Effects Of ⁶⁰Co ?-ray Irradiation And Low-temperature Thermal Cycling On The Microstructure And Properties Of AuSn20 Solder Joint

The AuSn20 solder provides not only excellent thermal and electrical conductivity,but also good oxidation resistance.Besides,the shear strength and fatigue resistance of the solder joint are significantly superior to traditional Sn-Pb solder and lead-free Sn-based solders,hence the solder has been widely used in the high-end packaging of microelectronic and optoelectronic devices.Despite of the high cost,the AuSn20 alloy still plays an indispensable role in the electronic package for military and aerospace applications,especially in the highly reliable connection between aerospace high-power chips and circuit substrates.Recently,rapid development of the aerospace technology from near-field exploration into vast deep space exploration makes the space electronic equipment more lightweight and miniaturized,and thus higher requirements for solder joint reliability of future aerospace electronic devices have been put forward.If the resistance of solder joints to space environment such as space radiation and thermal shock can be improved,many radiation hardening structures and active thermal control equipments will be eliminated,which helps to realize the lightweight and high-reliability design of the aerospace electronic systems.Therefore,it's imminent to explore the impact laws of space environment effects on the solder joint reliability of electronic devices.In this paper,⁶⁰Co ?-ray as an irradiation source and-55?125?temperature as thermal cycling conditions were adopted to study the influences of radiation total dose effect on the solder matrix,as well as the low-temperature thermal impact and irradiation coupling with thermal impact.Additionally,the interfacial microstructure and mechanical properties of AuSn20 solder joints had also been investigated.Firstly,Cu/Ni/Au/AuSn20/Au/Ni/Cu solder joints were prepared by eutectic friction soldering process to simulate the bonding between chips and circuit substrates in practical applications,thereafter the soldering rate and cross-sectional microstructure of AuSn20 solder joints were observed via X-ray detector and SEM equipped with EDS.The results showed that the as-soldered joint provided high soldering rate,dense joint microstructure and well-connected interfaces.In addition,the plated Au layers of substrates were completely dissolved into the solder matrix,while the palted Ni layers still remained.A thick Au layer from the top substrate resulted in the formation of(Au,Ni)₅Sn/Ni interface structure on the top,which is favourable to better chip protection.For the underlying interface,an IMC(intermetallic compound)layer of(Ni,Au)₃Sn₂ was produced,forming the AuSn20/(Ni,Au)₃Sn₂/Ni interface structure.Afterwards,during the ⁶⁰Co ?-ray irradiation with a dose rate of 0.2 Gy(Si)/s,it was found that the matrix morphology of the AuSn20/Au/Ni/Cu solder joints was not apparently altered,indicating that the absorption of?-ray in both the AuSn and Au₅Sn phases were unobvious.Nevertheless,irradiation significantly accelerated the dissolution and diffusion of the Ni atoms in plating layer into the solder matrix,the growth of Ni-Sn compounds at interfaces was therefore promoted.After 1000 h of irradiation,a thin and smooth(Ni,Au)₃Sn₂ layer was generated at the(Au,Ni)₅Sn/Ni interface on top.While the original(Ni,Au)₃Sn₂ IMC layer at the underlying interface continued to grow into the solder layer,resulting in an increased thickness and roughness,and a new(Ni,Au)₃Sn phase was formed near the plated Ni.In comparison,the growth rate of IMC phases at the underlying interface was significantly higher than that of the top interface.The irregular growth of the interfacial Ni-Sn compounds caused a significant deterioration in the shear forces of AuSn20 micro-joints with increase of irradiation durations.Finally,the shear force of the joint irradiated for 1000 h decreased by 17.5%compared to that of the as-soldered sample(88.12 N).The shear fracture of AuSn20 solder joints was also transformed from ductile failure in the solder layer into brittle failure at the solder/IMC interface.During the thermal cycling for 0?200 cycles,it was observed that the solder matrix of AuSn20solder joints slightly coarsened,and the interfacial IMC phases grew rapidly.After 200 thermal cycles,a thick and irregular(Ni,Au)₃Sn₂ layer was formed at the top interface,in which some dispersed granular phases identified as(Ni,Au)₃Sn compound were also detected.Regarding the underlying interface,the original(Ni,Au)₃Sn₂ phases abnormally grew and shaped like long needles or rods.Moreover,a(Ni,Au)₃Sn IMC layer occurred at the(Ni,Au)₃Sn₂/Ni interface.The growth of interfacial IMC layer during thermal cycling was attributed to the increased atomic interdiffusion rate,so that(Ni,Au)₃Sn₂ particles near the interface first grew and then merged with interface layer.And the growth rate was evidently faster than that of the irradiated solder joints.In addition,owing to the CTE(coefficient of thermal expansion)mismatch between Ni and Cu,the connection zone at Ni/Cu interface generated thermal strain and residual stress during temperature cycling,thereby inducing many micro-cracks and voids as well as partial spalling of Ni layer from Cu substrate,which reduced the reliability of solder joints.Therefore,the shear forces of AuSn20 solder joints constantly declined with the increased number of thermal cycles,the value of the joint after 200 thermal cycles decreased by 25.3%compared to that of the as-soldered sample.And the structural defects including micro-cracks and voids became new crack sources during the shearing process.After 0?200 thermal cycles of the AuSn20 solder joints irradiated for 1000 h,SEM micrographs revealed a continuous growth of the interfacial IMC layers and the interface structures similar to that of the solder joints after thermal cycling.However,the growth rate of interfacial IMC layer was lower than that of the solder joints after thermal cycling.This is because the existence of the initial IMC layer in the irradiated joints reduced the atomic interdiffusion rate at interface and then inhibited the rapid growth of intermetallic compounds.Similarly,some serious micro-cracks and voids defects emerged at the Ni/Cu interfaces of solder joints,which deteriorated the adhesion of plated Ni layer onto substrate.Consequently,the shear forces of the irradiated AuSn20 solder joint with thermal cycling decreased with the increased number of thermal cycles,the value of the solder joints after 200cycles was reduced by 23.74%by comparison to that of the irradiated joints of 1000 h.Additionally,the shear fracture positions of solder joints were also gradually transferred to the interfacial IMC layer,IMC/Ni and Ni/Cu interfaces.The shear forces here were lower than that of the solder joints after thermal cycling,which indicated that the coupling effect of irradiation and thermal cycling had seriously deteriorated the mechanical reliability of AuSn20 solder joints.However,it still remained the technical index that was higher than 60%of the original solder joint strength.