Study on the Interfacial Reactions of Ag-alloy Stud Bumps for Flip-Chip Assembly
Date Issued
2015
Date
2015
Author(s)
Chen, Chun-Hao
Abstract
The pursuit of fine pin and precisely electronic package demand of developing trend lead first level packaging evolving wire bonding into flip chip packaging. Due to the progress of TSVs technology, 3D IC technology becomes a remarkable issue of our semiconductor industry. To make a breakthrough advance on flip chip technology, we have to choose a proper material of flip chip bump and make improvement of its manufacture process. Traditionally we use gold and copper bump. In real case, due to its hardness property, copper bumps may cause fracture on low k chip during its manufacturing process. In thermal compression bonding, due to its low yield of plastic deformation, coplanar issue may occur. As for the gold bump, gold atom will diffuse into solder base quickly and form AuSnx intermetallic compound causing gold embrittlement issue. According to the above problems, these two kinds of materials are not good candidates for flip chip bumps. Therefore, a newly developing Ag alloy stud bump is introduced. First we examine the hardness of Ag alloy. Experimental result shows that according to its low hardness, it can provide enough plastic deformation to suit the thermal compression bonding. Owing to Ag alloy’s soft property, we can use less strength to fabricate on low k chip process without causing fracture and crack. Furthermore, its interfacial reaction with solder is stable and good, remains reworkable and self-alignment property. At the same time its manufacture process has no pollution issues. To sum up, we expect Ag alloy as a good candidate with high reliability for stud bump. In this research, first part begins with its wettability discussion. If the material we choose doesn’t have well wetting phenomenon with solder, it won’t be able to assemble during flip chip process. In order to know the wettability, we tested pure gold, pure silver, pure copper and Ag alloy with SAC305 solder by wetting balance experiment. It shows that Ag alloy has good wettability which is near Au, due to the addition of Palladium and Gold. The wettability trend is: pure gold > Ag alloy > pure silver > pure copper. Next we decided to test the Ag alloy with SAC305 solder interfacial reaction properties. Flip chip industry used copper and gold bump in the past time. In copper’s case, it will form Cu3Sn and Cu6Sn5 intermetallic compound. But these two kinds of intermetallic compound is so fragile that it leads to weak bonding issue and forms Kirkendall void and crack after long time aging, reveals itself is not a good choose. As for the gold bump, due to the high diffusion coefficient between gold and solder, gold-embrittlement may occur, damage the interfacial texture. On the other hand, we tested pure Ag, Ag-2Pd, and Ag-4Pd reflowed with SAC305 solder under same condition. Ag alloy forms Scallop-shaped intermetallic compound, which is, Ag3Sn with proper thickness and well interfacial texture. Then we examine its property and reliability with high temperature aging test, under 100℃, 125℃, 150℃, 175℃, 200℃ and 100hr, 300hr, 500hr, 1000hr conditions. The result shows that it only forms Ag3Sn intermetallic compound and grows smoothly and slowly after aging without diffusing into solder base. Comparing to gold case, no Kirkendall voids or cracks formed. However, after aging under 200℃ for a long period of time, Pd forms PdSn4 with Sn. Though it thicken the IMC layer and results some minor cracks, but it still better than the gold and copper ones. Owing to these excellent behaviors, the Ag alloy stud bumps have the potential to replace the traditional copper and gold bumps.
Subjects
Ag alloy wire
Ag alloy stud bump
wettability
Solder
Intermetallic compounds
Type
thesis