Characterization and Mechanism of Silver Sintering on Different Electroless Ni(P) Metallization for Electronics Packaging

As a cutting-edge die attachment technique for electronics packaging, the low-temperature silver (Ag) sintering is attracting increasing interest. Electroless nickel (phosphorus), often known as Ni(P), is a cost-effective and widely used metallization for substrates and baseplates when compared to silver (Ag) or gold (Au). However, oxidation of Ni, low inter-diffusivity and complete immiscibility between Ag and Ni, and complex effects of P content make it challenging to achieve robust sintered-Ag bonding on Ni(P). In this study, IGBT assemblies were packaged for the first time via pressureless sintered-Ag die attachment on electroless Ni(P)-plated substrates featuring different P contents, including 2.74 wt. % of low-P (LP), 7.65 wt. % of medium-P (MP), and 13.10 wt. % of high-P (HP). Afterwards, a comprehensive exploration was firstly conducted regarding the effects of different P content in Ni(P) on the bonding properties. Based on the Ni2p spectra derived from the XPS analysis, the atom ratios of NiO in LP-Ni(P), MP-Ni(P), and HP-Ni(P) are 44.6 at. %, 35.4 at. %, and 20.3 at. %, respectively, indicating the amorphous HP-Ni(P) experiences a lower degree of Ni oxidation than the other two polycrystalline Ni(P) counterparts. Consequently, as the P content increases, there is an enhancement in the mechanical, electrical, and thermal properties. In particular, the high-P Ni(P) yields outstanding results, including a strong bonding strength reaching 39.7 MPa, a small <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_CE</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(sat)</sub> of 2.00 V close to the 1.99 V of samples using substrates with Ag-plating, and a low <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th_jc</sub> of 0.95 °C/W. Furthermore, a deeper investigation into the bonding mechanism between sintered-Ag and Ni(P) has been carried out through chemical and microstructural analyses via EDS, SEM, and TEM. Due to the massive twins within sintered-Ag and reduction of lattice misfit between sintered-Ag and NiO from 2.5% to 0.8%, a robust bonding was achieved by the formation of tightly-knit Ag-NiO-Ni chemical bonds along the (111) crystallographic orientation within the identical face-centered-cubic (FCC) structures. The findings in this study not only contributes to a deeper understanding of the underlying mechanism but also offer valuable guidance for the application of sintered-Ag bonding on Ni(P) metallization in electronics packaging.