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Reference Information (continued)
The Preheat Soak zone holds the component/PCB temperature at 130-170°C for 30-60 seconds, continuing the
drying process to prevent out-gassing and possible spattering of the solder paste (if used). This zone is also where the
flux begins to remove the oxides from the surfaces of the leads, pads and the powder itself. The resins and/or higher
boiling solvents remain as a cover to prevent the re-oxidation that would readily occur at the elevated temperatures.
Rework operations do not always include a soak zone, but theoretically they should do.
Reflow Zone - In the reflow, or spike zone, the temperature is more quickly raised 25-40°C above the melting point of
the bonding alloy. Known as the point at which reflow occurs, or 'reflow', the melting point is normally between
183°C and 188°C for standard solders (Lead-free solders may have melting points normally 217°C to 228°C). It is
above reflow that the solder wets the surfaces and forms the intermetallic bonds. The maximum temperature to be
reached is normally 220-230°C for tin/lead based solders, 240-250°C for typical Lead free applications. The maximum
ramp rate in this zone is typically 2-4°C/second.
Reflow Soak Zone - The Reflow Soak time (the period of dwell time at the peak reflow temperature, typically 20-60
seconds) should be long enough to allow for all of the joints to reach temperature and form the bonds. Too long a
soak time can lead to excessive intermetallic formation. Typically, intermetallics are brittle, and if they make up a large
portion of the fillet, can lead to premature failure of the joint.
The optimum profile is not the same for all PCB assemblies. In the real world, almost every assembly has different
thermal characteristics across the board due to different components or component densities. Variations in the board
itself can lead to large differences in thermal mass. However, it is easily possible to establish a pattern and develop a
range of profiles to take into account all the factors.
Summary - It is perfectly possible to establish a cost effective post production, soldering/rework process. A range of
equipment is often needed. Consideration of the basic science is vital. As well as the method of heating, it is as
important to conduct extensive research, testing to get the ability to control a process in a repeatable manner.
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