http://support.spring8.or.jp/Report_JSR/PDF_JSR_27A/2015A1675.pdf

2015A1675

BL20XU

In-situ Real-time Observations of Reactions between Different Lead-free
Solder Pastes and Surface Finish Substrates
M.A.A. Mohd Salleha, H. Yasudab, A. Sugiyamac, T. Nagirad, K. Uesugie, S. D. McDonalda, K. Nogitaa
aNihon Superior Centre for the Manufacture of Electronic Materials (NS CMEM),
School of Mechanical and Mining Engineering, The University of Queensland,b
Department of Materials Science and Engineering, Kyoto University,c
Department of Mechanical Engineering for Transportation, Osaka Sangyo University,dDepartment of Adaptive Machine Systems, Osaka University,eThe Japan Synchrotron Radiation Research InstituteReal time solidification observation of Sn-0.7wt%Cu, Sn-0.7wt%Cu-0.05wt%Ni, Sn-3.5wt%Ag and Sn3wt%Ag-0.5wt%Cu solder pastes on different FR4 substrates with Cu organic soldering preservative (CuOSP) and electroless nickel immersion gold (ENIG) surface finish were achieved using a synchrotron X-ray
imaging technique. It was found that, upon wetting, the rapid dissolution of Cu adjacent to the solid–liquidinterface was followed by near-instantaneous interfacial intermetallic compound (IMC) formation and laterby the independent nucleation and growth of primary intermetallics in the solder alloys. The presence ofindependent primary intermetallics is dependent on the dissolution of Cu from the substrate. The results
provide direct experimental evidence of the effect of different substrate metallization on the formation andgrowth behaviour of primary intermetallics in solder alloys during soldering.
Keywords: In-situ X-ray imaging, soldering, solidification
Background and aim:
Recently electric and electronic devices have rapidly become more advance with denser assemblies
requiring stronger and higher reliability interconnects. The advances of electric and electronic devices havestimulated research to achieve higher quality solder alloys with good reliability. A major consideration related
to the properties of solder alloys is how the solder reacts with the substrate. Recently, several industrial
common Cu substrates with varying surface finish such as organic soldering preservative (OSP) andelectroless nickel immersion gold (ENIG) have been widely adopted [1][2]. Solder joint reliability is known
to be dependent on the formation of an intermetallic compound (IMC) layer which itself is dependent on
factors such as surface finish and substrate and solder composition [1]. Although many researchers havestudied the metallurgical reactions of the solder material itself, there is a need to understand the reactions
with different substrates and surface finishes.
Experimental:
Four solder pastes, Sn-0.7wt%Cu, Sn-0.7wt%Cu-0.05wt%Ni (SN100C solder), Sn-3.5wt%Ag, Sn3.0wt%Ag-0.5wt%Cu (SAC305 solder) paste and a fire retardant 4 (FR4) substrate with Cu organic solderingpreservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finishes were used for thisexperiment.
The experiments were performed at BL20XU beamline in the SPring-8 synchrotron using an in situ X-ray
real-time solidification observation setup developed from previous research [3][4]. Figure 1 shows (a) The
synchrotron real-time in-situ observation heating furnace setup, (b) the soldering sample cell setup, and (c)the FR4 substrate with 0.5 mm ball grid array (BGA) pitch. FR4 substrates were grounded horizontally until
100 μm thickness. With a high degree of coherence, absorption contrast and phase contrast are observed ontransmitted images with enhanced boundaries. The collected signals are then converted into a digital formatof 2000 x 2000 pixels. This area represents a 1 mm x 1 mm area on the BL20XU experiments (giving aresolution of 0.5 μm per pixel). A planar undulator was used as a light source and the radiation wasmonochromatized with Si double-crystal monochromators. An exposure time of 1 s per frame was used tocapture the images. To mimic the process of reflow soldering, a furnace with graphite heating elements where
heat is transferred through radiation in an enclosed sample chamber was used. The solder paste and substrate
were used at 100 µm thickness. The sample was then placed in a sample cell with an observation window

area of 10 x 10 mm2 with a vent for flux outgassing created using a 100 μm thickness polytetrafluoroethylene
sheet placed between two SiO2 plates. Samples were set to be heated from room temperature to approximately
250C at 0.17 C/s and cooled down at 0.33 C/s.

Figure.1．(a) Synchrotron real-time in-situ observation heating furnace setup, (b) soldering sample cell
setup, and (c) FR4 substrate with 0.5 mm ball grid array (BGA) pitch.
Results and discussion:
Reactions of the full soldering process were observed for each sample focusing on the nucleation andgrowth of the interfacial and primary intermetallic compounds.

Figure.2．In-situ real-time observations of reactions between different solder paste and the Cu organic
soldering preservative (Cu-OSP) surface finish substrate. Sn-0.7wt%Cu solder paste at
experimental time of (a) 369 s, (b) 510 s, (c) 568 s; Sn-0.7wt%Cu-0.05wt%Ni solder paste at
experimental time of (d) 390 s, (e) 520 s, (f) 590 s; Sn-3.5wt%Ag solder paste at experimental
time of (g) 340 s, (h) 465 s, (i) 640 s; Sn-3.0wt%Ag-0.5wt%Cu solder paste at experimental
time of (j) 346 s, (k) 492 s, (l) 520 s.
Figure 2 shows the reactions between different solder pastes and the Cu/organic soldering preservative(OSP) surface finish substrate. Sn-0.7wt%Cu solder paste at experimental time of (a) 369 s, (b) 510 s, (c) 568

s; Sn-0.7wt%Cu-0.05wt%Ni solder paste at experimental time of (d) 390 s, (e) 520 s, (f) 590 s; Sn-3.5wt%Ag
solder paste at experimental time of (g) 340 s, (h) 465 s, (i) 640 s; Sn-3.0wt%Ag-0.5wt%Cu solder paste atexperimental time of (j) 346 s, (k) 492 s, (l) 520 s.
It was observed that upon wetting, the rapid dissolution of Cu adjacent to the solid–liquid interface wasfollowed by the near-instantaneous precipitation/growth of an interfacial intermetallic compound (IMC) andlater by the independent nucleation and growth of primary intermetallics in the solder alloys away from thesubstrate. From figure 2, the primary intermetallics formed on a Cu-OSP surface finish are finer on Sn0.7wt%Cu-0.05wt%Ni solder paste. On the other hand, Figure 3 shows the reactions between different solder
pastes and the electroless nickel immersion gold (ENIG) surface finish substrate. Sn-0.7wt%Cu solder paste
at experimental time of (a) 363 s, (b) 510 s, (c) 640 s; Sn-0.7wt%Cu-0.05wt%Ni solder paste at experimental
time of (d) 185 s, (e) 319 s, (f) 350 s; Sn-3.5wt%Ag solder paste at experimental time of (g) 363 s, (h) 455 s,(i) 534 s; Sn-3.0wt%Ag-0.5wt%Cu solder paste at experimental time of (j) 205 s, (k) 340 s, (l) 364 s. It isclearly observed that with ENIG surface finish, primary intermetallic formation in the bulk solder of Sn3.5wt%Ag and Sn-3.0wt%Ag-0.5wt%Cu could be eliminated.

Figure.3．In-situ real-time observations of reactions between different solder paste and the electrolessnickel immersion gold (ENIG) surface finish substrate. Sn-0.7wt%Cu solder paste at
experimental time of (a) 363 s, (b) 510 s, (c) 640 s; Sn-0.7wt%Cu-0.05wt%Ni solder paste at
experimental time of (d) 185 s, (e) 319 s, (f) 350 s; Sn-3.5wt%Ag solder paste at experimental
time of (g) 363 s, (h) 455 s, (i) 534 s; Sn-3.0wt%Ag-0.5wt%Cu solder paste at experimental
time of (j) 205 s, (k) 340 s, (l) 364 s.

Conclusions and future works:
From real-time soldering observations using a synchrotron radiation facility it is clear that substrate plays
a significant role in the formation of primary intermetallics in the bulk solder. In addition to that, certain
element addition such as with Ni can suppress the growth of the primary intermetallic as obtained in theexperiment results. This observation gives a useful insight into the metallurgical reactions during solderingon various solder materials and substrates. Based on these results, investigation of the most dominant
contributioning factor relating to intermetallic formation will be investigated further.
Acknowledgments:
Synchrotron in-situ solidification observations were performed at the Japan Synchrotron Radiation
Research Institute (JASRI) on BL20XU of the SPring-8 synchrotron, under Proposal No. 2015A1675. We
acknowledge travel funding provided by the Australian Synchrotron International Synchrotron Access
Program (AS/IA151/9538) managed by the Australian Synchrotron and funded by the Australian government.
References:[1] Wang, T.H., C.-H. Tsai, and Y.-S. Lai, Effect of the combination of surface finishes and solder balls on
JEDEC drop reliability of chip-scale packages. Microelectronic Engineering, 2012. 98 (0): p. 1-5.
[2] Liu, Y. et al., Interfacial reaction and failure mode analysis of the solder joints for flip-chip LED on ENIG
and Cu-OSP surface finishes. Microelectronics Reliability, (0).
[3] Salleh, M.A.A.M. et al., Rapid Cu6Sn5 growth at liquid Sn/solid Cu interfaces. Scripta Materialia, 2015.
100: p. 17-20.
[4] Salleh, M.A.A.M. et al., In-situ soldering process technique by synchrotron X-ray imaging. Applied
Mechanics and Materials, 2015. 754-755 p. 508-512.