Fenglian Sun

Solderability, IMC evolution, and shear behavior of low-Ag Sn0.7Ag0.5Cu-BiNi/Cu solder joint

The solderability, intermetallic compounds (IMC) evolution, and shear behavior of the low-Ag Sn0.7Ag0.5Cu-3.5Bi-0.05Ni (SAC0705-BiNi)/Cu solder joint was investigated by comparing with Sn0.7Ag0.5Cu (SAC0705)/Cu and Sn3.0Ag0.5Cu (SAC305)/Cu solder joints. Experimental results demonstrated that the melting temperature of Sn0.7Ag0.5Cu-BiNi solder alloy was lower than that of SAC0705 and SAC305 solder. But the melting range of Sn0.7Ag0.5Cu-BiNi was wider. Compared with the other two kinds of alloys, SAC0705-BiNi showed the best wettability. SAC0705/Cu, SAC0705-BiNi/Cu, and SAC305 solder joints appeared similar IMC morphologies and grain size at the beginning of soldering, but evolved to different appearance as the soldering process proceeded. The growth rate of the IMC grains in SAC0705-BiNi/Cu solder joint was the lowest because of the refinement of Ni. SAC0705-BiNi/Cu solder joint showed the highest shear strength before and after being aged, mainly due to the enhancement of solid solution strengthening and dispersion strengthening of Bi and Ni in the bulk solder, as well as the refinement of Ni at the soldering interface.

Comparing drop impact test method using strain gauge measurements

Abstract In this paper, strain gauges are used to measure the dynamic response during drop impact. A set of high-speed data acquisition system is used to capture the real-time strain response of critical locations. Finite Element Modelling is used to determine where to place the strain gauges. Two board level drop impact test methods, A and B, are compared by the strain gauge measurements. Results show significant differences in strain amplitude, strain rate, frequency, rebound, and damping. Method B produces higher strain amplitude, stronger rebound, and less damping, while method A produces higher frequency and strain rate. Damping and rebound are important factors for both test methods. The differences of two test methods are due to the contributions from test condition and test board. Combination test were conducted to investigate the contribution of test condition and test board. Results show that test board dominates strain amplitude, frequency, mode and damping, while test condition shows influence on strain amplitude and rebound. Test condition A produces higher strain amplitude, while test condition B produces stronger rebound. Test board A produces higher frequency, while test board B produces higher strain amplitude and less damping. Test board contributes more to the strain amplitude than test condition. Strain rate is dependent on both the test condition and test board.

Effect of Ni addition on the Sn-0.3Ag-0.7Cu solder joints

It is gradually recognized that Sn-Ag-Cu is better alloy system for application prospect in the lead-free solders. In order to cut down the cost and improve properties, researchers have always been searching for proper compositions and adding elements to improve properties actively. This paper has investigated the effect of Ni addition on Sn-0.3Ag-0.7Cu (SAC0307) low-Ag solder joint. The results of EDS analysis showed that the major IMC formed between the SAC0307 solders and Cu substrate was Cu6Sn5, while for solders with Ni addition the main IMC was (NixCu1-x)6Sn5. After aging, in the case of 0.05% Ni addition, the thickness of IMC layer was the thinnest regardless of aging time. SAC0307-0.05Ni solder is effective to reduce the formation of IMC at the interface during the reflow process and to inhibit the growth of IMC after aging. Additionally, the atom ratio of Ni to Cu in the (NixCu1-x)6Sn5 phase after reflow and after aging were also studied. Some findings of this study can be rationalized by the Cu-Ni-Sn isotherm.

Effect of elevated temperature on PCB responses and solder interconnect reliability under vibration loading

Abstract In this paper, vibration tests are conducted to investigate the influence of temperature on PCB responses. A set of combined tests of temperature and vibration is designed to evaluate solder interconnect reliability at 25xa0°C, 65xa0°C and 105xa0°C. Results indicate that temperature significantly affects PCB responses, which leads to remarkable differences in vibration loading intensity. The PCB eigenfrequency shifts from 290xa0Hz to 276xa0Hz with an increase of test temperature from 25xa0°C to 105xa0°C, during which the peak strain amplitude is almost the same. Vibration reliability of solder interconnects is greatly improved with temperature rise from 25xa0°C to 105xa0°C. Mean time to failure (MTTF) of solder joint at 65xa0°C and 105xa0°C is increased by 70% and 174% respectively compared to that of solder joint at 25xa0°C. Temperature dominates crack propagation path of solder joint during vibration test. Crack propagation path is changed from the area between intermetallic compound (IMC) layer and Cu pad to the bulk solder with temperature increase.

Characterization of interfacial IMCs in low-Ag Sn–Ag–xCu–Bi–Ni solder joints

This research investigated the effects of Cu content on the interfacial IMCs in low-Ag Sn–0.7Ag–xCu–3.5Bi–0.05Ni (xxa0=xa00.3, 0.5, 0.7, and 1.5xa0wt%, respectively) solder joints by deep-etching method and SEM observation. Experimental results indicated that as Cu content increased in the solder, the grain size of the IMCs increased and the thickness of the IMCs decreased on Cu substrate. When the concentration of Cu in the solder was 0.3xa0wt%, the IMC on the soldering interface was (Cu, Ni)6Sn5. The concentration of Ni in (Cu, Ni)6Sn5 IMC was significantly suppressed by the increase of Cu content in the solder. As Cu content increased to 1.5xa0wt%, the concentration of Ni in the IMC decreased to 0 and the IMC transformed from (Cu, Ni)6Sn5 to Cu6Sn5. Due to the increase of Cu content, more and more (Cu, Ni)6Sn5 grains nucleated on Ni substrate, and the morphology of (Cu, Ni)6Sn5 transformed from polyhedrons to tiny prisms.

Effects of Bi and Ni addition on wettability and melting point of Sn-0.3Ag-0.7Cu Low-Ag Pb-free solder

Bi and Ni were added to Sn-0.3Ag-0.7Cu low-Ag solder, to fabricate new low-Ag solders, Sn-0.3Ag-0.7Cu-XBi (X= 1.0, 3.0, 4.5) and Sn-0.3Ag-0.7Cu-XNi (X=0.05, 0.10, 0.15). Melting point tests were carried out with DSC (differential scanning calorimetry) instrument. Wettability tests were conducted on a wetting balance instrument. Test results of the two new solders were compared with that of Sn-0.3Ag-0.7Cu respectively to study the effects of the adding elements on the melting point and wettability of the low-Ag Pb-free solder. It shows that Bi addition has striking positive effects on decreasing the melting point and improving wettability. Ni addition could improve the wettability as well, although not as much as Bi does. And Ni has a negative effect on melting point. With proper adding amount as X=3.0, Bi could significantly improve the wettability and decrease melting point at the same time. However, too much Bi addition could increase the melting range between liquidus and solidus, which may lead to the initiation of solidification crack of the solder joints.

High temperature creep properties of low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu solder joints by nanoindentation method

Purpose n n n n nThis paper aims to investigate the creep properties of the bulks of low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu micro solder joints from 298 to 358 K. The creep constitutive modelling was developed. Meanwhile, the creep mechanism of the bulks of Cu/Sn-Ag-Cu-Bi-Ni/Cu micro solder joints was discussed. n n n n nDesign/methodology/approach n n n n nThe creep properties of the bulks of low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu micro solder joints from 298 to 358 K were investigated using the nanoindentation method. n n n n nFindings n n n n nThe results of the experiments showed that the indentation depth and area increased with increasing temperatures. At the test temperature of 298-358 K, the creep strain rate of the bulks of the micro solder joints increases with the rising of the tested temperature. The values of creep stress exponent and activation energy calculated for the bulks of Cu/Sn-Ag-Cu-Bi-Ni/Cu micro solder joints were reasonably close to the published data. At the tested temperatures, dislocation climb took place and the dislocation climb motion was controlled by the dislocation pipe mechanism, and the second-phase particles enhancement mechanism played a very important role. n n n n nOriginality/value n n n n nThis study provides the creep properties of low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu solder joints at different temperatures. The creep constitutive modelling has been developed for low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu solder joints.

Nanoindentation properties of as-soldered low-Ag SAC-Bi-Ni/Cu

In this study, the nanoindentation properties of the as-soldered low-Ag SAC-Bi-Ni solder alloys on Cu substrate were characterized. The overall evaluations on the elastic modulus, hardness, and creep properties of the new solder alloys were proposed based on the comparisons with low-Ag solder SAC0307, SAC0705, and high-Ag solder SAC305. Experimental results indicated that the hardness of the SAC solders without Bi and Ni addition varied from 127MPa to 196MPa. Meanwhile, the elastic modulus was between 29-36GPa, and the creep stress sensitivity was from 15 to 19. However, the hardness of low-Ag SAC-Bi-Ni solders was between 305-403MPa. Additionally, with Bi and Ni elements addition, the elastic modulus of SAC0705 increased to 35-63GPa, and the value of creep stress sensitivity reached to 25-45. It is clear that the addition of Bi and Ni in low-Ag SAC solder greatly improved its nanoindentation properties, which were critical for the solder joint reliability.

Nanoindentation for measuring mechanical properties of Sn-Ag-Cu-RE BGA solders joints

Creep behavior of the Sn-0.3Ag-0.7Cu-RE(rare earth) BGA (ball grid array) solder joints on copper pad containing 0.05La wt.% and 0.05 Ce wt.% respectively was studied by Berkovich nanoindentation tests with different loading rates at room temperature. The results of the two new solder joints were compared with that of BGA solder joints of Sn-0.3Ag-0.7Cu and Sn-3.0Ag-0.5Cu. The load-depth curves were rate dependent. As the rate increased, the creep depth was increased at same hold time. The derived Youngs modulus with Oliver-Pharr method from unloading curves was loading rate-independent. The Youngs modulus of BGA solder joints of Sn-3.0Ag-0.5Cu, Sn-0.3Ag-0.7Cu, Sn-0.3Ag-0.7Cu-0.5La and Sn-0.3Ag-0.7Cu-0.5Ce are 19.3±1.0 GPa, 17.8±1.2 GPa, 25.23±1.3 GPa and 29.3±1.0 GPa, respectively. In addition, the Youngs modulus of Sn-0.3Ag-0.7Cu-0.05Ce low-Ag BGA solder joint is 52 percent higher than that of Sn-3.0Ag-0.5Cu BGA solder joint. The results indicated that RE additions significantly improve Youngs modulus of BGA solder joints, whereas RE additions have inconspicuous effect on increasing indention hardness of Sn-0.3Ag-0.7Cu BGA solder joint. The stress exponents of 15.5424, 11.5741, 17.6897, and 20.0401 are obtained for BGA solder joints of Sn-3.0Ag-0.5Cu, Sn-0.3Ag-0.7Cu, Sn-0.3Ag-0.7Cu-0.5La and Sn-0.3Ag-0.7Cu-0.5Ce, respectively. Results shows that adding trace amounts of RE elements into low-Ag solders have notable effort to improve the creep resistance and RE elements will play an important role in providing better electronic interconnections.

Effects of adding some elements on solderability of Sn-0.7Ag-0.5Cu solder

Some alloy elements were added to Sn-0.7Ag-0.5Cu solder, to fabricate two new solders, Sn-0.7Ag-0.5Cu-0.05Ni-XBi(X=0, 2.0, 2.5, 3.0, 3.5, 4.0wt %) and Sn-0.7Ag-0.5Cu-XIn(X=0, 0.2, 0.5, 0.8, 1.0, 1.2, 1.5, 1.8 and 2.0wt %). The solderability tests of the two solder alloys have been conducted to study effect of bismuth, nickel and indium content on melting temperature and wettability of solder alloys. Melting point test was carried out with DSC (Differential Scanning Calorimetry) instrument. Wetting area is referred as the index to evaluate the wettability. The results of melting point experiment showed that the peak melting point of Sn-0.7Ag-0.5Cu solder was 220.05°C. The peak melting point of Sn-0.7Ag-0.5Cu-0.05Ni-3.5Bi solder was 215.74°C. And the peak melting point of Sn-0.7Ag-0.5Cu-1.5In solder was 215.27°C. The melting points of two new solders were lower than the recommendatory that of Sn-Ag-Cu solders. The results of wettability experiment of Sn-0.7Ag-0.5Cu-0.05Ni-XBi showed that with the increasing amount of Bi addition, wetting area of alloy solder increased from 54.93 mm2 to 63.27 mm2. The Sn-0.7Ag-0.5Cu-0.05Ni-3.5Bi solder showed best wettability in accordance with wetting area. It also showed that with the increasing amount of indium addition in Sn-0.7Ag-0.5Cu-XIn, wetting area increased from 64.26 mm2 to 77.96 mm2. The Sn-0.7Ag-0.5Cu-1.8In solder showed best wettability in accordance with wetting area.