Research On Forming Method And Microstructure And Properties Of CGA Device Solder Pillar Based On Capillary Filling Effect

CCGA devices need to use auxiliary molds to locate the solder pillars for connection during the packaging process.The auxiliary molds have problems such as high production costs,complex procedures,and easy to cause solder pillar damage when removed.This article attempts to propose a new method of planting the pillars.With the help of a micro drilling machine,the drill bit is rotated and drilled into the solder balls arranged on the pads through the reflow soldering array to a certain depth,leaving behind the solder balls arranged in the array.A certain depth of positioning hole,insert the two ends of the short copper pillars with a diameter of 0.8mm into the positioning holes of the solder balls on both sides of the substrate,and use the capillary caulking effect of the melted low-melting solder paste to fill the positioning holes of the solder pillars and the solder balls The gap realizes the connection of the planting column.When Sn58 Bi solder paste is heated and melted at 170℃ and fills the 25μm capillary gap of Sn63Pb37 solder ball/Cu pillar,the soldering time is extended from1 min to 4min,and the void ratio at the interface of Sn58Bi/Cu pillar is reduced from23.4% to 2.1%;soldering time is 4min Under the conditions,the capillary gap increases from 25μm to 75μm,and the interface void ratio decreases from 2.1% to0.04%;the rule of "filling effect increases as the capillary gap decreases" is not followed in the range of 25μm-75μm brazing gap.At 170℃/4min/75μm,the IMC interface connecting layer of 0.62μm will be formed at the interface of Sn58Bi/Cu column;because the Sn58Bi/Sn37Pb solder has the same matrix phase,the two solders will fuse well at the interface.However,due to the increase in soldering time compared with traditional reflow soldering,the average grain size of β-Sn phase and Bi-rich phase increased slightly.The capillary gap has been increased from 25μm to 75μm,and the average pull-off load of the joint has been increased from 114.85 N to 126.71 N.This method of planting columns can obtain reliable planting column connections.After low temperature aging at 80℃ and 90℃ for 1/4/9/16/25 days,there is only the thickening of Cu6Sn5 phase at the interface of Sn37Pb/Sn58Bi/Cu pillar solder joints,and no Cu3 Sn phase is formed.At 80℃ and 90℃ aging temperature The IMC layer thickened with growth rates of 0.604μm/d1/2 and 0.843μm/d1/2,respectively.As the aging temperature increases,the diffusion rate of atoms increases,and the growth rate of the IMC layer increases.During the aging period,the Pb and Bi phases are obviously coarsened.The average pull-off load of the solder joints after aging is above 126.71 N,and the connection strength of the solder joints of the new planting column process is higher after aging.When Sn58 Bi solder paste is heated and melted at 190℃ and fills the 75μm capillary gap of Sn3.0Cu solder ball/Cu pillar,the Cu pillar/Sn58 Bi interface forms an IMC layer with a thickness of about 1.24μm,which is better than traditional single solder CGA solder joints.The average thickness of the IMC layer is about 0.72 μm,an increase of 0.52 μm.In addition to the presence of the network-like Bi-rich phase in the Sn3.0Cu/Sn58 Bi transition zone,there is also a part of the granular Bi phase dispersed in the Sn phase.There are intermetallic compounds in the Sn58 Bi solder in the transition zone.The combination of Sn3.0Cu/Sn58 Bi solder increases the melting point gap between the two solders and expands the process window.The average pulloff load of the solder joint can reach 124.84 N.The hardness values of the transition zone of the Sn3.0Cu/Sn58Bi/Cu column joints of 75μm,50μm and 25μm gap filling gaps are 408.72 MPa,419.82 MPa and440.69 MPa,respectively.The hardness values of the solder near the interface are341.82 MPa,367.6MPa and 392.19 MPa,respectively,showing a trend of increasing with the decrease of the capillary gap and the increase of the indentation hardness.In the nanoindentation test,the maximum load is 30 m N and the loading rate is 2m N/s.The creep rate sensitivity index m of the solder near the solder joint interface of 75μm,50μm and 25μm gap is 0.245,0.2168 and 0.1863 respectively,and the transition zone is m.The values are 0.1898,0.1543,and 0.1385,respectively.