Preparation And Bonding Of Micro-Bumps With Flexible Arrayed Cu Nanowires

Flexible electronic devices have broad application prospects in information,energy,and medical fields.Driven by information technologies such as big data,cloud computing,and artificial intelligence,multifunctional and highly integrated flexible electronics have become future development trend.The key technology of flexible electronics is integrated packaging,and the key structure of flexible electronics is the micro-joints.However,the current microjoints cannot meet the need for flexibility.In this study,two diameters of arrayed Cu nanowire micro-bumps were prepared by direct current electroplating.The effects of main parameters on the growth and morphology of nanowires were studied,and the growth mechanism of nanowires was discussed.The interconnection of Cu nanowire micro-bumps was achieved by direct sintering and solder bonding method,and its properties were characterized.The research results are as follows:(1)The electricity density has a significant impact on the growth and morphology of the Cu nanowires.As the electricity density decreases,the growth rate of the Cu nanowires slows down,the diameter increases,and the height consistency improves.By regulating the electricity density and using the freeze-drying method,arrayed Cu nanowire micro-bumps with a diameter of 221 nm and 45 nm were prepared.The effect of electroplating time on the growth rate of Cu nanowires increased first and then slowed down.It is related to the difficulty of liquid mass transfer and the size of the template pore size.In addition,within the optimized electricity density range,arrayed Cu nanowire micro-bumps with consistent height were prepared by grinding and polishing process.It was 195 min shorter than nanowires selfformation,and the preparation efficiency of micro-bumps was improved.But there was a problem with the uniform dispersion of nanowires.The compression test results showed that the 221 nm and 45 nm arrayed Cu nanowires produce 26.09 % and 50.96 % elastic deformation,respectively,exhibiting a good flexibility.(2)The 221 nm and 45 nm micro-bumps as the base layer,the arrayed Cu-Sn nanowire micro-bumps with diameters of 49 nm and 228 nm were prepared by adjusting the electricity density and freeze-drying method.The melting points of the two Sn nanowires are about232.43 °C and 166.83 °C,respectively,and the latter can achieve low-temperature interconnection below 200 °C.Otherwise,Cu pads with a grain size of about 20 nm were prepared by direct current electroplating,which was used as interconnect pads for arrayed Cu nanowire micro-bumps.(3)Through the direct sintering interconnection approach,the 45 nm arrayed Cu nanowires were fused with Cu nano-pads,achieveing direct sintering interconnection.The shear strength is about 10 MPa,the resistance value is about 2.878 mΩ,and it keeps well after2000 times of bending with a 5 mm bending radius.Through the solder welding interconnection approach,the 49 nm and 228 nm arrayed Cu-Sn nanowire micro-bumps and pads form a uniform Sn interconnection layer and an interface intermetallic metallurgical connection layer,achieveing the solder welding interconnection.The shear strength is about15 Mpa and 12 Mpa,respectively.The resistance values is about 2.518 mΩ and 2.268 mΩ,respectively,and keeps well after 2000 times of bending with a 5 mm bending radius,which meets the most demanding medical component standard in the national standard GB/T 42471-2023 "Flexible nano energy-storage device bending test method".