Study On Brittle Fracture Mechanism Of Sn-based Solder Alloys At Low Temperature

In space exploration and aerospace applications,some electronics and devices in spacecraft have to suffer the cabin environment,such as extremely low temperature and large temperature ranges.Solder joints in the electronic circuits are the most sensitive parts to the thermal and mechanical environment.The mechanical properties of solder joints have changed dramatically in the extreme environment and the solder joints lead to failure suffering the damage.Therefore,it is very important to study the mechanical properties and failure modes of the interconnection materials for electronic components in extremely low temperature environment.Sn-based solders are the most commonly used electronic interconnection materials at present.Studies have shown that the ductile-to-brittle transition of fracture modes will occur for Sn-based solders at low temperatures.And the addition of Ag elements has a great impact on the failure temperatures and modes.Therefore,the purpose of the project is to study the brittle fracture mechanism of Sn-based solder.Pure Sn and 5 Sn-based solders,including 99.3Sn0.7Cu solder,96.5Sn3Ag0.5Cu solder,62Sn36Pb2 Ag solder,63Sn37 Pb solder,10Sn90 Pb solder,were selected as the study objects to be performed in the Charpy impact test from 20? to-150?.The impact energy was 150 J and the impact velocity was 5.24m/s.The Ductile-to-Brittle Transition Temperature(DBTT)of each solder could obtain by observing the impact curve or the ratio of ductile and brittle fracture areas on the fracture morphology after the Charpy test.The fracture microstructures at different temperatures were observed under Scanning Electron Microscope(SEM).And the second phase observed was analyzed by Energy Dispersive Spectrometer.At last,the fracture mechanism of pure Sn and Sn-based solders at different temperatures was obtained.The results showed that the DBTT of pure Sn is about-50?.And there was a large fiber area in the range from 20? to-40? on the fracture surface.The fracture mode was mainly intergranular fracture mode which is crystal-like fracture morphology from-50 ?,and there was local transgranular fracture mode occurring that cleavage morphology was observed.For 99.3Sn0.7Cu solder,the addition of Cu element in the Sn-based solder made a slightly increase in fracture toughness when it compared with pure Sn solder,but the DBTT were almost the same for the 2 solders.Cu6Sn5,the fibrous second phase,was formed in the Sn matrix,which facilitated the formation of cleavage fracture.The DBTT of 96.5Sn3Ag0.5Cu solder increased to-40? because of the addition of Ag element.The Ag3 Sn particles of dendritic crystal structure and fibrous structure were observed and the fracture mode was transgranular fracture.Due to the addition of Pb element,the ductile-to-brittle transition temperature range of 62Sn36Pb2 Ag solder was wider and the transition occurred gradually.The DBTT of it was-50? and the fracture mode was quasi-cleavage fracture.Ag3 Sn particles,as the crack sources,were observed in the vicinity of the tearing ridges around the small planes that they facilitated the occurrence of that fracture mode.There was no ductile-to-brittle transition for 63Sn37 Pb solder and dimples were the main fracture morphology while the brittle fracture morphology only occurred in some regions.It was ductile fracture mode all the time for 10Sn90 Pb solder,in which the content of Pb increased to a certain extent.The Sn-rich phase was observed that could facilitate the occurrence of the microporous aggregation fracture.