This is part three in my series on semiconductor fluxes. As someone new to learning about semiconductor fluxes, these posts will help others to better understand how fluxes work.
The most important function of a flux is to clean the surface of the substrate you are soldering to ensure proper wetting. This is done during reflow. The combination of heat and time is important. You need to get to the appropriate temperature to activate the flux and then you need to allow enough time for the solder to form an intermetallic species. During the "cleaning" action or soldering process, the flux reacts with the metal oxides to form a salt, which then gets contained within the flux residue when it solidifies. Once the flux "cleans" the surface, the solder can easily reach the surface of the substrate and soldering begins.
So when it comes to flip-chips, the flux needs to do all of the above. It needs to be able to clean the surfaces and not get exhausted during the process, and when the residue cools it needs to encapsulate all other parts of the flux. At the same time, you also have to worry about warpage and die tilt. Thermocompression bonding (TCB)can help to overcome these challenges. TCB would be used instead of the standard reflow process. The best flux to use for the TCB process would be an ultra-low residue no-clean flux. These ultra-low residue fluxes completely evaporate the extra flux and leave little residue behind.
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