| Surface Mounted Technology(SMT)is main technologies used in electronics packaging industry.Electronic manufacturing industries have utilized advanced Integrated Circuits(IC)packages to achieve the manufacture of smaller,lighter,faster and cheaper products.However,the assembly of these surface mounts packages and the reliability of the products are challenged by solder paste printing process.A novel solder paste jet printing technology can solve the problem well,which demands a new-type solder paste suitable for jetting process on the basis of solder paste for stencil printing.This means that the paste flow and deformation behavior is very important in defining the jet printing performance of any solder paste.The solder paste is a dense suspension,composed of metal powder dispered in a continuous organic phase(flux).Both metal powder and flux system will great influence rheological properties of pastes.In this paper,the viscosity and stability properties of lead-free solder pastes with different solder powder,Sn58Bi and Sn3Ag0.5Cu,have been systematically investigated by viscosity test,slump test and other material analysis methods.The impacts of different metal content,alloy type,size distribution and different kinds of flux on the viscosity,thixotropy and slump properties have also been discussed.The requirements of solder powder used for jet printing have also been concluded by jet printing test.In addition,the influence of thixotropic agents was explored preliminarily.The effect of solder paste to adaptive jet printing performance has been researched,which will provide theoretical and experimental basis for developing solder paste for jet printing.The study found that the higher the metal content is,the greater the viscosity of solder paste is.What is more,the recoverable priority of its viscosity becomes worse.In large range of metal content,the viscosity of solder pastes increase into different degrees with the increase of metal content.Adopting a flux formula called F1 in this paper,the content of solder powder,87wt%(about 42vol%)is the transition zone of rheological property of solder paste,which makes the acting force between particles and flux transit to force of friction in the internal system of solder paste.The solder paste jetting test shows that the lower viscosity of solder pastes caused by lower metal content makes it easier to realize injection.However,when the metal content increases to more than 42vol%,flow blockage occurs under high shear rate,it cannot make the solder paste jetting continously.Compared with Sn58Bi solder pastes,Sn3Ag0.5Cu solder pastes perform similar viscosity under the same condition of particle size and volume fraction,which proves that the rheological property of pastes has nothing to do with the surface of solder powder when the acting force between particles and flux plays a leading role in the system.However,Sn3Ag0.5Cu,with a smaller hardness,will easily occur deformation and blockage of nozzle under high-speed impact.Adopting a special design of striker structure can realize the jet printing of solder pastes with Sn3Ag0.5Cu.solder powder.The size distribution of solder powder also has a great influence on the rheological properties.The viscosity of solder paste increases with the decrease of solder powder diameter.According to the jetting test,solder paste with smaller particles is easier to implement continuous injection.For polydisperse system,large particles mixtured with small particles can significantly reduce the solder paste viscosity,while the influence is little to thixotropic performance.Nevertheless,this kind of paste is not suitable for injection because of serious splashing,which may casued by inhomogeneous size.According to Stoke's law,whether the sedimentation progress takes place or not mainly related to the viscosity of flux system.To F1 flux system in this paper,the sedimentation rate of single solder particle is 1.59?m/min when metal content is over 85wt%(about 38vol%),which can be ignored in the system.A 48 hours sedimentation experiment also proves the truth.Additionally,cold and thermal slump resistances relate to the viscosity of solder pastes.When metal content reaches to or over 87wt%(about 42vol%),solder paste is better able to maintain its shape and fundamentally meet the normal functional use.Another factor to solder paste properties is flux system.The results show that the viscosity for the corresponding solder paste is higher when the flux system exhibits higher viscosity.And the law of thixotropic performance is the same.Solder paste with greater thixotropic coeffcient can firmly wrap solder powder,to prevent the separation between flux and solder powder and guarantee the stability of solder paste.Compared with two mature solder paste products,the solder pastes studied in this paper showed a worse jetting performance.It is found that thixotropic property will affect the shape of paste droplets by injection.A long trail and poor shape will easily appear when the thixotropic coefficient of solder paste is low.In this paper,two kinds of thixotropic agent were adopted to improve thixotropy,which is hydrogenated castor oil(HCO)and ethylene bis stearamide(EBS).The solder paste exhibits higher viscosity at lower shear rate and thixotropy at higher shear rate by adding 5%of HCO,which makes the continuous jetting of solder paste with 160Pa.s viscosity come ture.At the same time,the slump resistance after jet printing also gets guarantee.On the contrary,the addition of EBS decrease the viscosity and thixottopy. |
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