10 Fundamentals of Bonding Technology and Process Materials …
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heterogeneous atoms. Microstructures of solder joints are investigated and characterized using scanning electron microscopy (SEM). These SEM images are analyzed
to determine the morphologies of brittle Cu6Sn5 IMCs, which is one of the factors
that accounts for premature failure upon EM-aging test. The β-Sn grain size distributions and its crystal orientation are characterized using electron back-scatter
diffraction (EBSD), and the established linkages between the process-structure and
structure-property by Principle Component Analysis (PCA) are discussed. Lastly,
EM degraded flip-chip samples are characterized using optical microscopy to verify
our hypothesis stated in the previous paragraph. This study will help us elucidate the
impact of TCB process on β-Sn microstructures on SAC-305 solder joint reliability.
Also, the statistical analysis such as PCA would enhance the understanding of the
differences between reflow and TCB processes, which can be used to model and
optimize the bonding parameters and processes that can be utilized in high volume
manufacturing.
10.5.2 Experimental Section
10.5.2.1 Test Coupon and Process Materials
The flip-chip dies, 5.03 mm by 5.07 mm and 625 μm in thickness, PB8 (8 mil
perimeter pitch) with the SAC 305 solder spheres was provided by Engent Inc. The
flip-chips consisted of under bump metallurgy (UBM) layers consisting of sputtered
layers of Al, NiV, and Cu, and the bump size and height were 130 μm in diameter and
107 μm, respectively. A standard high temperature FR4 PCB, 782 μm in thickness
with a T g greater than 170 °C was provided by Engent Inc. The PCB substrate bond
pads, 85 μm by 85 μm, were surface finished with a standard electroless nickel
immersion gold (ENIG) over the Cu bond pads, which are 12 to 20 microns in
thickness. During the bonding of the flip-chip to the substrate, Indium NC510 flux
was used, and after the bonding, H. B. Fuller FH8310 underfill material was used to
remove flux residues. EpoxiCure 2 Resin, EpoxiCure 2 Hardener, and Conductive
Filler (nickel powder) were purchased from Buehler, USA. MetaDi Supreme (1 μm
diamond suspension) and MasterMet (0.05 μm colloidal silica suspension) were
purchased from Buehler, USA.
10.5.2.2 Conventional Reflow Process
During the reflow process, the thermal profile was monitored, and the temperature
was measured at each zone in the conveyer from Zone 1 to Zone 9. There were four
major steps at specific temperatures as shown in Fig. 10.52a. Between the second
and third stage, just before solder reflow, degassing was observed and followed by
collapse during solder reflow as the solder balls melted and formed interconnections.
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heterogeneous atoms. Microstructures of solder joints are investigated and characterized using scanning electron microscopy (SEM). These SEM images are analyzed
to determine the morphologies of brittle Cu6Sn5 IMCs, which is one of the factors
that accounts for premature failure upon EM-aging test. The β-Sn grain size distributions and its crystal orientation are characterized using electron back-scatter
diffraction (EBSD), and the established linkages between the process-structure and
structure-property by Principle Component Analysis (PCA) are discussed. Lastly,
EM degraded flip-chip samples are characterized using optical microscopy to verify
our hypothesis stated in the previous paragraph. This study will help us elucidate the
impact of TCB process on β-Sn microstructures on SAC-305 solder joint reliability.
Also, the statistical analysis such as PCA would enhance the understanding of the
differences between reflow and TCB processes, which can be used to model and
optimize the bonding parameters and processes that can be utilized in high volume
manufacturing.
10.5.2 Experimental Section
10.5.2.1 Test Coupon and Process Materials
The flip-chip dies, 5.03 mm by 5.07 mm and 625 μm in thickness, PB8 (8 mil
perimeter pitch) with the SAC 305 solder spheres was provided by Engent Inc. The
flip-chips consisted of under bump metallurgy (UBM) layers consisting of sputtered
layers of Al, NiV, and Cu, and the bump size and height were 130 μm in diameter and
107 μm, respectively. A standard high temperature FR4 PCB, 782 μm in thickness
with a T g greater than 170 °C was provided by Engent Inc. The PCB substrate bond
pads, 85 μm by 85 μm, were surface finished with a standard electroless nickel
immersion gold (ENIG) over the Cu bond pads, which are 12 to 20 microns in
thickness. During the bonding of the flip-chip to the substrate, Indium NC510 flux
was used, and after the bonding, H. B. Fuller FH8310 underfill material was used to
remove flux residues. EpoxiCure 2 Resin, EpoxiCure 2 Hardener, and Conductive
Filler (nickel powder) were purchased from Buehler, USA. MetaDi Supreme (1 μm
diamond suspension) and MasterMet (0.05 μm colloidal silica suspension) were
purchased from Buehler, USA.
10.5.2.2 Conventional Reflow Process
During the reflow process, the thermal profile was monitored, and the temperature
was measured at each zone in the conveyer from Zone 1 to Zone 9. There were four
major steps at specific temperatures as shown in Fig. 10.52a. Between the second
and third stage, just before solder reflow, degassing was observed and followed by
collapse during solder reflow as the solder balls melted and formed interconnections.