Porous Copper Electroforming By Dynamic Hydrogen Bubble Template Using Continuous and Pulse Currents

Abstract

Copper is the most widely used materials for microelectronics applications due to their electrical and thermal conduction properties. In power electronics converters, it plays several roles and is the main constituent of cooling systems, but is also used for joints to be thermocompressed directly on power semi-conductor device. The possibility to get a 3D patterns of porous copper electroformed “in-situ” on the metallized ceramic substrate is an attractive development area which constitute a task in the COPPERPACK project (ANR-19-CE05-0011-04).Different possibilities still exists such as copper-zinc or copper-manganese electrodeposition followed by an anodic dissolution, but the faster and simpler way resides in electroforming deposition made by Dynamic Hydrogen Bubble Template (DHBT) [1]. This process works in unusual potentials and current densities where the competition between hydrogen discharge (commonly avoided) and copper reduction is very high. In this case, this method achieves thick deposits in very short time (few seconds) with high currents allowing a very fast growth of copper around hydrogen bubbles. Pores and ligaments sizes can be controlled by several operating parameters i.e. current densities, temperature, deposit time as well as by chemical additives [2].With the use of a copper sulphate acid bath at various concentrations, with or without additives (acetic or hydrochloric acids) and by applying current densities between 100 and 400 A.dm-2, it is possible to obtain thick and porous coatings, with pores diameters between 10 and 500 µm and ligaments length between 10 and 200 µm. An example of a SEM image shows the morphology of the deposits obtained in top view and in cross section (Figure 1). Another possibility to control the 3D growth resides in the use of pulsed currents, with an effect on bubbles size. Furthermore, reverse pulses are useful for edge effects limitation.[1] Abdel-Karim, R.; El-Raghy, S. Chap 4 in Advanced materials and their applications - micro to nano scale. One Central Press, United Kingdom, pp 69–91, 2017[2] Shin, H.-C.; Liu, M. Chem. Mater. 2004, 16, 25, 5460–5464 doi.org/10.1021/cm048887b Figure 1