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88 SMT Magazine • July 2014 fect is observed between the ramp to peak and soak profiles for the water soluble formulation. This result is consistent with prior experience and due to lower oxidation resistance of water soluble fluxes. It is true that water soluble for- mulations are more active, but the oxidation resistance is poor in comparison to rosin/resin based no-clean formulations. Activity refers to the ability to remove existing surface oxides; oxidation resistance refers to the ability of the flux chemistry to prevent re-oxidation, which occurs later in the reflow process. Rosin/resin provides an excellent oxidative barrier, widen- ing the reflow process window. This is espe- cially true if a ramp to peak type profile is not attainable. Summary The utilization of ultrafine solder powders can improve transfer efficiency in the stencil printing process and effectively reduce the area ratio acceptance level to 0.50. Some con- tributing factors need to be considered such as square vs. circle and solder mask defined pads vs. copper defined to attain desireable results. A trade-off when choosing ultrafine solder pastes is the premature flux exhaustion due to an increase in total surface oxide, and sensitiv- ity of the smaller paste deposit-utilized for the oven environment. The total heat excursion that the solder paste endures may need to be optimized. Striving for a ramp to peak type profile with a peak temperature 240–245˚C (SAC305), time above liquidus 45–60s, and an average ramp rate 1–1.5 ˚C/s is ideal. No-clean flux formulations provide more oxidation resis- tance than water soluble formulations and can widen a reflow process window, especially if a ramp to peak profile is not attainable. SMT Acknowledement Special thanks to Eric Moen and Laserjob for providing the stencil utilized in this study. MEETING FuTuRE STENCIl PRINTING CHAllENGES continues ARTiCLE Ed Briggs is a green Belt Six Sigma certified technical support engineer with indium Corporation. To contact him, click here. Thermal systems use heat to produce cold, and vice versa. To do so, a material must dissipate water vapor particularly well and quickly. A new method applies this property as a layer onto the components. Refrigerators have the human body as an ex- ample: When we perspire, water evaporates on our skin and cools it. if the process is transferred to a vacuum, water evaporates at a few millibar and a temperature of 10 degrees. for the devices to continu- ously cool, vapor must be removed. This is achieved, for example, by an electric compressor. An alternative is the thermal compressor; the operating power is not electrical, but thermal. Heat pumps or chill- ers produce cold from heat, and vice versa. So far, however, these have not been able to prevail en- tirely over their electricity-powered counterparts. Researchers at the fraunhofer institute for Solar Energy Systems iSE have now closed this gap. Their metal organic frameworks (Mofs) are well suited to absorb water vapor. These layers can be directly applied without further auxiliary layers. in prototypes, Mofs are di- rectly crystallized onto metals. for other materials, such as ceramics, the scientists have accomplished this with binder-based coatings. in both methods, the compo- nents of the device are immersed in a fluid containing the essential components of the material. The temperature needed for direct crystallization occurs only on the surface of the component, with minimum waste. efficient thermal Cooling and Heating

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