K. Suganuma et al.: Heat resistance of Sn–9Zn solder/Cu interface with or without coating
alloy. In addition, because of poor oxidation resistance of
heat exposure was observed by optical microscopy
(OM), scanning electron microscopy (SEM), and elec-
tron probe microanalysis (EPMA).
Sn–Zn alloys, only a limited number of works have been
focused on the Sn–Zn alloy system. Recently, some of
the present authors have developed a new Sn–Zn solder
paste with which an air atmosphere reflow process can be
6
III. RESULTS AND DISCUSSION
applied for surface mount technology (SMT). This new
solder paste is expected to break through in promoting
the lead-free soldering technology. Then the aim of
the present work is to examine the heat resistance of
the Sn–Zn/Cu interface with or without coating on a Cu
substrate.
A. Wetting of new Sn–Zn paste and strength of
solders/Cu interface and their changes after
high-temperature exposure
The applicability of an air reflow process is the most
advanced feature of the present solder paste. Figure 1
shows the comparison of wetting properties for Sn–9Zn
and Sn–8Zn–3Bi alloys with various lead-free and lead-
bearing solders in air. Even though all lead-free solders
in the figure have wettability worse than the Sn–Pb eu-
tectic solder, the present Sn–Zn with or without Bi ex-
hibited good wettability almost equivalent to Sn–Ag and
Sn–Cu eutectic solders, which have been recognized as
the two of the best choices as lead-free solders. The addition
of Bi into Sn–Zn seems to improve wettability slightly. In
fact, the several reflow trials in air showed the excellent
solderability of this alloy as shown in the photograph in-
serted in Fig. 1. A quadra flat package (QFP) is connected
on a circuit board without any serious misconnection.
Figure 2 summarizes the tensile strength of the Cu
joints soldered with various solders with or without heat-
treatment at 125 and 150 °C. It is noteworthy that the
joint soldered by the present Sn–9Zn solder paste has
quite high strength just after soldering. The strength ex-
ceeds about 50 MPa, which is far beyond the strength
II. EXPERIMENTAL PROCEDURE
A. Materials and soldering
The new Sn–9 wt% Zn alloy paste was supplied by
Senju Metal Industry Co. Ltd., Saitama, Japan. Other
alloys such as Sn–3.5 wt% Ag, Sn–37 wt% Pb eutectic
alloys, and Sn–2 wt% Ag–7.5 wt% Bi–0.5 wt% Cu (Al-
loy-H) were also prepared for comparison in the same
3
process mentioned in the previous paper.
Cu used was 99.99 wt% pure and was a 15-mm-
diameter rod. The rod was cut into 15-mm-long short
columns, and one of their ends becomes a face to be
soldered. The short columns were used for tensile test
after soldering. The soldering face was polished to an
optical flatness. Some of the Cu columns were electroless
plated with a Ni/Pd/Au coating layer. The average thick-
ness of each sub layer was 0.6 m/Ni, 0.1 m/Pd, and
0.06 m/Au.
Solder sheets and Cu columns were ultrasonically
3
obtained by a Sn–9Zn solder in the previous work, i.e.,
cleaned in ethanol before soldering. Soldering was
carried out in air at 250 °C for 5 min. For a tensile speci-
men, two Cu columns were soldered together with the
solder paste or with the solder sheets in between them.
The length of the joint becomes about 30 mm by this
treatment.
2
5 MPa for the joint soldered in vacuum. Such low
strength in the previous work was primarily attributed to
poor wetting between Sn–Zn and Cu even in vacuum.
Many defects were found at the interface. In contrast, the
excellent strength obtained by the present solder paste is
achieved by the improvement of wetting property by
modifying the flux in the paste.
B. Evaluations
Tensile specimens, which were rectangular plates
4
mm wide × 1 mm thick × 30 mm long, were cut from
the joint. Tensile strength vertical to a soldering interface
was measured at a crosshead speed of 0.5 mm/min at
room temperature. The influence of high temperature ex-
posure on joint strength was evaluated. The joints were
heat-treated at 125 and 150 °C for 100 h in air, which
have been recognized as the typical highest temperatures
for the electronic circuits of consumer electronics and for
vehicles engines. After the exposure, tensile strength was
also evaluated in the same manner mentioned above. For
each piece of data, more than two samples were tested.
Wettability of solders on a Cu plate was evaluated by
the spreading area of solders in air followed by the JIS Z3
1
97 standard. The number of samples was 6 for each
FIG. 1. Spreading of various solders on Cu with a photograph of air
reflow QFP on a printed circuit board with Zn–8Bi–3Bi paste.
piece of data. Interface microstructure before and after
J. Mater. Res., Vol. 15, No. 4, Apr 2000
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