Printed Circuit Design & Fab - July 2008 - (Page 32)

EMERGING tEchnoloGiES ‘Warm’ manufacturing heATs UP Nanotechnology poses an attractive solution for lower-temp soldering. by Dr. alan rae, Dr. anDrew sKiPor and MarC CHason Higher reflow process temperatures are causing concern among electronics manufacturers. Product reliability can be diminished by the residual stresses in PCB assemblies that these higher temperatures cause. Higher temperatures also require tougher qualification requirements for components and sometimes a significant change in manufacturing processes.1 Low-temperature or room temperature assembly processes have the potential to improve field reliability, streamline manufacturing and reduce cost. As nano-structured and more sensitive components are introduced, some with biological components that have to interface with electronic detection systems, lower-temperature assembly processes will become a necessity. “Warm manufacturing” is a term coined by the International Electronics Manufacturing Initiative (iNEMI) to describe processes that can be used to assemble electronic devices at temperatures lower than traditional solder reflow. The need for warm manufacturing stems from: ■ Higher Pb-free solder reflow temperatures increased failure rates of existing components and devices. ■ Increased thermal sensitivity of newer devices that contain nanoscale semiconductor structures, MEMS devices, proteins and other low-temperature organic materials. Several novel nanotechnology applications have shown great promise as solutions for warm manufacturing. Encompassing many diverse disciplines that permit the manipulation of matter at the atomic level, nanotechnology enables radically new approaches to material property enhancement and synthesis. Nanomaterial solutions have the potential to augment and enhance traditional manufacturing processes, improve existing products, enable new product concepts, and disrupt industry. Although none of the technical solutions currently available is sufficiently developed to provide a universal, easily integrated process for lower temperature assembly, initial work has shown encouraging results. This 32 article discusses some of these applications. nano-solder: melting point depression. An example of how nanotechnology can be used to modify assembly process temperatures is the excellent work over the past several years on melting 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 the studies by Buffat and Borel on gold nanoscale particles demonstrated well over 50% melting point depression, compared to the bulk melting point of gold. More recently, other researchers3,4,5,6 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 nano-Joules of heat can be measured. Based on these nanocalorimetry studies, a simple expression was developed to relate melting point to particle size.5 Tm(r) = 156.6 - (220/r) (EQ. 1) where Tm(r) = melting temperature (°C). r = radius of the particle (nm). This equation reveals that 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, JULY 2008 printEd circuit dESign & fAB

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Printed Circuit Design & Fab - July 2008

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