A new multilayer metallization, ENEPIG (electroless nickel electroless palladium immersion gold) with 0.1㎛ thin Ni(P) layer (thin-ENEPIG), was plated on a Cu PCB substrate for fine-pitch package applications. We evaluated interfacial reactions and mechanical reliability of a Sn-3.0Ag-0.5Cu (SAC305) solder alloy on a thin ENEPIG coated substrate during various reflow times. In the initial soldering reaction, (Au,Cu)Sn 4 intermetallic compound (IMC) formed at the SAC305/ENEPIG interface. After prolonged reflow reactions, the Pd and Ni layers were consumed, and (Cu,Ni) 6 Sn 5 IMC formed on the Cu layer. As the reflow time increased, the Cu and Ni contents in (Cu,Ni) 6 Sn 5 IMC increased and decreased, respectively, due to the limited Ni layer in the ENEPIG plating layer. In the low-speed shear test, all fractures occurred in the bulk solder regardless of reflow times. In the high-speed shear test, the fracture mode was changed from ductile to brittle with increasing reflow time, due to the formation of the thick interfacial IMC.

In this paper, we evaluated the interfacial reactions and brittle fracture behaviors of thin electroless nickel-electroless palladium-immersion gold (ENEPIG) plating layers with different Ni and Pd thicknesses for fine-pitch package applications. Firstly, the interfacial reactions and mechanical reliability of Sn-3.0Ag-0.5Cu (SAC305)/thin ENEPIG solder joints were evaluated. (Cu,Ni) 6 Sn 5 intermetallic compound (IMC) was formed at all of the thin ENEPIG interfaces, and P-rich Ni layer was also observed at the joint interface of the Pd substrate with 0.3 ㎛ Ni thickness. The interfacial IMC thickness decreased with increasing Ni and Pd thicknesses. In addition, the IMC thickness was affected by the contents of the Pd plating layer. The IMC thickness for the Pd-P substrate was thicker than that for the Pd substrate. In the high-speed shear test, the brittle fracture rate decreased with increasing Ni and Pd thickness. Also, the brittle fracture rate was affected by the components of the Pd plating layer, and the brittle fracture rate for the Pd substrate was lower than that for the Pd-P substrate. The ENEPIG joint with thicker Ni plating layer had superior interfacial stability and mechanical reliability.

Non-conductive paste (NCP) formulation for chip-on-board (CoB) flip chip packages should contain a large amount of inorganic filler to achieve the low CTE (coefficient of thermal expansion) of cured joint and resultantly the reliability of overall packages. As the interconnection pitch decreases, the filler particle size should also be decreased because the particles trapped between bump and pad at the joint may deteriorate the joint reliability. In this study, effects of submicron-sized silica filler additions on the uncured and cured properties of NCP formulation and the void formation behavior at the flip chip joint were investigated. Two kinds of spherical fused silica fillers having particle size of 1 ㎛ and 100 nm were used. The abrupt increase point of viscosity of NCP formulation was shorten in the finer silica addition due to the increased probability of agglomeration and collision of particles. It resulted in the maximum filler content of 60 wt% and 30 wt% for 1 um and 100 nm silica, respectively. The addition of 100 nm silica showed lower effect on the CTE reduction and increased void formation at the flip chip joint compared to 1 ㎛ silica. It was considered that the defects at the interface between epoxy resin and silica filler originated from the surface contaminants, such as moisture and adsorbed gases, of silica particles was mainly responsible for the bad effects of the finer silica particles on CTE reduction and void formation behaviors.