The document describes the different zones of a reflow oven and their purposes and parameters. It discusses the preheat, soak, reflow, and cooling zones. In the preheat zone, the temperature is raised slowly to avoid thermal shock, while the soak zone brings the board to a uniform temperature and activates flux. The reflow zone melts the solder at the proper peak temperature for adequate wetting. The cooling zone sets the solder joint grain size, with faster cooling producing stronger joints. Achieving the desired time and temperature uniformity within 5-10°C across all zones is important for developing an effective profile.

1. REFLOW OVEN ZONES
Preheat Zone:
In the preheat zone, the temperature is 30°–175°C, and component suppliers
generally recommend 2°–3°C/sec.ramp rate to avoid thermal shock to sensitive
components such as ceramic chip resistors. This guideline is too conservative because
the same capacitors are wave soldered where they go from preheat temperature of
about 120°C to wave-pot temperatures of 260°C. The fast ramp rate increases the
potential for solder balls. However, it is safe to use a 5°C/sec. ramp rate
Soak Zone:
This zone brings the entire board up to a uniform temperature. The ramp rate
in this zone is slow, almost flat, when raising the temperature from 75°– 220°C. The
consequences of being too high in the soak zone are solder balls and solder splatter
due to excessive oxidation of paste. The soak zone also acts as the flux-activation
zone for solder paste. The purpose of the long soak zone is to minimize voids,
especially in BGA’s. It also is common practice not to use the soak zone, but ramp the
temperature steadily from preheat zone to peak reflow. However, the likelihood of
voids is increased when ramping steadily to the peak reflow temperature.
Reflow Zone:
In this zone, boards may char or burn if the temperature is too high. If the
temperature is too low, cold and grainy solder joints will result. The peak temperature
in this zone should be high enough for adequate flux action and good wetting.
However, it should not be so high as to cause component or board damage,
discoloration, or charring of the board. The peak temperature in this zone should be
230°–245°C for lead-free. The time above liquidus (TAL) should be 30-60 seconds.
Extended duration above the solder melting point or TAL will damage temperature-
sensitive components. It also results in excessive inter metallic growth, which makes
the solder joint brittle and reduces solder joint fatigue resistance.

2. Cooling Zone:
The cooling rate of the solder joint after reflow also is important. The faster
the cooling rate, the smaller the solder grain size and the higher the fatigue resistance.
The cooling rate should be as fast as possible. However, there is no control on the
cooling rate other than ensuring that the cooling fans are operational. If they are not,
the cooling rate will be slow, increasing grain size and causing weaker solder joints.
Achieving the desired time, temperature, and uniformity of temperature across the
board in all four zones within 5°–10°C is vital in developing a reflow profile. Adjust
the panel settings and belt speed until these objectives are met. The right profile with
tight bandwidth is like using a good design for manufacturing (DFM).
Unique Profile for Each Product
Once the desired profile is achieved, run a production board with solder paste
and components for reflow. After reflow, inspect the solder joint quality. A random
problem in only a specific section of the board may be related to solder ability; a
consistent problem in a given section may be related to the solder profile due to non-
uniform heating. Consistent problems may also be related to paste quality and land
pattern design. Once the profile gives the desired results (assuming design and other
material variables have been optimized), document the profile. After this point, no
changes should be allowed in the profile