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Novel Fluxes with Decreased Viscosity After Reflow for Flip Chip and SIP Assembly
Keywords: Flux, viscosity, decrease upon reflow
At assembly of flip chip or SIP, often use of a tacky flux is necessary to secure the chip during handling and reflowing the chips. On the other hand, it is critical to remove the flux residue after reflow in order to achieve high reliability. A series of flux system has been developed which would result in a reduced viscosity after reflow. This enables a high viscosity high tack flux to be used to secure component at component placement and reflow stage, but ends up with a low viscosity flux residue after reflow, thus facilitate the flux residue to be cleaned. A technique for forming such special fluxes is to establish a temporary association force within the materials themselves, such as an acid-base association. This kind of association force can increase the apparent molecular weight and cause material viscosity to increase. After a heating process, one of the critical ingredients got evaporated, thus eliminated the association force, causing a decrease in the apparent molecular weight, and consequently a decrease in viscosity or an increase in mobility. The evaporation of one ingredient can be the result of one ingredient having a lower boiling point, or the decomposition of one ingredient during heating. Volatile ingredient approach was less effective than decomposable ingredient approach, presumable due to formation of some permanent chemical bond between volatile ingredient and non-volatile ingredient.
Ning-Cheng Lee, VP Technology
Indium Corporation
Clinton, NY
USA


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