Dr. Rainer Dohle, Micro Systems Engineering GmbH
Based upon the paper “Accelerated Life Tests of Flip-Chips with Solder Bumps Down to 30 µm Diameter,” by Dohle, R., Härter, S., Goßler, J., Franke, J., Proceedings of the 44th International Symposium on Microelectronics, Long Beach, California.
This tutorial examines the reliability of ultra fine-pitch flip chip assemblies with lead-free solder bumps for which there are two major reliability aspects to consider: Thermo cycling performance and electromigration (EM). Isothermal aging and humidity should not cause significant problems, providing that suitable materials have been selected, see  and chapter “Results”. First, dry heat storage, temperature humidity bias testing, and temperature cycling were conducted, showing excellent reliability of these ultra fine-pitch assemblies. Second, electromigration (EM) performance of the flip chip assemblies (with solder sphere sizes mentioned above) was investigated.
The test coupons used in this study are specially designed flip chip packages. SAC305 Solder spheres with 40 µm, 50 µm, or 60 µm diameter were attached to silicon chips 10 x 10 x 0.8mm in size, using the wafer level solder sphere transfer process (often called “gang ball placement”) and 30 µm SAC305 spheres using a laser based single-sphere process, respectively . Even exotic solder compositions are readily accommodated using these methods.
Figure 1 shows the layout of the flip chip and the bumps on the chip. A circumferential daisy chain connection is integrated for each of the solder sphere sizes and each chip can be connected for online measurements during reliability or EM testing. The AlCu0.5 trace interconnection on the die is 1 µm thick, the Ni-P UBM 5 µm (with flash gold on top).
Figure 1: All-purpose test chips with 50 µm solder bumps (top) or 40 µm solder bumps (bottom). Solder spheres with 60 µm diameter would be placed at the outer row, solder spheres with 30 µm diameter at the 4th row.
…… To continue with the complete tutorial, please click on Reliability of Ultra Fine-Pitch Flip Chip Assemblies / Lead-Free Alloys PDF (1,700 k)
Dr. Rainer Dohle
Micro Systems Engineering, GmbH
Berg / Germany
For further information, please contact:
Micro Systems Technologies, Inc.