Mechanical strength of thermally aged Sn-3.5Ag/Ni-P solder joints

This work investigates the effect of reflow and the thermal aging process on the microstructural evolution and microhardness of five types of Sn-Ag based lead-free solder alloys: Sn-3.7Ag, Sn-3.7Ag-1Bi, Sn-3.7Ag-2Bi, Sn-3.7Ag-3Bi, and Sn-3.7Ag-4Bi. The microhardness and microstructure of the solders for different cooling rates after reflow at 250°C and different thermal aging durations at 150°C for air-cooled samples have been studied. The effect of Bi is discussed based on the experimental results. It was found that the microhardness increases with increasing Bi addition to Sn-3.7Ag solder regardless of reflow or thermal aging process. Scanning electron microscopy images show the formation of Ag 3 Sn particles, Sn-rich phases, and precipitation of Bi-rich phases in different solders. The increase of microhardness with Bi addition is due to the solution strengthening and precipitation strengthening provided by Bi in the solder. The trend of decrease in microhardness with increasing duration of thermal aging was observed.

Section: Introductionmentioning

confidence: 99%

Effect of reflow and thermal aging on the microstructure and microhardness of Sn-3.7Ag-xBi solder alloys

Acoff

2006

“…Indeed there is plenty of evidence for interface strength degradation with either extended aging or reflow. [20][21][22][23][24][25][26] The interface reaction was found to smoothen the interface and/or generate interface voids, causing the degradation. The passage of electric current is likely to aid interdiffusion during the interface reaction, expediting the degradation process.…”

Section: Effect Of Electromigration On the Mechanical Properties Of Smentioning

confidence: 99%

Effect of Electromigration on the Mechanical Performance of Sn-3.5Ag Solder Joints with Ni and Ni-P Metallizations

Kumar

Yang

Wong

et al. 2008

Self Cite

The effect of moderate electric current density (1 9 10 3 to 3 9 10 3 A/cm 2 ) on the mechanical properties of Ni-P/Sn-3.5Ag/Ni-P and Ni/Sn-3.5Ag/Ni solder joints was investigated using a microtensile test. Thermal aging was carried out at 160°C for 100 h while the current was passed. The interfacial microstructure and intermetallic compound (IMC) growth were analyzed. It was found that, at these levels of current density, there were no observable voids or hillocks. Samples aged at 160°C without current stressing failed mostly inside the bulk solder with significant prior plastic deformation. The passage of current was found to cause brittle failure of the solder joints and this tendency for brittle failure increased with increasing current density. Fractographic analysis showed that, in most of the electrically stressed samples, fracture occurred at the interface region between the solder and the joining metals. The critical current density that caused brittle fracture was about 2 9 10 3 A/cm 2 . Once brittle fracture occurred, the tensile toughness, defined as the energy per unit fractured area, was usually lower than $5 kJ/m 2 , compared with the case of ductile fracture where this value was typically greater than $9 kJ/m 2 . When comparing the two types of joint, the brittle failure was found to be more severe with the Ni than with the Ni-P joint. This work also found that the passage of electric current affects the IMC growth rate more significantly in the Ni than in the Ni-P joint. In the case of the Ni joint, the Ni 3 Sn 4 IMC at the anode side was appreciably thicker than that formed at the cathode side. However, in the case of electroless Ni-P metallization, this difference was much smaller.

“…[24][25][26][27][28][29] The phosphorus-rich layer was believed to be the cause of the weakening in shear strength found by Alam et al 24,25 In another study, the brittleness of Ni 3 Sn 4 and formation of Kirkendall voids were also cited as reasons for the decrease of shear strength. 26 Relatively little research has been reported for tensile strength degradation.…”

Section: Effect Of Thermal Aging On Solder Joint Fracture Behaviormentioning

confidence: 99%

“…Another distinct feature of the tensile test is that the stress distribution in all interface layers is the same, so the test can be used to reveal the weakest layer or interface. 27 In the present work, tensile testing and the fracture behavior of Sn-3.5Ag/Ni-P and Sn-37Pb/Ni-P joints in different thermal aging conditions were investigated, and the influence of interface reaction and solder composition on the solder joint strength will be discussed.…”

Section: Introductionmentioning

confidence: 99%

Effect of Interfacial Reaction on the Tensile Strength of Sn-3.5Ag/Ni-P and Sn-37Pb/Ni-P Solder Joints

Chen

Kumar

et al. 2006

This work investigates the effect of interfacial reaction on the mechanical strength of two types of solder joints, Sn-3.5Ag/Ni-P and Sn-37Pb/Ni-P. The tensile strength and fracture behavior of the joints under different thermal aging conditions have been studied. It is observed that the tensile strength decreases with increasing aging temperature and duration. Associated with the tensile strength decrease is the transition of failure modes from within the bulk solder in the as-soldered condition toward failures at the interface between the solder and the intermetallic compounds (IMCs). For the same aging treatment, the strength of the Sn-3.5Ag/Ni-P joint degrades faster than that of Sn-37Pb/Ni-P. The difference between the two types of joints can be explained by the difference in their interfacial reaction and growth kinetics. An empirical relation is established between the solder joint strength and the Ni 3 Sn 4 intermetallic compound thickness.