Investigation Of Solder Paste For Laser Applications In Electronic Packaging

With the further development of electronic packaging,the higher the integration of electronic components,the higher the requirements of the laser process,any one solder joint fail to form reliable joint will lead to the failure of the whole product.In recent years,laser soldering catches more and more eyes from researchs and the public,because of the huge technical advantage in microelectronics packaging industry.In this paper,the influence of solvent,filmogen and active agent of Sn58Bi solder paste on the solder splash during laser soldering process was studied.The microstructure and mechanical properties of laser soldering solder joint were analyzed finally.The results show that different solvents is related to varying degrees of solder splash during the laser soldering process.The splash of solder caused by solvent gasification.The higher the solvent gasification temperature,the less the amount of solder splash.In addition,the effect of the gasification characteristics of the solvent at the low temperature stage and high temperature stage of the heating process is different for the solder splash.The less the amount of gas that occurs at the front end of the soldering process,the less the amount of solder splash produced by the solder paste.At the rear end of the soldering process,the amount of gas produced by solvent has no significant effect on the amount of solder splash.The viscosity of the solder paste depends primarily on the filmogen,and there is a close relationship between the amount of solder splash and the viscosity of the solder paste.The higher the solder paste viscosity,the less the solder splash.The type of the solder splash is also related to the viscosity of the solder paste.Tin beads are lager size solder splash.If the solder paste viscosity above 60Pa.s,the probability of tin beads splash is very low;and solder paste viscosity below 60Pa.s,the probability of tin beads splash increases greatly.During high heating rate process,the active agent may fail to be activated and even be pushed out of the bonding pad which lead to make ineffective use of the active agent,so the laser soldering process requires higher flux activity compared with the reflow soldering process.If the flux activity does not meet the need of laser soldering process,solder paste will produce a lot of tin bead splash.In a certain range,enhancing the activity of the flux can effectively reduce the solder splash.However,increaseing the amount of active agent continually does not help to reduce the solder splash completely.The microstructure of laser soldering joint is typical Sn + Bi eutectic structure.Due to the high cooling rate,the eutectic structure is very dense,the Sn-rich phase and the Bi-rich phase are both fine,and there has no secondary-precipitated phase in the microstructure.The soldering time had no obvious effect on the microstructure of solder joint.If the power of laser is set to be 1w,soldering time must be longer than 300ms,in order to ensure the firm combination formed between weldments(chip resistance)and the substrate.The results show that if the welding time is 300ms,the shear strength of the joint can reach 68.1N and far more than industry standards(standard)requirements.Extending the soldering time continually to improve the strength of the joint is inefficient.The interface compound layer soldered by laser is found between solder and bonding pad observed by SEM.The thickness of the interface compound layer gradually increased with the extension of the soldering time.Soldering for 300 ms to 1500 ms,the thickness of the interfacial compound formed is less than 1 ?m,which is much thinner than the thickness of the interface compound layer soldered by reflow soldering.