ACCOMPLISHING THE REFLOW PROCESS
Process Zones
11-120 Bravo™ 8105 Programming and Operations Guide Rev. 2
11
It is important to monitor the maximum temperature. Temperatures above 230°
C may cause damage to the internal dies of SMT components as well as foster
intermetallic growth. Conversely, a temperature that isn’t hot enough may
prevent the paste from reflowing adequately.
Time above reflow, or liquideous, measures how long the solder is a liquid. The
flux reduces surface tension at the juncture of the metals to accomplish
metallurgical bonding, allowing the individual solder powder spheres to combine.
If the profile time exceeds the manufacturer’s specification, the result may be
premature flux activation or consumption, effectively “drying” the paste before
formation of the solder joint. An insufficient time/temperature relationship causes
a decrease in the flux’s cleaning action, resulting in poor wetting, inadequate
removal of the solvent and flux, and possibly defective solder joints.
Experts usually recommend the shortest TAL possible, however, most pastes
specify a minimum TAL of 30 seconds. There appears to be no clear reason for
that specific time. One possibility is that there are places on the PCB that are not
measured during profiling, and therefore, setting the minimum allowable time to
30 seconds reduces the chances of an unmeasured area not reflowing. A high
minimum reflow time also provides a margin of safety against oven temperature
changes.
The wetting time ideally stays below 60 seconds above liquidus. Additional time
above liquidus may cause excessive intermetallic growth, which can lead to joint
brittleness. The board and components may also be damaged at extended times
over liquidus. Too little time above liquidus may trap solvents and flux and create
the potential for cold or dull joints as well as solder voids.
Cooling Zone The last zone is a cooling zone to gradually cool the processed board and solidify
the solder joints. Proper cooling inhibits excess intermetallic formation or thermal
shock to the components. Typical temperatures in the cooling zone range from
30° — 100° C (86° — 212° F)
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