Reflow soldering is a process in which a solder paste is used to temporarily attach one or
several electrical components to their contact pads, after which the entire assembly is subjected
to controlled heat, which melts the solder, permanently connecting the joint. Heating
may be accomplished by passing the assembly through a reflow oven or under an infrared
lamp or by soldering individual joints with a hot air pencil.
Reflow soldering is the most common method of attaching surface mount components to a
circuit board, although it can also be used for through-hole components by filling the
holes with solder paste and inserting the component leads through the paste. Because
wave soldering can be simpler and cheaper, reflow is not generally used on pure through-hole
boards. When used on boards containing a mix of SMT and THT components, through-hole reflow
allows the wave soldering step to be eliminated from the assembly process, potentially reducing
assembly costs. The goal of the reflow process is to melt
the solder and heat the adjoining surfaces, without overheating and damaging the electrical
components. In the conventional reflow soldering process, there are usually four stages, called
"zones", each having a distinct thermal profile: preheat, thermal soak, reflow, and cooling.
Preheat zone Maximum slope is a temperature/time relationship
that measures how fast the temperature on the printed circuit board changes. The preheat
zone is often the lengthiest of the zones and often establishes the ramp-rate. The ramp–up
rate is usually somewhere between 1.0 °C and 3.0 °C per second, often falling between
2.0 °C and 3.0 °C per second. If the rate exceeds the maximum slope, damage to components
from thermal shock or cracking can occur. Solder paste can also have a spattering effect.
The preheat section is where the solvent in the paste begins to evaporate, and if the
rise rate is too low, evaporation of flux volatiles is incomplete.
Thermal soak zone The second section, thermal soak, is typically
a 60 to 120 second exposure for removal of solder paste volatiles and activation of the
fluxes, where the flux components begin oxide reduction on component leads and pads. Too
high a temperature can lead to solder spattering or balling as well as oxidation of the paste,
the attachment pads and the component terminations. Similarly, fluxes may not fully activate if
the temperature is too low. At the end of the soak zone a thermal equilibrium of the
entire assembly is desired just before the reflow zone. A soak profile is suggested to
decrease any delta T between components of varying sizes or if the PCB assembly is very
large. A soak profile is also recommended to diminish voiding in area array type packages.
Reflow zone The third section, the reflow zone, is also
referred to as the “time above reflow” or “time above liquidus”, and is the part
of the process where the maximum temperature is reached. An important consideration is
peak temperature, which is the maximum allowable temperature of the entire process. A common
peak temperature is 20–40 °C above liquidus. This limit is determined by the component
on the assembly with the lowest tolerance for high temperatures. A standard guideline
is to subtract 5 °C from the maximum temperature that the most vulnerable component can sustain
to arrive at the maximum temperature for process. It is important to monitor the process temperature
to keep it from exceeding this limit. Additionally, high temperatures 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 liquidus, or time above reflow, 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, although 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, and most components have a well-defined time limit for how long they
may be exposed to temperatures over a given maximum. 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. A fast
cooling rate is chosen to create a fine grain structure that is most mechanically sound.
Unlike the maximum ramp-up rate, the ramp–down rate is often ignored. It may be that the
ramp rate is less critical above certain temperatures, however, the maximum allowable slope for any
component should apply whether the component is heating up or cooling down. A cooling rate
of 4°C/s is commonly suggested. It is a parameter to consider when analyzing process results.
Etymology The term "reflow" is used to refer to the
temperature above which a solid mass of solder alloy is certain to melt. If cooled below
this temperature, the solder will not flow. Warmed above it once more, the solder will
flow again—hence "re-flow". Modern circuit assembly techniques that use
reflow soldering do not necessarily allow the solder to flow more than once. They guarantee
that the granulated solder contained in the solder paste surpasses the reflow temperature
of the solder involved. Thermal profiling
Thermal profiling is the act of measuring several points on a circuit board to determine
the thermal excursion it takes through the soldering process. In the electronics manufacturing
industry, SPC helps determine if the process is in control, measured against the reflow
parameters defined by the soldering technologies and component requirements.
See also Wave soldering
Reflow oven Restriction of Hazardous Substances Directive
Thermal profiling References