Fabrication Of Cu@Sn Core Shell Powder And Performances And Microstructure Evolution Of The Formed High Melting Point Bondlines

The electronic products with high integration and high power promote the development of the third generation semiconductor materials such as SiC,Ga N,and so on.The electronics based on SiC semiconductor material could sustain high operation temperature up to 600?,however,the matched bonding materials are still lacking.High reflow temperature will induce large thermal stress and do harm to other temperature-sensitive components in the system.So it is desiable to accomplish the refolw process shortly under low temperature but achieve the bondline with a much higher re-melting temperature.Additionally,thicker bondline could release stress concerntration.By successfully fabricating Cu@Sn core shell particles and innovatively using them as bonding material,this thesis presents a new effective solution for die attachment in the third generation power devices,meeting the demands of being a good die attach material.Cu@Sn core-shell structured particles are successfully fabricated with the following characteristics: Sn coating layers merge and connect the Cu cores during reflow processing,sufficient Cu atoms sources and higher surface activities ultimately reduce the time needed to completely comsume pure Sn and transformed them into intermetallics.In this way,the bonding material can be reflowed at a low temperature but resulted into interconnections with a high re-melting point.Bondline thickness could be adjusted by the added amount of Cu@Sn particles.More detailed tests were conducted to particles in diameters of 1?m?5?m and 30?m.The effects of particle size to coating layer thickness and bondline properties were analyzed;the smaller of the Cu@Sn particles,the worse of the bondline's conductivity.A series of electroless Sn plating phenomenon were summarized and the corresponding formation mechanisms were explained.Both of solder paste and preform are fabricated by using Cu@Sn core-shell structured particles with different Cu diameters and Sn plating thicknesses.The preparation process is simple and flexible,but the vacancies and voids are inevitable in the bondlines due to the evaporation of flux and poor flowability.The preform bonded samples show compact microstructures and high strength,and the optimum properties were achieved when 30?m Cu particles with 2?m Sn coating were used as the preform.After reflow soldering at 250? for 8min,16 min and 40 min,the phase compositions of Cu@Sn particles transformed into Cu+Cu6Sn5,Cu+Cu6Sn5+Cu3Sn and Cu+Cu3Sn,respectively.Average shear strength of the bondlines fabricated by Cu@Sn paste was only 2.3MPa at room temperature,while the strength of the bondliens fabricated by the preform could reach 29.35 MPa at 400? and 18.78 MPa at 500? after reflow for 8min and 40 min,respectively.Preforms exhibit excellent electrical resistivity(6.5??·cm)and thermal conductivity(154.26,130.64 and 127.99W·m-1·K-1 at 30,150 and 250 ? respectively)due to the Cu particles embedded in bondline.Vertical cracks occurred during thermal shock cycling test did not affect the performances of the bondline as a whole.IGBTs(PCG40N65SMW)were bonded with DBC by the preform,and Ic-Vce curve and breakdown voltage measurements were conducted after thermal shock cycling.The results show that the bondline fabricated by the preform could satisfy the requirements of IGBT 's data sheet.Microstructure evolution of Cu@Sn particles with different coating layer thickness was studied under different high temperature service conditions.Volume expansion was calculated to be 2.60% when 2?m Sn coating layer completely transformed into Cu6Sn5,while the volume shrinkage of this transformation is estimated to be 5.60% when Cu6Sn5 continued to transform into Cu3 Sn.Newly formed ?-Cu41Sn11 phase was detected after heating at 450? for 1h.Upon heating the Cu@Sn particles with different coating layer thicknesses to high temperature,two phase transformation pathways occur: one is Cu@Sn ? Cu@?-Cu6Sn5 ?Cu@?-Cu3Sn?Cu@?-Cu41Sn11?Cu@? phase(520?)?Cu@? phase(586?)for Cu@Sn particles with thin coating layer(<2?m),and core shell structure still exists in bondline because the thin Sn coating layer could not consume Cu at the core completely,and phase transformation occurred only at the outer core region;another is Cu@Sn?Cu@?-Cu6Sn5?Cu@?-Cu3Sn??-Cu3Sn+?-Cu41Sn11 for Cu@Sn with thick coating layer(?4?m).The corresponding core shell structure disappeared,and a bondline with a uniform microstructure was formed finally.