17 Interconnect Quality and Reliability of 3D Packaging
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17.3 Quality and Reliability of Microbumps
We present four types of microbumps that are widely used in the industry for 3D IC
technology, in which the under-bump-metallization (UBM) is in different metallurgical combinations. For each type of microbump, there are case studies and discussion
of quality and reliability concerns. The aim is to understand the impact of different
UBM or surface finishes on the mechanical reliability of microbump.
Microbump materials are typically Sn-based Pb-free solder, e.g. SnAgCu, SnAg,
SnCu or SnAgCu-X where X represents the fourth elements. We simplified the
discussion here by using “Sn” to represent solder and focus on the impact of UBM.
No doubt the solder composition also plays a critical role to quality and reliability.
With a fixed UBM layer, modulating solder composition and bonding temperature
profile can significantly impact performance.
17.3.1 Type 1—Cu/Sn/Cu
17.3.1.1 Microstructure of Cu–Sn IMCs-Based Microbump
Cu/Sn/Cu is one of most widely used microbumps for chip-to-chip connection. When
the joint height is less than 10 μm, under typical assembly condition of 240 °C for
1 min bonding time, all the microbumps can be converted completely into Cu–Sn
IMCs. The kinetically favorable Cu 6 Sn 5 is preferentially nucleated at the Cu/solder
interface and continue to grow at assembly condition. Under further reflow or thermal
annealing, Cu 6 Sn 5 can be gradually transformed into the thermodynamically stable
phase of Cu 3 Sn. A typical microstructural evolution of Cu–Sn microbump in both
as-received and re-flowed condition is shown in Fig. 17.3.
17.3.1.2 Microstructural Characteristics of Cu 6 Sn 5 in Microbump
Li et al. prepared Cu/Sn/Cu microbump samples by controlled compression of
two polished copper surfaces sandwiching a tin foil in the middle with thickness
25 μm under different reflow temperature [5]. They monitored Cu 6 Sn 5 microstructural evolution during continuous liquid state reaction where Sn is in the molten
state. Shown in Fig. 17.4, Cu 6 Sn 5 was formed at the Cu/Sn interface in scallop-type
morphology. Growth and ripening of Cu 6 Sn 5 on two opposing interfaces happened
concurrently. Once the two opposing Cu 6 Sn 5 grains came into contact with each
other, the parallel grain boundaries surprisingly disappeared. No grain boundaries
in lateral direction could be identified and only vertical columnar grain boundaries
could be observed, Fig. 17.4c. The similar findings were also reported by Chuang
et al. [6] and Wang et al. [7], Figs. 17.5 and 17.6. The columnar Cu 6 Sn 5 was further
consumed by Cu into more stable phase of Cu 3 Sn in much finer columnar grain
533
17.3 Quality and Reliability of Microbumps
We present four types of microbumps that are widely used in the industry for 3D IC
technology, in which the under-bump-metallization (UBM) is in different metallurgical combinations. For each type of microbump, there are case studies and discussion
of quality and reliability concerns. The aim is to understand the impact of different
UBM or surface finishes on the mechanical reliability of microbump.
Microbump materials are typically Sn-based Pb-free solder, e.g. SnAgCu, SnAg,
SnCu or SnAgCu-X where X represents the fourth elements. We simplified the
discussion here by using “Sn” to represent solder and focus on the impact of UBM.
No doubt the solder composition also plays a critical role to quality and reliability.
With a fixed UBM layer, modulating solder composition and bonding temperature
profile can significantly impact performance.
17.3.1 Type 1—Cu/Sn/Cu
17.3.1.1 Microstructure of Cu–Sn IMCs-Based Microbump
Cu/Sn/Cu is one of most widely used microbumps for chip-to-chip connection. When
the joint height is less than 10 μm, under typical assembly condition of 240 °C for
1 min bonding time, all the microbumps can be converted completely into Cu–Sn
IMCs. The kinetically favorable Cu 6 Sn 5 is preferentially nucleated at the Cu/solder
interface and continue to grow at assembly condition. Under further reflow or thermal
annealing, Cu 6 Sn 5 can be gradually transformed into the thermodynamically stable
phase of Cu 3 Sn. A typical microstructural evolution of Cu–Sn microbump in both
as-received and re-flowed condition is shown in Fig. 17.3.
17.3.1.2 Microstructural Characteristics of Cu 6 Sn 5 in Microbump
Li et al. prepared Cu/Sn/Cu microbump samples by controlled compression of
two polished copper surfaces sandwiching a tin foil in the middle with thickness
25 μm under different reflow temperature [5]. They monitored Cu 6 Sn 5 microstructural evolution during continuous liquid state reaction where Sn is in the molten
state. Shown in Fig. 17.4, Cu 6 Sn 5 was formed at the Cu/Sn interface in scallop-type
morphology. Growth and ripening of Cu 6 Sn 5 on two opposing interfaces happened
concurrently. Once the two opposing Cu 6 Sn 5 grains came into contact with each
other, the parallel grain boundaries surprisingly disappeared. No grain boundaries
in lateral direction could be identified and only vertical columnar grain boundaries
could be observed, Fig. 17.4c. The similar findings were also reported by Chuang
et al. [6] and Wang et al. [7], Figs. 17.5 and 17.6. The columnar Cu 6 Sn 5 was further
consumed by Cu into more stable phase of Cu 3 Sn in much finer columnar grain