Surface insulation resistance (SIR) is a critical material property for electrically reliable assemblies using a no-clean surface mount technology process. It has been shown in recent studies that the reflow profile and atmosphere used can have significant effects on the SIR properties of no-clean solder paste. In this study, multiple reflow profiles were used in a vapor phase reflow process [1]. Vapor phase reflow is the process of subjecting an assembly with unreflowed solder paste and components to a temperature equal to the boiling point of the liquid used in the reflow process. This type of solder reflow process offers several advantages, especially for assemblies with a large amount of thermal inertia. Differences in temperature at different locations on the board can be reduced. In addition, lower peak temperatures can be used versus a typical reflow profile in a convection reflow oven. This can reduce the thermal stress on smaller components that otherwise could reach higher temperatures in a conventional convection oven process. Another advantage is that the reflow liquid vapor is extremely inert, reducing any potential for excessive oxidation which can increase the incidence of head in pillow defects. It is well known that an inert atmosphere also increases the spread and wetting of lead free alloys. This also contributes to the reduction in voiding under bottom terminated components.
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