Room-temperature Solder Bonding of Electronic Packages
By David van Heerden and Jai S. Subramanian, Reactive NanoTechnologies
Reactive bonding is a room-temperature process* for soldering packages or components directly to boards without using a conventional solder reflow. The process is lead-free, fluxless, and produces a bond with the same mechanical, thermal, and electrical properties as a conventional solder bond, without exposing components to reflow temperatures.
Applications for reactive bonding include mounting packages and silicon die directly to heatsinks, mounting packages or components on boards, and joining thermally mismatched materials. The process uses a multilayered foil composed of thousands of nanoscale layers that alternate between two elements with large negative heats of mixing. After initiation by a small electrical or thermal stimulus, a self-propagating exothermic mixing reaction occurs in the foil. As shown in Figure 1, the exothermic reaction is a local heat source that can reach 1,400°C and subside within milliseconds. The intense heat pulse melts the solder on both sides of the foil and bonds the components without exposing them to high temperatures, minimizing residual stress and thermal damage.
Figure 1. Numerical model of a reactive bonding temperature profile for joining stainless steel with indium solder, showing the limited distance and duration of thermal exposure.
Solder bonding high-power transistor packages to heatsinks demonstrates the benefits of reactive bonding. Currently, the transistor package is bonded or attached to a highly conductive copper heat spreader, which is then integrated into a larger heatsink. There are strong incentives to eliminate the costly heat spreader and reduce the number of interfaces that create thermal resistance.
Reactive soldering bonds the transistor package directly to the underlying heatsink, enhancing the thermal and electrical performance of the transistor. However, conventional reflow solder direct bonding is impractical because of increased cycle times and added cost of soldering the large thermal mass of the sink. The large thermal mass would also expose the transistor to longer times at high reflow temperatures. In this case, reactive bonding provides all of the thermal advantages of solder bonds, but with rapid cycle times and minimal thermal exposure of the transistor.
Direct solder bonding of silicon die to heatsinks is another reactive bonding application. The increasing power density of silicon die makes conventional thermal interface materials, such as greases and phase-change material, inadequate as heat conductors. Reactive bonding allows soldering the die to thermal management components using high thermal conductivity solders such as indium. Thermal conductivities of 30-40 W/mK are achieved routinely with reactively bonded joints. Thermal cycling evaluations show that such joints exhibit minimal degradation in thermal performance after 3,000 cycles. The joints have also passed extensive thermal and mechanical shock testing without failures.
Reactive bonding facilitates fabricating products that undergo multiple reflows. In conventional processing, each subsequent reflow must be performed at a lower melting temperature, and with a reflow profile that does not degrade previous solder joints. The thermally localized reactive bonding process allows greater freedom in the choice of solders and the sequence of operations.
Soldering connectors onto a PCB is one example. Conventional soldering methods require that all reflows be limited to temperatures below the glass transition temperature of the circuit board, restricting process and material flexibility. With reactive bonding, connectors can be mounted onto boards with high-melting-temperature lead-free solders such as AuSn. The torque strength of resulting joints are 3-4 times those of comparable tin-lead solder joints. In addition, testing shows that subsequent reflows do not degrade the strength of the joint. The joints have passed all post-bonding reliability tests, including thermal and mechanical shock and vibration testing.
Reactive bonding is particularly attractive for joining components with large mismatches in coefficients of thermal expansion (CTE), such as bonding optical components or ceramic packages to metal components. In summary, reactive bonding is a platform technology applicable whenever temperature sensitivity or CTE eliminates traditional reflow processes.
DAVID VAN HEERDEN, director, R&D, and JAI S. SUBRAMANIAN, director, applications engineering, may be contacted at Reactive NanoTechnologies, 111 Lake Front Drive, Hunt Valley, MD 21030; 410/771-9801; E-mail: firstname.lastname@example.org and email@example.com.