- Nanosolder on multi-segment nanowires have been successfully fabricated by electrodeposition method;
- Flux assisted environment enhanced reflow result and micron scale solder spheroids formed on non-wetting Si substrate;
- Nanosolder reflow performance on reactive Cu substrate was studied;
- 1-D interdiffusion on Cu-Sn two-segment nanowire were observed through the thermal heating and e-beam irradiation;
- Nanojoints formed between nanowires and a network was constructed through quasi-reflow process in liquid.

•What is a halogen?
•Impact on the environment
•Halides and halogens
•Halides in electronics
•Halogens in electronics
•Definition of halogen free
•Technical challenges to remove halogen
•Current ‘State of the Art’

There has been a rapid increase in the use of selective soldering equipment for PCB assembly
–Lower equipment costs
–Smaller equipment footprint
–Lower solder “inventory” cost (smaller pots)
–Decrease use of through hole devices
–Some technical challenges
•Tighter component spacing
•More complex board designs
•Increased desire to control flux spread
The selective soldering process is much different than wave soldering so there are different liquid flux considerations

Looking back twenty-five years ago,the solder pastes residues had to be cleaned after reflow due to their corrosive nature: two ways of cleaning were possible,either with solvent or by using water,with or without detergent. Now the assembly world is mainly no-clean: paste formulation is safer in terms of chemical reliability and process costs are reduced without cleaning. However,some applications,i.e. military,aerospace,high frequency,semiconductor require a perfect elimination of the residue after reflow. There are several options to achieve this result: the use of a no-clean paste which residue can be removed with the most suitable cleaning method or the use of a paste designed to be cleaned,as a water-soluble solder paste.
The water-soluble solder pastes generally show great wettability because of their strong activation but they are also known to have shorter stencil life and to be more sensitive to working conditions as temperature and humidity,compared to the no-clean pastes. Additionally,with the components stand-off getting smaller and smaller,washing residues with water only is more and more challenging due to its high surface tension: the addition of detergent becomes often necessary.
The purpose of this paper is to highlight the differences between these two families of solder pastes to guide users in their choice. This will be achieved through the comparison of several recent water-soluble and no-clean formulations as far as reliability is concerned. First the printing quality will be evaluated (viscosity,tack,cold slump,printing speed according to pressure,stencil life,idle time,printing consistency). Then the reflow properties will be compared (hot slump,solderballing,reflow process window,wetting ability on different finishes). Finally the residue cleanability will be assessed. The IPC SIR test (method IPC TM 650 2.6.3.7) will be also done to conclude the study. Both standardized tests and production tests will be used to evaluate the performance of these two kinds of solder pastes.

As the proliferation of modern day electronics continues to drive miniaturization and functionality,electronic designers/assemblers face the issue of environmental exposure and uncommon applications never previously contemplated.
This reality,coupled with the goal of reducing the environmental and health implications of the production and disposal of these devices,has forced manufacturers to reconsider the materials used in production.
Furthermore,the need to increase package density and reduce costs has led to the rapid deployment of leadless packages such as QFN,POP,LGA,and Micro-BGA. In many cases,the manufacturers of these devices will recommend the use of no clean fluxes due to concerns over the ability to consistently remove flux residues from under and around these devices.
These concerns,along with the need to implement a tin whisker mitigation strategy and/or increase environmental tolerance,have led to the conundrum of applying conformal coating over no clean residues.
The AIM Research & Development team has united with OEM electronics and conformal coating manufacturers in an attempt to characterize the different coating technologies currently available. In this study,various coating materials were tested with different chemistries of no clean fluxes. Results demonstrate possible combinations meeting the mission profile of the assembly with consideration for the assemblers’ capabilities and cost objectives.

The very small solder joints that now account for an increasing proportion of the connections on which modern electronics depends are typically made up of only a few grains,sometimes only a single grain. This combined with the high degree of anisotropy in the mechanical properties of the body-centred tetragonal beta-tin crystal is a significant factor in determining the response of the joint to the strain to which it is subject in service,with consequent implications for reliability. The superior reliability in joints with multiple small grains of random orientation suggests that it would be advantageous if the solder alloy could be made to solidify with that fine grain structure. In the study reported in this paper the effect of trace additions of selected elements on the grain structure of pure tin and lead-free solder alloys was observed. The elemental additions were chosen on the basis of previous research as well as an analysis of relevant binary phase diagrams. Solidification theory suggests that an objective of the addition should be to promote the rapid development of a constitutionally undercooled zone ahead of the advancing solid/liquid interface since this is known to favour the repeated nucleation required to achieve a fine grain structure. The results contribute to the growing body of knowledge on the development of microstructure in lead-free solder alloys.

SAC-Bi and Sn-Ag-Bi alloys have demonstrated superior performance in thermal cycling reliability tests of printed circuit boards,such as the National Center for Manufacturing Sciences programs in the 1990’s and the JCAA-JGPP program of the early 2000’s. They have not been widely used in electronics manufacturing despite these promising results due to their Bi content,which has raised concerns for the potential of forming the low-melting point Sn-Pb-Bi eutectic phase (Tm 96 °C) in mixed SnPb-Pb-free soldering. The recently concluded (December 2011) NASA-DoD program Phase II follow-on to JCAA-JGPP revived the possible use of Bi-containing alloys with the recommendation that lower reflow temperatures for ternary Sn-Ag-Bi and quaternary SAC-Bi could reduce potential for pad cratering. At the same time,an explanation for the observed performance in thermal cycling has not been provided,and basic aspects of the metallurgy of these alloys have not been explored to the same extent as more common SAC alloys. In this study,the relationship between microstructure,aging and mechanical behavior was studied for Sn-3.4Ag-4.8Bi and Sn-3.4Ag-1.0Cu-3.3Bi and compared to SAC305. The alloys were prepared in bulk form by rapidly quenching from 260°C resulting in an as-solidified microstructure similar to that observed in solder joints. As-solidified properties were compared to those for samples aged two weeks at 150°C. Tensile testing,constant-stress creep tests,and low-frequency dynamic mechanical analysis up to 50 Hz were performed at various temperatures for both microstructural conditions. Aging led to significant microstructural changes in all the alloys,but while aging was accompanied by changes in the tensile and power-law creep properties of SAC305,the corresponding differences in the as-solidified and aged Bi-containing alloys were either smaller or absent. For example,aging SAC305 led to an increase in the creep stress exponent and a nearly 50% reduction in the activation energy,while for SAC-Bi the reduction in activation energy was similar but the stress exponent was reduced,and in Sn-3.4Ag-4.8Bi neither activation nor stress exponent were changed by aging. These differences do not explain the performance of the solder joints in reliability testing but suggest that thermal fatigue reliability of solder alloys may be enhanced by addition of Bi.

1.Obtain suggestions from the solder manufactures
Low-Ag SAC Paste
•Increase in reflow temp
•Impact on thermal fatigue and wetting
Sn-Bi low-temp Paste
•Impact on shock strength
•Creep fatigue & max operating temp
Specialized Paste
•Effectiveness of additives
•Side effects
2. Quantification of properties using user-friendly methods
3. Licensing terms for alloy candidates are RAND basis

The growth of portable and wireless products is driving the miniaturization of packages resulting in the development of many types of thin form factor packages and cost effective assembly processes. Wire bonded packages using conventional copper lead frame have been used in industry for quite some time. However,the demand for consumer electronics is driving the need for flip chip interconnects as these packages shorten the signals,reduce inductance and improve functionality as compared to the wire bonded packages. The flip chip packages have solder bumps as interconnects instead of wire bonds and typically use an interposer or organic substrate instead of a metal lead frame.
The integration of these packages in high volume SMT assembly demands good assembly process controls at the package level and clear understanding of the failure modes to minimize defect escape to subsequent assembly operations. This challenge is enhanced with the transition to lead free reflow as the higher peak reflow temperatures results in more thermal and CTE mismatch between package and PWB.
The paper provides a general overview of typical defects and failure modes seen in package assembly and reviews the efforts needed to understand new failure modes during package assembly. The root cause evaluations and lessons learned as the factory transitioned to thin form factor packages are shared.

The successful integration of package-on-package (PoP) stacking utilizing through mold via (TMV) technology hinges on a robust assembly process. In this study,seven dip materials were investigated for high quality TMV PoP assembly by optimizing machine settings to achieve proper material transfer. Film thickness was varied for each material to transfer enough material (target of 50% ball coverage) while preventing parts from sticking within the film. Assemblies were reflowed in both air and N2 atmospheres and yields were quantified. It was determined that flux dipping provides for better TMV assemblies in air reflow due to the flux’s ability to wet to and subsequently protect the TMV solder ball during reflow. All paste dipped materials experienced significant fallout in air reflow due to a non-coalescing of the TMV solder joint. All materials provided 100% assembly yields in N2 reflow.