Mechanical Properties of Plated Copper

Abstract

Both electrodeposited and electroless copper are widely used in the electronics industry to form signal lines and plated-through holes in printed circuit cards and boards. Because of widely differing thermal expansion coefficients of copper and of the ceramic and polymeric substrates, large mechanical stresses develop in the metallization during thermal cycles, as e.g. during solder reflow. To safeguard against premature fracture it is imperative that the metallization is sufficiently ductile. Plated thin foil copper of poor ductility is known to be susceptible to cracks in plated-through holes which, besides causing problems in the manufacture, poses a threat to device reliability in service. In this paper we show that such cracks arise as a combination of reduced ductility due to presence of initial porosity in copper and due to grain boundary sliding and diffusional cavity growth during the soldering cycle. We emphasize that there exists strong interactions between these ductility reducing effects, such that their coupled action may exceed the effect when each mechanism operates independently. We review recent research on deformation and fracture of bulk and plated copper between RT and 300°C. Finally, we briefly discuss approaches to improve mechanical properties of plated thin foil copper.

Thermal effects in thin copper foil

“…An absence of embrittlement for the control and TCAM foils refutes an often made claim that the ED copper foils, in general, and perhaps also the R foils, embrittle at high test temperatures. [16][17][18][19] The (ED) GR3 foil is characterized by (a) a sharp anneal transition around 180°C; 2 (b) wellbehaved anneal kinetics; 1 (c) a stable grain structure during anneal transition; 4 (d) subgrain rotation or subboundary migration as the anneal softening rate-controlling mechanism (Table I); (e) moderate anneal-induced embrittlement, 1 high-temperature embrittlement (Fig. 10), and longitudinal/transverse elongation anisotropy (Fig.…”