Low-cycle fatigue behavior of Sn-Ag, Sn-Ag-Cu, and Sn-Ag-Cu-Bi lead-free solders

Low-cycle fatigue (LCF) tests on as-cast Sn-3.5Ag, Sn-3Ag-0.5Cu, Sn-3Ag-0.5Cu-1Bi, and Sn-3Ag-0.5Cu-3Bi solders was carried out using a noncontact strain-controlled system at 20°C with a constant frequency of 0.1 Hz. The addition of Cu does not significantly affect the fatigue life of eutectic Sn-Ag solder. However, the fatigue life was significantly reduced with the addition of Bi. The LCF behavior of all solders followed the Coffin-Manson relationship. The fatigue life of the present solders is dominated by the fracture ductility and can be described by the ductility-modified Coffin-Manson’s relationship. Steps at the boundaries of dendrite phases were the initiation sites for microcracks for Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-3Ag-0.5Cu-1Bi solders, while for Sn-3Ag-0.5Cu-3Bi solder, cracks initiated along both the dendrite boundaries and subgrain boundaries in the dendrite phases. The linking of these cracks and the propagation of cracks inside the specimen occurred both transgranularly through eutectic phases and intergranularly along dendrite boundaries or subgrain boundaries.     

Creep behavior of eutectic Sn-Cu lead-free solder alloy

Tensile creep behavior of precipitation-strengthened, tin-based eutectic Sn-0.7Cu alloy was investigated at three temperatures ranging from 303–393 K. The steady-state creep rates cover six orders of magnitude (10⁻³−10⁻⁸ s⁻¹) under the stress range of σ/E=10⁻⁴−10⁻³. The initial microstructure reveals that the intermetallic compound Cu₆Sn₅ is finely dispersed in the matrix of β-Sn. By incorporating a threshold stress, σ _th, into the analysis, the creep data of eutectic Sn-Cu at all temperatures can be fitted by a single straight line with a slope of 7 after normalizing the steady-state creep rate and the effective stress, indicating that the creep rates are controlled by the dislocation-pipe diffusion in the tin matrix. So the steady-state creep rate, , can be expressed as exp , where Q_c is the activation energy for creep, G is the temperature-dependent shear modulus, b is the Burgers vector, R is the universal gas constant, T is the temperature, σ is the applied stress, A is a material-dependent constant, and , in which σ _OB is the Orowan bowing stress, and k_R is the relaxation factor. An erratum to this article is available at .     

SnAgCuRE系钎料的钎焊接头组织与力学性能

以Sn2.5Ag0.7Cu为基础,添加微量的稀土(RE)r(Ce︰La)为4︰1,研究了钎焊接头的显微组织与力学性能。结果表明:添加微量的RE后,钎料与Cu试样间的界面层厚度明显减小,且界面处的组织更加平滑,相应地其剪切强度随微量RE的添加而增大,并在RE含量(质量分数)为0.1%时达到最大值36MPa。     

无铅焊膏用助焊剂的制备与研究

无铅焊膏用助焊剂需要一定的黏性,选择有机酸和有机胺混合作为活性物质,配合一定量的松香配制出一种助焊剂。通过控制助焊剂中松香的含量,降低不挥发物的含量,减少残留物对元器件的腐蚀。依据标准对所配制的助焊剂进行了性能测试,并与一种松香含量较高的助焊剂比较。结果表明:助焊剂的松香含量降低约10%,在260℃焊接后不挥发物含量降低了近6%,黏度较好,无卤化物,无毒,环保,符合焊膏用助焊剂的要求。     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Application of an asymmetrical four point bend shear test to solder joints

The asymmetrical four-point bend shear (AFPB) test method was used to measure the shear strength and creep properties through the stress relaxation experiments using three different Pb-free solder joint compositions in an assolidified condition. Since it was difficult to shear the uniform specimens and the local bending usually occurs at the inner loading points, the notches were introduced at the joint line to preferentially weaken this region. The stress analysis by finite element modeling showed that the straight notches transform the parabolic shear stress distribution in the uniform specimen into a relatively uniform shear distribution along the bond line in the notched specimens. Therefore, the shear strength results from the notched specimens are expected to be much more accurate. Experiments showed that both the Sn-3.6Ag-1Cu (wt.%) and Sn-3.6Ag-1Cu-0.45Co joints have superior strength and creep properties as compared to the Sn-3.5Ag joint. However, there was no statistical difference between the shear strength of the Sn-3.6Ag-1Cu and Sn-3.6Ag-1Cu-0.45 Co joints. Moreover, the difference between the creep resistance of these two types of joints was small.     

Microstructure and shear strength evolution of SnAg/Cu surface mount solder joint during aging

The effect of aging at 150°C on the microstructure and shear strength of SnAg/Cu surface mount solder joint has been investigated with comparison to 62Sn36Pb2Ag/Cu. It is found that the diffusion coefficient of intermetallic compounds at SnAg/Cu interface is smaller than that of intermetallic compounds at SnPbAg/Cu interface at 150°. The shear strength of SnAg solder joint is higher and decreases at a smaller rate during aging compared to that of SnPbAg solder joint. The fracture surface analysis shows that as the aging time increases, the fracture takes place along the solder/Cu₆Sn₅ interface with an extension toward the Sn−Cu intermetallic layer.     

Interfacial reactions between a Pb-Free solder and die backside metallizations

The interfacial reactions between Sn-3.0 Ag-0.7 Cu solder and backside metallizations on two semiconductor devices, field-effect transistors (FET) and diode, are studied. The metallizations on both devices were vacuum evaporated Ti/Ni/Ag. The intermetallic compounds (IMC) formed near the diode/solder and FET/solder joints during reflow, and the interdiffusion processes during solid state aging are characterized by the quantitative energy dispersive x-ray analysis and the x-ray mapping technique in a scanning electron microscope. Two different intermetallic compounds are found near the diode/solder interface. Both are in the form of particles, not a continuous layer, and are referred to as IMC-I and IMC-II. IMC-I corresponds to Ni₃Sn₄, with Cu atoms residing on the Ni sublattice. It is uncertain whether IMC-II is Cu₆Sn₅ or a Cu-Ni-Sn ternary phase. Near the as-reflowed FET/solder interface, both isolated scallops and a skeleton-like layer of Ni₃Sn₄ are observed. The primary microstructural dynamics during solid-state aging are the coarsening of IMCs and the reactions involving the Ni-and Ti-layer with Sn and Au. While the reaction with the Ni-layer yields only Ni₃Sn₄ intermetallic, the reaction involving the Ti-layer suggests the formation of Ti-Sn and Au-Sn-Ti intermetallics. The latter is due to the diffusion of Au from the substrate side to the die side. It is postulated that the kinetics of the Au-Sn-Ti layer is primarily governed by the diffusion of Au through the Ni₃Sn₄ layer by a grain boundary mechanism.     

Interconnection Technology Based on InSn Solder for Flexible Display Applications

A novel interconnection technology based on a 52InSn solder was developed for flexible display applications. The display industry is currently trying to develop a flexible display, and one of the crucial technologies for the implementation of a flexible display is to reduce the bonding process temperature to less than 150°C. InSn solder interconnection technology is proposed herein to reduce the electrical contact resistance and concurrently achieve a process temperature of less than 150°C. A solder bump maker (SBM) and fluxing underfill were developed for these purposes. SBM is a novel bumping material, and it is a mixture of a resin system and InSn solder powder. A maskless screen printing process was also developed using an SBM to reduce the cost of the bumping process. Fluxing underfill plays the role of a flux and an underfill concurrently to simplify the bonding process compared to a conventional flip‐chip bonding using a capillary underfill material. Using an SBM and fluxing underfill, a 20 μm pitch InSn solder SoP array on a glass substrate was successfully formed using a maskless screen printing process, and two glass substrates were bonded at 130°C.     

改进钎焊技术的有效途径

改进钎焊技术需从钎焊工艺和焊料选择两方面入手。选用Ｓｎ／Ｐｂ比为６３／３７，且在惰性气体保护下铸造或轧制的焊料并采用抗氧化磷片调整焊锡的流动性是改进钎焊技术的有效途径。