Like it or not, lead-free requirements and RoHS compliance are impending realities. While many companies have processes and products that meet these requirements right now, there are many reasons to be concerned about the use of lead-free solders. Conductive adhesives with nano-engineered fillers have shown some promise toward addressing those concerns. Much of this work is being done by Professor C.P. Wong's group at the Georgia Institute of Technology (Atlanta).
The main benefit to using a conductive adhesive instead of a lead-free solder is the lower process temperature. Lead-free solders tend to have higher melting temperatures than commonly used eutectic lead-tin solder, and components are becoming more sensitive to high temperatures. Flash memory, for example, is sometimes programmed prior to packaging, and excessive temperatures may scramble this programming, if not harm the devices. Low-k dielectrics may also degrade when exposed to lead-free solder temperatures. Beyond the 65 nm node, diffusion can be an issue. The temperature sensitivity of MEMS devices, sensors and optoelectronic devices may make them incompatible with lead-free solder processes as well.
One way to enhance the properties of a conductive adhesive is to load it with nano-sized metal powder. Currently used conductive adhesives contain silver flake with loadings up to 75 weight-percent. If loaded with metal powders with particle sizes in the 10-40 nm range, the particles need not be in contact with each other to facilitate conduction, due to tunneling and percolation (Fig. 1). According to Alan Rae of NanoDynamics (Buffalo, N.Y.), conduction can be achieved with loadings below 1 weight-percent using such powders.
The challenge to using such a powder in a conductive adhesive is keeping the particles from agglomerating. The small size and high surface energies of the particles make this difficult. According to Rae, "If they form clusters as big as a micron or more, the game is over."
If this challenge can be met, it would be possible to produce conductive adhesives whose rheological properties are determined by the adhesive rather than the conductive material. In this case, conductive adhesives could be tailored to optimize their handling properties, and an ink-jettable conductive adhesive would not be out of the question.
Also, work done by Wong's group shows that particles this small are swept away from contact points between metal pads during a die bonding process, if there are no larger particles present. The shrinking of the adhesive during the cure process actually draws the metal pad surfaces together. A die bond adhesive filled with such particles could possibly function as an underfill that enables electrical conduction in the same way that anisotropically conductive films do.
Another use for nano-sized metal powder is in solder paste. In this application, the reactivity of the smaller particles actually reduces the melting point of the metal, which lowers the melting point of the solder. After the solder is melted, its properties would be the same properties as regular solder. The use of nano-sized powders in solders is under investigation in a variety of applications because of this melting point property.
Another way of enhancing the properties of a conductive adhesive is to load it with carbon nanotubes. Wong's group recently announced some results of its work on this idea, and more results will be presented at the upcoming Electronic Components and Technology Conference (ECTC) in Lake Buena Vista, Fla.
Carbon nanotubes tend to stick to each other via van der Waals forces in much the same way that strands of spaghetti stick to each other. When conductive carbon nanotubes are loaded in an adhesive, the "ropes" formed by the nanotubes do more than provide conductive paths; they also reinforce the adhesive, making the bond even stronger.
One of the main challenges in the manufacture of carbon nanotubes for IC applications is to keep them from sticking to each other. In this application, sticking is actually desired. With the main challenge out of the way, Wong's group has demonstrated the ability to produce aligned carbon nanotubes suitable for use in loading conductive adhesives (Fig. 2).
While some are busy developing nanotechnology for use in advanced sensors and post-CMOS ICs, it can also provide inexpensive solutions in packaging applications right now. With some luck, this fact could help to build nanotechnology infrastructure faster, so that it might be ready when we need it after CMOS runs out of steam.