Pressureless Sintering And Properties Of Micron Silver Paste For Electrical Connections

With the rapid development of semiconductor industry,miniaturization and integration of electronic devices have gradually become a new trend in the industry development,which will inevitably generated more heat in the same area.In order to maintain the performance and stability of the device,we need a bonding materials with the higher heat dissipation capacity.Traditional bonding materials such as epoxy resin base materials and alloy solder can meet the application of devices in the low temperature,but its limited thermal conductivity and electrical conductivity can not meet the application of devices in the high temperature.So variety of alternative bonding materials have been developed to meet the application of devices in the high temperature.Among the various alternative materials,sintered silver joints have attracted more and more practitioners’attention due to their excellent thermal conductivity and electrical conductivity.The sintered silver paste can be divided into nano silver paste and micron silver paste according to the particle size.The sintering process can be divided into pressure sintering and pressureless sintering according to whether the pressure is needed.After many years research,a large mount of achievements on nano silver paste have been gained,and excellent performances have been achieved.However,some difficulties are also encountered,such as the high cost of nano silver paste,which is not conducive to the popularization of sintering process.The sintering of micron silver paste is difficult,because the particle size is larger than that of nano silver particles and the particle surface energy is lower.Sintering with pressure will complicate the sintering process,which is not conducive to automation of production process,and the pressure can also cause the damage and the defects on chips.Therefore,pressureless sintering process on micron silver paste becomes an attractive solution.In the recently publications on sintering process of micron silver without pressure,most of them were achieved by change the particle morphology,such as using chestnut-like silver particles,flake silver particles,etc.,or adding nano-sized particles.By changing the particle morphology,such as chestnut-like silver particles and flake silver particles,there is a large potential gradient at the tip of chestnut-like silver particles or the edge of flake silver.A large potential gradient becomes the main driving force of sintering.By adding the nano-sized particles,on the other hand,the micron silver particles connect with the easily sintered nanoparticles by the size effect of nanomaterials.In this paper,we prepared micron silver paste with the spherical micron silver particles,explored the most appropriate preheating process and sintering behavior of the self-made micron silver paste.The feasibility of another sintering micron silver paste without pressure was explored.First of all,thermogravimetric analysis was conducted to predict the range of the solvent and silver paste on preheating temperature,sintering temperature and the sintering time.To explore the most suitable preheating process,the morphologies of preheated silver paste were observed by optical microscope.The optimal sintering atmosphere was explored by comparing the morphologies of micron silver joints in different sintering atmospheres.The optimum sintering temperature of different sintering temperature was also explored.The experimental results show that the volatilization of the solvent mainly occurs below 190^oC and the sintering behavior occurs mainly in the range of 200-300^oC.With the sintering temperature increase,the sintering behavior was enhanced,and the density of sintered micron silver joint was higher.A higher heating rate led to the large voids in the sintered silver paste layer.N₂ sintering atmosphere would hinder the volatilization of solvent and the sintering of silver particles in silver paste,which led to the lag of sintering behavior.We sintered the micron silver paste under different temperature under the most appropriate preheating process,the results showed that the average shear strength of 260^oC sintered joint for 30 minutes was22 MPa,the interface bonding rate was 44%.Under the process of 300^oC for 30 minutes,the average shear strength of silver joint was 31 MPa,and the interface bonding rate was increased to 64%.Moreover,with the increase of sintering temperature,the variance of the average shear strength of the micron silver joint increases gradually.In order to evaluate the thermal properties and electrical properties of the sintered micrometer silver joint,we bonded the LED chip on the surface of the silver-plated ceramic substrate with the self-made micron silver paste,and sintered under different process.Then,we tested the optical parameters and electrical parameters of the chips.As a comparison,the same LED chips were bonding on the silver-plated ceramic substrate by a commercial Sn solder and a commercial nano silver paste according to their respective manufacturers specify the sintering process.The results showed that with increase the temperature,the electrical conductivity and the heat conductivity of the sintered micron silver paste joint are gradually improved.The electrical conductivity of the sintered micron silver joint sintered at 280^oC for 30 minutes was higher than that of solder paste,and even compared to that of the nano silver joint.The thermal conductivity of sintered micron silver joint was similar to that of silver nano silver joint,but higher than that of commercial solder paste in any sintering process.