Direct Die Attach Using a Room Temperature Soldering Process

As the power and power density of IC components continue to rise [1], the need to effectively dissipate the heat to ensure long-term reliability has increased [2]. The largest contributor to the total thermal resistance along the heat conduction path comes from the thermal interface material (TIM) used between the surfaces of the silicon die and the heat spreader or heat sink. Polymeric TIMs are typically used to increase the contact area between the two surfaces and to thus minimize the resistance. Replacing these low thermal conductivity polymeric materials with a metallic solder bond would enable better performance due to the higher conductivity of the solder. This technical brief will present a unique method of achieving such metallic bonds between the die and thermal management components, while minimizing the thermal exposure of the die during bonding.

Figure 1. Joining of a silicon die to a thermal management component using reactive multilayered foil. (a) Typical silicon chip and heat sink package. (b) Joint configuration used. c) Cross section of a multilayered reactive foil showing individual layers of Al and Ni.In this new bonding method, direct die attach to thermal management components is achieved by localized melting of solder layers by using a reactive multilayered foil. The foil consists of thousands of alternating layers of aluminum (Al) and nickel (Ni). A reaction in the foil is initiated by an electrical, thermal, or optical energy source, resulting in mixing between the Al and Ni layers [3, 4]. The reaction releases a highly controllable and predictable heat pulse, which reflows the adjacent solder layers and creates a strong metallic bond. The joining configuration and structure of the foil used for making the highly conductive joint appear in Figure 1. In the joint configuration shown, indium is used as the solder. Indium was chosen because it is a lead-free high thermal conductivity material. In addition indium is relatively soft and thus able to accommodate the thermal expansion mismatch between the chip and heat sink without degrading or applying significant stresses to the components.

The localized nature of this bonding process minimizes the thermal exposure of the components and the residual stresses in the resulting joint. Although the foil itself reaches a high temperature, the active surface of the silicon sees only a temperature of 120°C for five milliseconds [7]. This joining process is fluxless, thus minimizing voiding and subsequent clean-up operations.

Silicon dies as large as 25.4 x 25.4 mm have been successfully joined using this technology. The thermal performance of reactively bonded joints between silicon dies and copper heat spreaders was measured using a laser flash technique [5, 6]. Table 1 summarizes the thermal performance for various bond line thicknesses and die sizes. The bond between an 11 x 13 mm die and a copper heat spreader with a bond line thickness of 200 �m has a thermal conductivity of 43 W/m�K with a unit area thermal resistance of 0.04-0.05 K�cm²/W (0.006 K�in²/W). Although, in principle, comparable indium joints fabricated by reflow soldering should have better thermal performance (0.025-0.035 K�cm²/W), reflow soldering exposes components to relatively high temperatures for extended periods and results in high residual stresses in the joint.

Table 1. Thermal Performance on Various Reactively Bonded Joint ConfigurationsSamples with the joint configurations detailed in Table 1 were subjected to 3000 thermal cycles between 0 and 100°C and thermal measurements were taken periodically [8]. Thermal performance values of all the 200 �m bond line thickness (BLT) samples were within the measurement uncertainty of the laser flash system (estimated to be +/- 6%), while the thinner BLT samples showed an increase of 8-10%. Mechanical shock and random vibration testing were also performed on the bonded samples, and no degradation in thermal performance after testing was observed. Table 2 shows the results after mechanical shock and vibration testing.

Table 2. Copper-Silicon Joints Reliability Test Summary

Conclusion

Reactive multilayer bonding is a robust method for solder bonding silicon dies directly to thermal management components without exposing the dies to reflow temperature. The resulting joints have interfacial thermal conductivities up to 43 W/m�K and pass all reliability requirements for a typical microprocessor application.

References

1. International Technology Roadmap for Semiconductors (ITRS), 2003 Edition, Semiconductor Industry Association, 2003.
2. Iscoff, R., “Thermal Management: Smaller, Faster, Hotter,” Chip Scale Review, Gene Selven & Associates, San Jose, CA, January – February 2004
3. Weihs, T. P., “Self-Propagating Reactions in Multilayer Materials,” Handbook of Thin Film Process Technology, edited by Glocker, D.A., and Shah, S.I., IOP Publishing, 1998.
4. Wang, J., Besnoin, E., Duckham, A., Spey, S. J., Reiss, M. E., Knio, O. M., Powers, M., Whitener, M. and Weihs, T. P., “Room-Temperature Soldering with Nanostructured Foils,” Applied Physics Letters, Vol. 83 (19), November 2003.
5. Test Method E1461-01, “Standard Test Method for Thermal Diffusivity of Solids by the Flash Method,” American Society for Testing and Materials, Annual Book of ASTM Standards, Vol. 14.02, 2001.
6. Campbell, R.C, Smith, S.E., “Flash Diffusivity Method: A Survey of Capabilities,” ElectronicsCooling, Vol. 8, No. 2, 2002, p. 34.
7. Subramanian, J.S., et al., “Direct Die Attach Using a Room Temperature Soldering Process,” Proceedings of IMAPS Conference, Long Beach, CA, 2004
8. Subramanian, J.S., et al., “Room Temperature Soldering of Microelectronic Components for Enhanced Thermal Performance,” EuroSime, 2005.