|
Sticky nano-solutions for electronic assembly
The incorporation of nanopowder fillers improves the performance of adhesives for flip chip electronic circuit assembly, the FP5 project Nanojoining has found. The research results have also led to unexpected applications in other sectors.
 Flip chip electronics, in which bare integrated circuits and components are adhesively bonded face-downwards onto conductive bumps on printed circuit substrates, offer advantages in terms of size, performance, flexibility, reliability and cost over conventional wire-bonded circuitry.
To date, however, this mode of assembly has been prone to thermally induced cracking of the solder bumps between the chip and board, causing connection breakdown and even damage to the devices themselves. While commercially available underfill materials solve this problem, they fail to meet manufacturing constraints with respect to reliability and packaging costs. In addition, current filler particle sizes do not match with further miniaturization demands. A third problem is overheating of the chips as a consequence of miniaturisation, higher operating frequencies and higher working temperatures, resulting in higher thermal stresses which current polymer materials cannot adequately dissipate.
A key factor limiting progress has been the difficulty of producing adhesives that provide the appropriate electrical and thermal conductivity characteristics suitable for application at minimal layer thicknesses. Based on work carried out in the FP5 thematic network ‘Adhesives in Electronics', the team at the Netherlands Organisation for Applied Scientific Research (TNO) concluded that the incorporation of nanoparticle fillers could hold the answer. It therefore assembled a consortium, bringing materials and equipment suppliers together with industrial end users and other research institutes to form the 42-month Nanojoining initiative.
Targeting major property improvements
The objective was to develop new materials and processing techniques for the bonding and underfilling of flip chips and bonding of heatsinks. The partners targeted a 40% reduction in size, weight, material consumption and power consumption for products such as mobile telecommunication equipment, computers, monitors and automotive devices.
“A key factor limiting progress was the difficulty in producing adhesives that provide the appropriate electrical or thermal conductivity characteristics together with appropriate processing properties,” explains coordinator Mrs Nienke Bruinsma, formerly of TNO. “Using nanoparticles makes it possible to obtain combinations of properties that would not otherwise be attainable, but their high specific surface area causes the viscosity of dispersions to rise rapidly as filler concentrations increase.
Silver lining
In this project, a particularly fruitful collaboration between Metalor Technologies (CH), Microdrop Technologies (DE) and the Fraunhofer IFAM institute (DE) realised an acrylate-methacrylate-epoxy adhesive filled with up to 70 wt% of metallic silver particles of around 5 μm diameter, together with an ink-jet process capable of delivering glue dots having a diameter of just 130 μm. Although smaller dot sizes have been achieved with silver-filled inks, this is a world first in the domain of adhesives. A two-stage curing process enabling initial dot patterning and drying to be followed by chip placement and post-polymerisation lends itself particularly well to industrial exploitation.
TNO headed the investigation of underfilling materials, which add mechanical strength to chip assemblies and protect the connections from environmental hazards. The incorporation of filler particles of 20 nm diameter has made it possible to provide effective filling of inter-surface gaps as shallow as 20 μm, with a potential to go to much lower values. The industrial partners Bosch and Thales are both interested in integrating this technology into their future manufacturing strategies.
Polish SME Amepox Microelectronics, a company with just 15 employees, developed its own proprietary method for producing silver nanoparticles with diameters down to 3-8 nm (thought to have been achieved by only one other company in the world). It chose to focus initially on the manufacture of jet-applicable inks. These offer an economical means of printing antennas for smart card and mobile phones, for example, and are expected to be commercially available early in 2006.
Subsequently, Amepox aims to introduce nano-filled conductive adhesives within around one year. Meanwhile, it has also received expressions of global interest from potential customers seeking to exploit the known bactericidal properties of silver.
“This was something of a surprise to us,” says Managing Director Andrew Moscicky. “We were concentrating on the microelectronics market, and had not recognised the wider implications of our discoveries. With their huge surface areas, silver nanoparticles are extremely effective bactericides, which could be used as polymer fillers in applications as diverse as domestic appliances, air conditioning system components and floor cladding materials for hospitals and food factories.”
“Our probable approach will be to licence the technology, which would help to provide us with the means to expand in our own core business areas,” Moscicky adds. “All in all, participation in Nanojoining has been highly beneficial for us, in terms of making new contacts, gaining new knowledge and sharing experiences.”
Effort continues
Thermally conductive adhesives and moulding materials with nano filler particles proved less interesting, but valuable lessons were learned in all areas of the project's coverage. Several of the partners will continue with individual and collaborative research efforts in what has proved to be an extremely promising field, with major implications for Europe's future in the electronics sector and beyond. |
