As the integrated circuit technology advances, area array of solder bumps is the only existing technology to accommodate the high density of input/output numbers. In the case of chip-level interconnection, a multilayered thin film metallization of Au/Cu/Cr is being used for bonding pad on a Si chip and the typical thickness of Cu in the metallization is about 0.5μm. It has been reported that the spalling of Cu6Sn5 will happen just because the amount of Cu available is very limited for reaction. The spalling can lead to solder dewetting on the exposed underlayer Cr, and weaken the solder joints. This is an urgent problem because of the wide application of solder joints in flip-chip technology. However, the deweting behavior of Sn-based solders on thin film is very complicated and far from complete. solder wetting on thin films has currently attracted much more attention. However, at present it has not been reported how the film thickness affect the solder dewetting behavior especially reflowed in the vaccum or with H2 protection.
The dewetting behavior strongly depended on the film thickness. On the thinner Cu films with the thickness of ≤0.45μm, dewetting behavior occurred. However, on thicker Cu films with the thicknesses of 1.07μm and 1.40μm, dewetting did not occur even after a longer reflow time. A critical film thickness fell between 1.07μm and 0.45μm, which satisfied non-dewetting exactly. The non-spalled Cu-Sn compounds, at the wetting tips, were clearly revealed by Focused ion beam cross-sectional analysis. The special and peculiar structure was proposed to be formed due to a limited amount of Sn for Cu-Sn reaction, whose size strongly depended on the film thickness. When the Cu film thickness was thick enough, the anchored force at the tips would stabilized the solder cap equilibrium on unwettable Si. Another experiment was conducted to confirm the pinning effect of wetting tip during dewetting.
Reactive wetting of eutectic SnAgCu solder on electrodeposited FexNiy layers was also investigated in a vaccum furnace with H2 protection. In this reactive wetting system, we also found a pseudo-partial wetting phenomenon, that is, a liquid film attached to the main liquid front. When eutectic SnAgCu solders were reflowed on high-Fe FeNi layers, dense Fe-Sn comounds would be formed at the interface and inhibit further interfacial reactions. In contrast, low-Fe FeNi layers would be consumed fast without denser Fe-Sn compounds formed at the interface. Finally, eutectic SnAgCu reacted with elctrodeposited FexNiy layers much faster in aging treatment that that with Cu.
Reactive wetting and spreading of liquid SnPb have been examined on the structured Au/Cu film with H2 protection. The striped structures played an important role in determining the thermodynamics and dynamics of reactive wetting or dewetting. Special attention has been paid to the liquid film, which extended from the viscous flow front and spread fast on the gold-coated copper line. Strong affinity of Au-Sn was proposed to induce the fast spreading of liquid film on the layered Au/Cu copper line.
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