BY ALAN RAE
In today’s rapidly changing packaging environment, board assemblers find themselves caught between two converging trend lines. On one hand, the move to lead-free assembly requires reflow temperatures as high as 260°C and higher for rework temperatures. Many existing components and systems – such as optical modules, certain low-cost connectors, and some advanced semiconductors -simply can’t cope with that much heat. On the other, the use of thinner stacked die, thinner bond wire, and smaller pitch devices that require careful alignment, or include low-temperature materials in their construction, is increasing in MEMS, optoelectronics, flash memory, and other devices. Nanotechnology will help packagers meet these challenges in the near term and beyond. Nanomaterials provide a number of processing options that can enable packagers to reduce the process temperature at assembly using tools such as lower temperature alloys, enhanced adhesives, and novel attachment approaches.
In nanomaterials, surface energy is high relative to the bulk material and the ratio of surface-to-volume. For example, as the diameter of a particle is reduced by 50%, the area is reduced by 75%, but the volume is reduced by 87.5%. As a result, particles less than approximately 20 nm are reactive. For example, the melting point of tin is reduced dramatically in the nano range. Now under development are nano-sized interconnect powders whose properties will allow a low initial fusion temperature. However, once the alloy is formed and macrocrystalline, the powders will solidify and their properties will revert to that of the “normal” alloy.
Figure 1. A close-up of nanomaterials.
Most conventional isotropic adhesives use relatively large (several microns) silver flake in a resin base – typically epoxy – and have limitations in terms of conductivity, strength, and moisture resistance. They generally cure in the 120° to 175°C range, much lower than the reflow temperature of solders. Conversely, anisotropic adhesives are largely based on gold-plated polymer spheres dispersed in a resin. As the resin shrinks on curing, the conductive surfaces pull together and the spheres compress between them.
Nano-sized fillers have an advantage over conventional fillers, which generally need to be in physical contact before conductivity is significant. They can be effective, even if not in direct contact. Using nanomaterials, we have the opportunity to lower the filler loading in adhesives and to create conductive materials with curing temperatures less than 130°C that may be strong, and in some case translucent or transparent for displays or UV-curable adhesives.
As nanotechnology moves into industrial products, other phenomena are opening up lower-temperature assembly opportunities. Non-conductive adhesives such as underfills can be used in die attach and other interconnect applications. Shrinking the polymer pulls the bump towards the pad. Unfortunately, the conventional 500-nm-size fillers needed to modify CTE and other characteristics of the adhesive can interfere with the direct connection between bump and pad.
Nano-sized materials may also be applied directly by inkjet or screen printing to write conductors onto surfaces. On evaporation of the transport medium, the silver can cold-weld to itself, even at room temperature, if the particle size is small enough. The preparation of materials suspended in alcohol or other process fluids allows ready incorporation into inkjet formulations.
Mechanical fasteners based on carbon nanotubes, such as conductive hook-and-loop and other types of fasteners, offer other connection possibilities to replace conventional interconnections. The strong, entangled loops of a nano-sized hook-and-loop system promise thermal and electrical conductivity, compliance, and strength calculated at up to 30 times the strength of conventional adhesives on an area basis. Arrays of hooks that can entangle other hooks or a nanotube mat can be synthesized by doping or other techniques.
The future of electronics assembly looks quite different as development of nanomaterials and structures continues. Low-temperature assembly becomes possible using techniques ranging from altering the basic properties of metals, to enhancing the structure of adhesives composites and creating totally new structures and interconnect mechanisms. The creative use of new materials will help assemblers meet the processing challenges they face.
ALAN RAE, Ph.D., VP of Market and Business Development, may be contacted at NanoDynamics, 901 Fuhrmann Blvd., Buffalo, NY 14203; 716/853-4900, ext. 346; E-mail: email@example.com. Rae also holds the positions of Director of Research for iNEMI and chair of the JISSO North America Council.