR4, multifunctional epoxies), circuitized either subtractively or additively, in addition to ceramic substrates. A description of the process flow, bath parameters, bath performance and resulting metallurgy are described. The process acts as an excellent follow on to the conventional electroless nickel/immersion gold surface finishing systems currently practiced by the industry. Rate, purity, hardness, and wirebond data as a function of bath life (metal turnovers) are discussed. The plating rate can be tuned between 1.5 and 2.5 microns/hr. The resulting deposit remains pure (>99.93%) and soft (<70 knoop) even in the presence of metallic impurities and upon bath aging. Wirebond and solder data as a function of gold thickness and plating parameters are also presented. These two assembly methods typically have competing requirements and the working range for both using this process are described. The process also has the ability to uniformly plate fine lines (>50 microns) separated by small spaces (>45 microns) without loss of line definition or bridging. The bath exhibits excellent throwing power. Drilled PTHs with aspect ratios as high as 15:1 (hole diameter of 0.03 mm, length of 0.45 mm) can be uniformly plated.