A microindentation technique for determining strength of solder interface with silver metallization on co-fired multilayer ceramic substrate

Journal of Electronic Materials, Mar 2001 by Shang, Jian Ku, Huang, Rong-Fong, Wilcox, David L Sr

In addition to the interfacial crack, two sets of cracks developed in the ceramic at the corners of the indentation. As shown in Fig. 6, some of the ceramic cracks perpendicular to the interface penetrated into the silver metallization and joined with the interface while others terminated at the metallization without linking up with the interfacial crack or simply stopped short of reaching the ceramic-metal interface. The cracks growing out of the indentation diagonal parallel to the interface tended to curve towards the interface, and would eventually join together with the interfaces if the indentation load was sufficient and the stand-off distance was small. The same crack pattern was found when the indentation was made outside of the solder bump where the ceramic was separated from the thermoset resin prior to indentation. Since the indentation cracks in the ceramic were produced during unloading, the crack curvature is believed to be the result ofthe interaction between the already formed interfacial crack and the median crack in the ceramic.

The response of the interface to external stress was examined by systematically varying either the indentation load or the stand-off distance. In Fig. 7, the stand-off distance was fixed at 80 (mu)m, while the indentation load increased from 500 g to 2000 g. At both 500 g and 1000 g, no interfacial cracking was observed. However, when the indentation load was increased to 2000 g, interfacial cracking was found. Similarly, at a constant indentation load of 2000 g, moving the indentation closer to the interface resulted in interfacial cracking when the stand-off distance became smaller than a critical distance (Fig. 8). On the failure diagram shown in Fig. 9, the response ofthe interface to indentation may be divided into the cracking vs. non-cracking regions. The boundary between the two regions marks the onset of the interfacial cracking. At the onset of the interfacial cracking, a critical load may be determined from Fig. 9 as the minimum load required to induce interfacial cracking or a critical stand-off distance as the maximum standoff distance at which interfacial cracking is observed. In those applications where only the relative strength of an interface is needed, the position of this boundary could be used for comparative purposes.

Since the elastic stress in the ceramic scales with the indentation load, P, and the square of the distance, S, as given in Eq. 1, lines with a slope of -2 on the log P vs. S plot represent iso-stress conditions. An iso-stress line passing through the boundary between interfacial cracking and non-cracking regions then gives the strength of the interface. As shown in Fig. 9, an iso-stress line could be drawn as the solid line that passes through nearly all of the transitional points from non-cracking to the cracking region, except for the data obtained at the highest indentation load. From this line, the strength of the solder interface was determined to be 196 MPa. The deviation of the data at the highest indentation load from this line requires further investigation but may have been caused by the bifurcation of the median crack at the interface, promoted by a long median crack or high indentation load.