Research On Single Sn Crystal Micron Bump For The3D-TSV Packaging

With the rapid development of high-density electronic packaginginterconnection,three-dimension(3D) integrated packaging technology has becometheresearch focus in the electronic packaging industry. Lead-freesolder interconnection is animportant technology for3D-TSV stacked packaging. In order to deal with the rising TSVdensity, solder joint volume is getting smallercontinually.Eventually,the solder jointwillcontain only one Sn grain.Theinterfacial reactions will become more complicated withthe decreasing size of solder joints.Due to the anisotropic mechanical properties of Sngrain and the lack of grain boundaries’ coordination in deformation process, single Sngrain microbumps will show different micro behaviors compared to solder joints withmultiple grains.In the present paper, single Sn grain microbumps were fabricated by photolithography,electroplating and reflow.The diameter and height are about10μm and8μm,respectively.This paper takes single Sn grain microbumps as the research object.Thefollowing research work was carried out.The interface interaction between the Cu substrate and liquid Sn solder during thereflow process was studied. The quantitative relationship of the Cu concentrationdissolved in the liquid Sn solder with the time and solder temperature was revealed. TheCu-Sn interface morphology was analyzed by using the theory of pure metal solidification.The value of Jackson factor calculated was1.2, which meant the interface was rough. Thisanalysismatched up with the observedscallop-shaped IMC morphology. Themicrostructure evolution and IMC growth behavior of single Sn grain microbumps under150℃isothermal aging conditions were investigated. According to the empirical formulaof the thickness ofIMC layer growth, time constant n was calculated through a linearregression fit, which is0.5.The growth model of the IMC layer was parabolic. The valueof IMC growth rate coefficient was approximately0.14by calculating. The comparative analysis of single Sn grain micro solder joints and multi-grain solderjoints with limited grain number under thermal cycling conditions of-55℃~125℃wasconducted by using finite element method. The high stress concentration region of singleSn grain micro solder joints was on the connection interface of the solder and the substrate.The highest stress values appeared in the outermost edge of the connection interface.Multi-grain micro solder joints with limited grain number showed a different stressdistribution compared to single Sn grain micro solder joint.The high stress concentrationregion appeared at the grain boundaries of different grains. Thehigh stress concentrationregion was priority areas that crack initialized and grew, thus making it the focus ofreliability analysis.