Abstract
The continuing thrust toward high-density and high-performance electronic devices has spurred development of more reliable solder joints in flip-chip technology. Successful solder joints not only give rise to metallurgically stable and mechanically robust but also ensure the electrical and signal delivery with excellent quality. Recently, Electroless Ni(P)/Electroless Pd(P)/Immersion Au (ENEPIG) has been widely used as surface finish for metal bond pad because of its many superior comprehensive performances. However, the amorphous electroless Ni(P) layer in ENEPIG dramatically increase the electrical resistance of solder joints and lead to pronounced signal degradation and conductor loss. Thus, it is urgently needed to search for another alternative surface finish which is suitable for low impedance soldering. ENEPIG with ultrathin electroless Ni(P) deposit (ultrathin-ENEPIG) was used to decrease the electrical impedance. The Ni(P) layer in ultrathin-ENEPIG was designed in submicron meter scale (0.05-0.31 µm) and expected to be completely exhausted after the first reflow process. The electrical impedance in ultrathin-ENEPIG was about an order magnitude lower than that in conventional ENEPIG. The next question is that what the optimal Ni(P) thickness is in ultrathin-ENEPIG regarding both the stability of mechanical bonding strength and superior electrical conductivity. In this study, the results of high speed impact test vehicle depicted that ultrathin-ENEPIG with 0.18 and 0.31µm electroless Ni(P) layer performed well owing to their limited growth of interfacial IMC. However, after 1000 hr thermal aging, the bonding strength of ultrathin-ENEPIG with 0.31 µm Ni(P) layer degraded abruptly because of the Kirkendall voids formation resulted from the huge difference in the diffusivity between Cu and Sn. On the other hand, the phase transformation from Ni3P to Ni2Sn1+xP1-x in ultrathin-ENEPIG with 0.18 µm Ni(P) layer eliminated the Kirkendall voids and further avoided the bonding strength degradation after thermal aging. Moreover, the needle-like interfacial IMC was proved to provide interlocking mechanics from fast crack propagation and improved the mechanical performance in the final part of the study. In summary, although ultrathin-ENEPIG indeed provided ultra-low electrical impedance, the mechanical bonding strength may decay faster than conventional ENEPIG. Notwithstanding, it is suggested that 0.18 µm would be the optimal Ni(P) thickness due to its limited growth of interfacial IMC, better bonding strength maintenance after prolonged thermal aging, and interlocking mechanics caused by the needle-like (Cu,Ni)6Sn5 IMC morphology.