| Flip chip(FC) technology is becoming more and more popular to meet the requirements of high integration, high power and miniaturization trend of electronic products. With the forbidden of Pb, leed-free solder alloys are attracting more and more attentions due to the development of study of solder alloy and experimental concerns. However, there are many reliability problems for leed-free solder joints, such as poor wettability, poor electromigration resistance, and IMC having bad effect on the reliability of the joints. Stud bump bonding(SBB) FC can be ultilized to solve this problem, during which the metal studs are firstly bonded onto the pad of chip, and then the studs will be bonded with substrate by leed-free solder in reflow. SBB FC is rapidly developing as a high reliability, lead-free, finer pitch-size and popular FC packaging. Many studies of reliability and properties on SBB have been repoted, of which gold is the most widely used stud material. What'more, almost all of the research reported that AuSn4 was the main couse of failures of mixed joints. Thus, there are three parts have been researched in this paper, including study of the mechanism and technology of Au stud bump bonding, study of the mechanism and technology of mixed joints with different amount of Sn0.7Cu, and study the influce of AuSn4 on reliabilities and properties of mixed joints.Experimental results are as follows. The Au stud was well bonded onto Cu pad with the diameter of 105 ?m and height of 65?m using the optimized parameters,of which force was 2, pressure was 2, and temperature was 150?. The amout of Sn0.7Cu was controlled by the stencil with diameters of 120 ?m, 150 ?m, and 180 ?m. The kinds of IMC of different mixed joints were same, including AuSn, AuSn2, AuSn4,(Au,Cu)Sn4, and(Cu,Au)6Sn5. The microstructures of AuSn4 were different, which were shorter and thicker, could be more easily thicken in smaller mixed joints, deciding the reliability of mixed joints. During 150? thermal aging, the larger mixed joints presented better reliability for the more slowly thicken of AuSn4 and appeared comparable shear force with the Sn0.7Cu joints. And larger mixed joints had nearly the same life with Sn0.7Cu joints in thermal cycling. While smaller mixed joints had better resistance for electromigration, for which the reason was that larger mumber of IMC blocking the path of migration.During Au stud bump bonding, the Au ball with Cu pad produced friction under the ultrasonic and pressure, then the ultrasonic softening effect promoted plastic deformation of interface materials, at the same time, the thermal effect promotes the mutual diffusion between atoms, and not flat Au/Cu interface formed in a mechanical mixed joining. Au and Cu had a competitive relationship with Sn in the mixed joints, and the diffusion of Au in Sn is faster, which could inhibit the formation of Cu-Sn IMC. AuSn4 grew along the maximum temperature gradient direction during the mixed joints, and thus grew from Au stud to Cu pad on substrate and through the wholed mixed joints. The amounts of Sn in different joints and the different temperature gradient s resulted in different microstructure of AuSn4. |
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