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The increasing durability requirements for aluminum heat exchangers involve the use of higher levels of magnesium in the core material in order to increase the strength of its components. The challenge is to braze these aluminum alloys by the preferred CAB process. During the CAB process magnesium diffuses out, forming a Magnesium Oxide and Magnesium Fluorides and the use of conventional flux becomes ineffective for disrupting it. When wetting occurs, the brazed joint is discontinuous and the presence of leaks becomes more probable. It was found that using a modified aluminum flux that includes some cesium in its composition for aluminum controlled atmosphere-brazing process, it is possible to obtain strong brazed joints on the heat exchangers. Additionally, the inclusion of cesium in the flux makes possible to braze aluminum alloys with higher levels of magnesium providing stronger and more durable heat exchangers assemblies.