Silver Flip-chip Technology:The Infinitesimal Joint Possibility

Integrated circuit chips are traditionally connected to the packages by tiny wires. As the number of I/O pin count on the chip increases, the electronic industry turns to flip-chip interconnect where the chip is flipped over with active surface facing down and connects to the package with numerous small solder joints. In 2013, the joint size is at 20 to 50µm range. Small solder joints create several reliability and manufacturing issues including intermetallic growth and bridging of adjacent joints during the re-flow process. We thus have developed the pure silver (Ag) flip-chip interconnect technology. Ag has the highest electrical and thermal conductivities among all metals. For demonstration, Ag columns are fabricated on Si chips and bonded to copper substrates at low temperature of 250oC using solid-state atomic bonding. In 2012, the Ag joints have been shrunk down to 10µm in diameter and 20µm in pitch. Compared to solder-based interconnect, this new Ag flip-chip technology possess fourteen (14) potential advantages.

Chu-Hsuan Sha*, Wen-Pei Lin**, and Yi-Ling ChenAdvisor: Professor Chin C. Lee

Electrical Engineering and Computer ScienceMaterials and Manufacturing Technology

*Now with Intel, **now with Broadcom

Abstract

Potential advantages of this Ag flip-chip by solid-state bonding:

1. High electrical conductivity, 7.7 times of that of Pb-free solders.

2. High thermal conductivity, 5.2 times of that of Pb-free solders.

3. Completely fluxless, thus 100% green.

4. No IMCs, all reliability issues associated with IMC and IMC growth do not exist.

5. Ductile Ag can manage CTE mismatch between chips and packages.

6. Ag joints can sustain high operation temperature because Ag has high melting temperature.

7. No electro-migration problem.

8. No molten phase involved, the joint can better keep its shape and geometry.

9. No molten phase involved, bridging of adjacent joints is less likely to occur.

10. Aspect ratio of bumps can be made greater than 1 if needed.

11. No under-fill is needed, greatly reducing processing steps.

12. No solder mask is needed either because there is no molten phase.

13. Alignment tolerance can be as high as the size of the joint.

14. The size of the joint is only limited by the lithographic process.

Summary

Silver flip-chip joints (10µm)

• Surface of Ag column and the surface of the Cu substrate deform to achieve atomic contact so that Ag atoms and Cu atoms share their conduction electrons.

• Inter-diffusion does not decrease the bonding ability so far as A atoms and B atoms share electrons after the inter-diffusion.

Bonding Mechanism

Si

Cu

Ag

Cross-section BSE images of 10µm Ag flip-chip joints made between Si chip and Cu substrate by solid-state bonding at 250°C with static pressure of 800 psi (0.044gm per joint).

Si

Cu

Ag

Si

Ag

Cu

Si

Ag

Cu

Ag

Cu

Ag to Cu bonding interface

Present first-level interconnect methods

Wire bonding

Solder flip-chip joints

T J Green Associates, LLC www.flicker.com