Pressureless Low-Temperature Sintering Of Silver Nano-Solder Paste Based On Surface Activation

At present,the interconnection materials of electronic packaging are undergoing revolutionary changes.The traditional Sn base brazing material can not meet the requirements of high temperature service of the third generation semiconductor in high power devices because of its low melting point.The sintering temperature of metal nanoparticles is much lower than the melting point of their block materials,and the nano-solder paste prepared can achieve the goal of"low temperature connection,high temperature service",which has been paid more attention in recent years.However,because the metal nanoparticles i n the solder paste often cover organic matter on the surface,it is necessary to apply a large auxiliary pressure(>1 MPa)and a higher sintering temperature(>250℃)in the sintering process,which is not only easy to damage the semiconductor chip,but al so makes it very difficult to interconnect the laminated chip.Therefore,the search for a pressure-free cryogenic sintering process is an effective way to promote the wide application of nano-solder paste.Based on the principle of surface activation,the use of oxygen plasma and formaldehyde/sodium hydroxide reductive steam two-step activation of silver nano-solder paste,can effectively remove the organic matter and metal surface oxides coated with nanoparticles,the successful realization of nano-silver solder paste pressureless low-temperature sintering,and ensure that the prepared solder joints have good mechanical,electrical and thermal properties.Through material characterization and theoretical analysis,the effects of different activation proces s parameters on the activation effect of Ag and Cu-Ag nano-solder paste are studied.The mechanism of pressureless low-temperature sintering is expounded,which provides a new solution for three-dimensional integration of power semiconductor devices.The effects of single-step activation(oxygen plasma)and two-step activation(oxygen plasma→reduction steam)on the pressureless sintering process of silver nano-solder paste are studied,and it was found that two-step activation had better effect.Using oxygen plasma activation 10 s and formaldehyde/sodium hydroxide reductive steam purge 30 s of two-step activation process,at 200℃ of low-temperature can achieve low porosity(<3%)without pressure sintering,solder joint shear strength up to 25MPa,and has a good thermal conductivity and electrical conductivity.Through surface analysis,morphological characterization and comprehensive thermal behavior analysis,the results show that two-step activation pretreatment can not only remove the excess organic components in silver nano-solder paste,but also eliminate the negative effects of silver oxide on the sintering layer of silver nano-solder paste,so as to reduce the sintering temperature from 225℃ to 194℃,The mechanism model of pressureless low-temperature sintering of silver nano-solder paste is presented.The above method is suitable for 5x5 mm~2 and 10x10 mm~2 chip-level pressureless sintering of,which can effectively reduce the hole area in the sintering layer and improve the density of its connecting layer.The Cu-Ag nano-solder paste prepared with a particle size of two nm was studied,and the low temperature and non-pressure sintering effect of the solder paste in the second-step activation process.The results show that the two-step activation can also effectively reduce the pore area in the sintering layer of Cu-Ag nano-solder paste and improve the density and shear strength of the 350℃ under pressureless sintering.