Metal Foil Gets It Together

Assembly difficulties can certainly foil design engineers, particularly those trying to join heat-sensitive components. But engineers can now foil their tough-to-join assemblies-literally. RNT (www.RNTfoil.com) of Hunt Valley, MD has developed a metal foil that can help create soldered joints in applications ranging from heat sink attachment in delicate microelectronics to the robust joints found on armored vehicles and planes.

The company creates the foil from up to 4,000 alternating layers of aluminum and nickel. "It looks a lot like aluminum foil," notes Tim Weihs, CEO and co-founder of RNT. But this high-tech foil is produced through a proprietary sputtering process that is capable of depositing the foil's nanoscale layers.

As a joining aid, the foil would be placed between the mating components with a layer of solder on either side of it. The foil would then be zapped with a small burst of energy, which would cause the aluminum and nickel to mix on an atomic level. An exothermic chemical reaction between the two materials ensues, generating enough heat to melt the solder and bond the mating materials. As the foil cools it cracks, allowing the solder to flow through it, leaving pieces of foil in a mostly-solder interface. "The solder, not the foil, determines the strength of the joint," Weihs notes.

This reactive joining process, which needs no flux, takes place at room temperature, usually in less than a second. And Weihs points out that the localized, short-term heating averts thermal distortion or damage of the mating components. The process also requires very little special equipment-just a fixture to hold the mating parts in place during joining and a small energy source. "A 9V battery can generate enough of a spark to initiate the chemical reaction," Weihs says.

RNT is pursing several kinds of applications for its reactive foil, which it will start shipping in commercial quantities early next year. The first applications for the foil will likely be as thermal interface material for microelectronics packaging, including heat sink attachment. In this application it has a couple advantages beyond keeping the heat in check during attachment. According to Weihs, the reactively joined interface has great thermal properties-including a thermal resistance of 1.8 Kmm^2/W versus 35 to 45 Kmm^2/W for conventional thermal interface materials. What's more, reactive joining works with lead-free solders and could cut the number of reflow operations when soldering multiple components.

RNT has also been working to apply its foil to beefier joints. The reactive foil is under investigation as a way to bond ceramic armor plates to the steel frames of military vehicles. And RNT is investigating metal-to-metal joining applications too. "We can join just about any metal as long as you can get solder to wet it out," Weihs says, citing aluminum, stainless steel, copper, titanium, and nickel-based super alloys as a few possibilities. He adds, however, that some metals do need a surface treatment in order for the solder to stick.

As for joint strength in these metal-to-metal joints, RNT has already achieved shear strengths as high as 100 MPa with common metals and joint designs. "People tend to think 'welding strong, solder weak,' " says Weihs, who holds a doctorate in materials science. But he makes a case that soldered joints can be quite strong if properly designed. "It's actually hard to compare these two because it all depends on the area of the weld versus the area of the solder joint," he says, explaining that welds are typically beads around the outside of a lap joint or butt joint while solder can cover the whole lap or butt area. Which can hold a greater load? It depends on the ratio of areas. "If the solder area is large enough, it can make up for the fact that solders have lower strengths than welds in general," he says.