George A. Riley
Two IMAPS 2005 papers by Reactive NanoTechnologies reported room temperature soldering of electronic components with heat supplied by nano-structured foils [1, 2].
The structured foils consist of thousands of alternating nanoscale layers of vapor-deposited aluminum and nickel. These elements have high negative heats of mixing. Triggering the foil by a pulse from a heat source initiates a self-propagating, strongly exothermic chemical reaction as the aluminum and nickel combine. The result is a brief, intense burst of localized heat, sufficient to melt solder or brazing metal without significantly increasing the temperature of the surrounding area.
For soldering, the foil is sandwiched between solder pre-forms which separate the components to be soldered, as shown schematically in Figure 1. The stack is held under light pressure, to control joint thickness and maintain close contact as the solder flows. Ignition of the foil melts the solder, forming the joint.
Figure 1. Schematic of room temperature hermetic cavity nanosoldering process.
Numerical modeling is crucial in determining the optimum combination of foil thickness, solder thickness, and applied pressure. These are governed by the amount, shape, composition, and thermal characteristics of the solder and the objects being joined. Earlier papers described joining a wide range of metal-to-metal and metal-to-ceramic materials, including silicon die attach and copper to silicon heat sink attach. [3, 4].
The key feature of this method is the rapid rise and fall time of local temperature. Figure 2 shows a numerical modeling of temperature profiles in creating a hermetic indium solder seal between a stainless steel package and lid. The peak temperature of 1,400 °C occurs 0.1 milliseconds after ignition. Component temperature falls to 100 °C within 20 ms.
Solder melt durations are less than 10 ms. The soldered component temperature drops below 50 °C within 25 ms. of ignition. Because of this brevity, temperature-sensitive components are unaffected by the heat pulse. Board temperatures 0.2 mm away from the component never rise above 100 °C.
Figure 2. Numerical modeling of a soldering temperature profile, showing the limited duration and distance of thermal exposure.
The nanosoldering process is fluxless, and requires no special atmosphere. The resulting joints have low mechanical stress, since thermal expansion between dissimilar materials is so time-limited during soldering. Joint strengths are higher than for equivalent oven-soldered joints, because the briefer time at high temperature results in smaller grain size. The more uniform joints improve electrical and thermal conductivity.
Nanosoldered hermetic seals have been tested to leak rates of 1 x10 –10 atm-cc/sec. Figure 3 shows a cross-section of a gold-tin solder hermetic seal joining stainless steel. The solder has filled and sealed all cracks in the reacted foil, which are a normal result of the volumetric contraction of the reaction product.
Figure 3. Cross-section of a hermetic stainless steel joint after gold-tin soldering. Cracks have filled with solder during the joining.
The second paper described a similar controlled melting in attaching connectors to a printed circuit board with gold-tin solder. The PCB could not long withstand temperatures above its glass transition temperature (Tg) of 280 °C. This precluded oven reflow of Au-Sn solder, which would require temperatures above 300 °C for several minutes.
The foil soldering technique kept the board above Tg for less than 10 msec. The soldered connectors tested well in electrical characteristics, bond mechanical strength and environmental stress testing.
In summary, nanosoldering creates stronger, low-stress joints in a wide range of similar or dissimilar materials, providing better thermal and electrical conductivity through a room temperature processing without special atmospheres. When it reaches production, nanosoldering will offer the added commercial benefits of potential productivity gains and lower equipment costs than alternative processes.
 T. Rude et al., “Hermetic sealing of microelectronics packages using a room temperature soldering process,” Proc. IMAPS 2005, Philadelphia, PA September 2005.
 J. Levin et al., “Room temperature soldering of connectors to PCB using reactive multilayer foils,” J. Levin et al., Proc. IMAPS 2005, Philadelphia, PA September 2005.
 J. S. Subramanian et al., “Direct die attach with indium using a room temperature soldering process,” Proc. IMAPS 2004, Long Beach, CA, November 2004.
 T. P. Weihs et al., “Ten-fold reduction in interface thermal resistance for heat sink mounting” Proc. IMAPS 2003, Boston, MA, 2003
PROCEEDINGS may be purchased from www.IMAPS.org .
PHOTOS courtesy of Reactive NanoTechnologies.
FOR MORE INFORMATION
Reactive NanoTechnologies Inc. www.RNTfoil.com