Circuits Assembly - July 2008 - (Page 31)

Emerging Technologies ing point depression. The phenomenon of melting point depression of nanoscale metal particles has been studied since the 1960s, when it was noticed that extremely thin evaporated particles of metal have a lower melting point than the bulk material.2 Melting point reduction of tin evaporated particles was studied by Wronski2, and Buffat and Borel3 studied gold nanoscale particles, demonstrating well over 50% melting point depression, compared to the bulk melting point of gold. More recently, other researchers4,5,6 have developed alternate experimental methods, such as nanocalorimetry, to measure the latent heat of fusion as a function of temperature. This new calorimetric technique has been developed where nanoJoules of heat can be measured. Based on these nanocalorimetry studies, a simple expression was developed to relate melting point to particle size5: Tm(r) = 156.6 - (220/r) (Eq. 1) where Tm(r) = melting temperature (°C). r = radius of the particle (nm). Silver exhibits a dramatic increase in sinterability at temperatures below 200°C. Nano-silver powders can be produced in essentially monodisperse form using a “bottom-up” approach, in which nuclei grow under a protective polymer coat that permits metal atoms to accumulate while acting as a charge director to keep the embryo crystals separated. The crystals are permitted to grow until the desired particle size is achieved; then the reaction is stopped and crystals suspended in an appropriate vehicle or dried. The polymer coating, which can be made hydrophilic or hydrophobic, aids re-dispersion and prevents spontaneous sintering of the dried silver particles. Pure nano-metals such as silver can be deposited by a range Automatic In-line Wire Bond Inspection High accuracy inspection of wire bond connections Viscom’s in-line automatic optical inspection (AOI) system S6053BO guarantees reliable defect detection on typical bond connections. With its modular camera technology and standard resolutions of 8.3µm or 5.6µm, the system inspects over 1,000 aluminum or gold wire connections per minute as well as SMD components, solder joints and die damages. For wire diameters down to 17µm, the new VHR camera module with resolutions < 3µm is now available. Viscom’s field-proven S6053BO AOI system is able to reliably inspect features such as ball or wedge position and completeness, loop-height, spacing, shape and geometry and ensures highest quality in zero defect environments. See us at Semicon West, July 15-17 West Hall - Level 1, Booth No. 7347 This equation reveals significant melting point suppression happens when the particle radius approaches the sub-20 nm range. Many materials exhibit a change in properties as they move toward the nanoscale. This is because of the increase in the relative proportion of higher energy surface atoms. This change can be exhibited as a change in reactivity (e.g., sinterability or electromagnetic properties), driven by band gap changes that cause dramatic changes of electronic properties or optical properties such as color and transparency. Where these changes occur – the tipping point – is a function of the individual element or compound and its environment, and normally occurs below 100 nm (Figure 1). Solder materials containing nanosized metals exploit the high surface area and high surface energy (think of it as stored energy) of nano-sized particles to lower the apparent melting point below the conventional melting point. Thus the melting points of tin, silver and copper, the ingredients of most popular Pb-free solders, all can be depressed below 200°C, well below the eutectic melting point of 217°C.7 circuitsassembly.com Wire Bond 4.5 x 7.indd 1 Circuits Assembly JULY 2008 6/16/2008 31 4:00:27 PM http://www.viscom.com http://www.viscom.com http://circuitsassembly.com

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Circuits Assembly - July 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Tech Tips Will Electronics Follow the Sun? ‘Warm’ Manufacturing Heats Up Reputation Trumps Recession, China ‘Web Circuits’ A Novel Non-VOC Conformal Coating Krypton: Benchmarking Customer Satisfaction 2008-09 CIRCUITS ASSEMBLY Buyers Guide Wave Soldering Pb-Free Lessons Learned Process Doctor Materials World SMTA Int'l Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - July 2008

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