The solder joint will only form once the solder alloy and solderable surfaces (the component leads and the board lands) are free from metal oxide and organic contaminants. This is the second job of the flux and is achieved by use of special chemicals called activators which react with the various metal oxides. Since the activator is consumed in the reaction it is essential that sufficient activator is present to clean all surfaces. As the assembly is being heated during the reflow profile, the metal surface oxide is reforming, therefore sufficient activator is required to survive the desired reflow profile. Once the solder has melted, the solder particles will flow together to form a single filet. The tin in the solder will then dissolve into the metal of the lead or pad to form an intermetallic, it is this intermetallic layer that gives the joint its strength. The exact intermetallic that is formed will depend on the metallization of the lead or pad. Some common surfaces and the associated intermetallic are shown here. The thickness of the intermetallic layer will depend on the metals involved and the time the solder is above liquidous as well as the peak reflow temperature. An intermetallic layer may be as thin as half a micron up to a couple of microns.