After the paste has been deposited, the components must be placed. At this time the assembly will be ready to be “reflowed”. This can be considered the actual soldering process and involves heating the assembly to make the solder alloy melt and form a solder joint. The most common way to achieve this is to use a forced convection oven which blows heated air across the top and bottom of the assembly as it passes through the oven. A similar effect can be achieved by condensing hot vapour onto the assembly to heat it up (called vapour phase or condensation reflow). The exact rate that an assembly heats up and the time that it spends at a given temperature is referred to as the reflow profile. The profile is achieved by splitting the oven into a series of zones that can be heated to different temperatures and by controlling the time an assembly spends in each zone. There are two basic types of reflow profile. The simplest is a linear ramp to the maximum or peak temperature, this is determined by the alloy that is being used and must be above the melt point of that alloy (for a lead containing alloy this would typically be between 205°C - 215°C and for a lead-free alloy between 235°C - 245°C). This type of profile is called a “linear” or “ramp to peak” profile. The second profile which is shown here is a “soak” or “ramp soak peak” profile. Here the assembly is heated to an intermediate temperature where it is held prior to being raised to the final peak temperature. This is usually done when there is a significant variation on the thermal mass of the components on the board leading to a large difference in the heating rates and peak temperature (see the red and blue profiles). The soak reduces the ultimate difference in temperature (so called delta T) between the large and small components allowing all the board to spend the necessary time above the melt point of the alloy, this is called the liquidous point.