Study On Microstructure And Properties Of Sn-based Interconnect Solder In Three-dimensional Packaging

With the emergence of emerging industries such as 5G networks,artificial intelligence(AI),and autonomous driving,the demand for high performance,small size,high reliability,and ultra-low power consumption in chips and electronics continues to grow.Solder provides electronic connections in microelectronic packages,ensuring mechanical support and reliability of solder joints,and is an essential material for the operation of electronic equipment.Over the past 50 years,traditional Sn-Pb solders have been widely used because of their excellent performance in electrical connection and mechanical support.Due to the toxicity of Pb,many environmentally friendly Sn-based lead-free solders have been proposed,such as Sn-Bi,Sn-Ag,Sn-Cu,Sn-Ag-Cu,etc.The eutectic Sn58Bi solder alloy has a lower melting point(138℃)than the Sn-Pb solder alloy(183℃).A low melting point will reduce the thermal damage of electronic equipment at high soldering temperatures.In addition,Sn58Bi alloy has higher mechanical strength,good wettability,larger creep resistance and a lower coefficient of thermal expansion,which can effectively ensure the reliability of electronic packaging.However,Sn58Bi solder has some disadvantages.First,due to the inherent brittleness of the Bi element,Sn-Bi solder joints are prone to brittle fracture.Second,the microstructure of Sn58Bi alloy is easy to coarsen under the condition of thermal aging,which reduces the mechanical strength and affects the reliability of the solder joint.Third,the phenomenon of Bi segregation and the excessive growth of the IMC layer easily lead to the generation of holes,which will become the source of cracks and eventually cause the failure of electronic products.In this paper,the composite solder was prepared by adding particles to the Sn58Bi alloy to improve the comprehensive properties of the Sn58Bi alloy.The specific research content is as follows:(1)The wetting,melting and mechanical properties of Sn58Bi-x Fe(x=0,0.03,0.05,0.1,0.2 and 0.4 wt%)composite solder was investigated.The spreading area method was used to characterize the wettability of the solder.The results showed that when x=0.05 wt%,the spreading area was increased from 107.88 mm~2to 122.80 mm~2.Differential scanning calorimetry was used to characterize the melting characteristics of the solder.Fe particles had no significant effect on the melting point of the solder.By measuring the shear strength of the solder joint,it was found that when the Fe particle content was 0.05 wt%,the shear strength of the solder joint was increased from 30.10MPa to 49.90 MPa,reaching a maximum value.(2)The scanning electron microscope was used to study the microstructure and interfacial structure of the solder joints after soldering and aging.Compared with the original solder,Sn58Bi-0.05Fe solder has a relatively uniform and finest microstructure,and the thickness of the interface IMC is reduced from 0.79μm to 0.62μm.During aging,compared with Sn58Bi solder,the coarsening of the microstructure in Sn58Bi-0.05Fe solder was suppressed,and the growth of the interfacial IMC was slower.(3)The original solder and composite solder were applied to transient liquid phase(TLP)bonding.The effect of Fe particles and Cu Zn Al particles on the microstructure of Cu/Sn Bi/Cu TLP solder joints was studied.The solders will form full IMC solder joints during the TLP bonding process,indicating that Sn58Bi solder,Sn58Bi-0.05Fe solder and Sn58Bi-0.5Cu Zn Al solder are suitable for low-temperature bonding and high-temperature applications.The addition of Cu Zn Al particles and Fe particles can inhibit the growth of IMC and the generation of voids at the Cu/Sn58Bi/Cu interface.The optimization effect of adding Fe particles is more significant.(4)The finite element model of the 3D package is established,and the stress distribution of the package under the thermal cycling load is analyzed.The results show that the maximum stress occurs at the lower surface of the IMC solder joints at the corners in contact with the chip.After optimization,the von Mises stress was reduced by 69.96%,the signal-to-noise ratio was increased by 10.46 d B,and the fatigue life of the solder joint was increased from 415 cycles to 533 cycles,indicating that the reliability of the 3D IC has been significantly improved.