Formation behavior of intermetallic compound in solder joint assembly within fluxless bonding

In this paper, we develop the advanced package technology, 2.1D flip chip, to assembly multi-chip modules on the thin film of organic substrate. The thermal compression bonding is employed to interconnect with flux and fluxless techniques, respectively. The formation behaviors of intermetallic compound (IMC) between micro Cu pillar bumps and micro pads with surface finish as electroless Ni/ electroless Pd/ immersion Au (ENEPIG) are investigated. In fine bump pitch design of chip, the thermal compression bonding with flux has the flux residue defects remaining between micro pads at center region of substrate after the flux clean procedure. However, assembly performance by the thermal compression bonding with fluxless technique is as well as the traditional bonding with flux and satisfies the residue free condition in advanced packages. Since assembled solder joints, interfacial reactions of the intermetallic compound discovery the nucleation of (Cu,Ni,Pd)
₆
Sn
₅
composition inside the eutectic region, which exists nearby the interface of the micro bumps and micro pads. Under thermal aging test, thickness of the (Cu,Ni,Pd)
₆
Sn
₅
composition initially increases by depending on thermal cycles increasing, whereas slowly grows after thermal aging of 800 cycles, and gradually trends to stable. The additions of Pd atoms play the important roles in assembled solder joints, which dissolves in the (Cu,Ni)
₆
Sn
₅
composition and further inhibits the interfacial reaction. In addition, the lattice strain can be varied by the grain structure refinement from different atomic radius of the intermetallic compound, and hinders grain growths of the (Cu,Ni,Pd)
₆
Sn
₅
composition to slow down. By reliability test, results show that solder joints by the thermal compression bonding with fluxless are passed MSL4 preconditioning and thermal cycling of G-condition (TCG) by 1200 cycles without any underfill delamination defect. The detailed formation behavior and influence of the intermetallic compound with ENEPIG surface finish are investigated and discussed.