Next- Calibration Blocks for 3-D Aramis for MEMS Devices

ROLE OF INTERMETALLICS IN THE ELECTROMIGRATION SOLDER BUMPS FOR LEAD FREE SOLDER SOLDER STRUCTURE AND ITS SOLDER PAD COMBINATION

Prof. S.B. Park & Ganesh R. Iyer

Introduction

It is attempted to quantify the role of intermetallics for the electromigration in flip chip solder interconnects (C4). Intermetallics are formed at the interface of solder and metallic pad where they serve as metallic bonding agent. For the transition to the Pb-free solder interconnects, electromigration is one of the prime reliability concerns. It is observed that some flip chip solder interconnect fails earlier than the estimated time to failure. It is explained by the effect of intermetallics formed by solder and different matching pad metallurgies. This paper quantifies the role of the intermetallics in the time to failure for a system. A series of experiments are being conducted to determine the current exponent and activation energies of the Black’s equation for different solder and pad metallurgy combinations. Two test structures are proposed. The first test structure was made to study the role of the intermetallics and the second structure was made to characterize the pure solder’s electromigration.

Approch to find mean time to failure

Using Blacks equation which is given as [1]

were,

• = time to failure, hours.
• B = Materials constant
• J = Current density
• n = Current density exponent
• = Activation energy of diffusion, eV
• K = Boltzmann’s constant = 8.63x10 -5 eV/ oK
• T = Absolute temperature, oK.

Conventionally, a characteristic behavior of electromigration, which results in material constants, for a certain solder was used in the prediction of electromigration life regardless of the pad metallurgy and this could be totally misleading. In this study, the role of intermetallics is quantified and the two different test structures were proposed to discern the differences in material properties between with and without the existence of intermetallics.

Structure 1: Solder-Pad Structure - Role of Intermetallic

Fig. Solder-Pad Structure mimicking the C4 interconnect

(a)

(b)

Fig. (a) Solder/Pad structure with two copper wires & (b) Cross-sectional view of the V groove. [2]

Structure 2: Pure Solder structure

Fig. Pure Solder structure with copper wires at both end

Fig. Pure solder structure

Fig. Cross-sectional view of the V grooves.

Details of Sn-Pb system.

Details of Pb-free system.

Test Matrix

Test matrix to determine current exponent

Test matrix to determine current exponent

Experiments are still on.

Conclusion

• Two test structures are proposed: The first one is to quantify the role of intermetallics in solder-pad system and the other is to measure the material constants such as activation energy and current density exponent for pure solder system.
• The detailed and well-controlled experiment scheme was presented and this test scheme and procedures are for the benefit of technical communities dealing with Pb free transition. This study would show the role of the intermetallics for eletromigration and provide true current exponent and activation energy of the Pb free solder itself.

* The structure was fabricated at Cornell Nano Fabrication Center.

References:

[1] J. R. Black “Mass transport of aluminum by momentum exchange with conducting electrons” Proceedings of 6 th Annual International Reliability Physics Symposium, page 148-159, IEEE, 1967.

[2] Q. T. Huynh, C. Y. Liu, Chih Chen, and K. N. Tu, “Electromigration in eutectic PbSn solder lines,” J. Appl. Phys. , 89, 4332-4335 (2001).