What is the Reflow Profile of Solder?

Reflow profiling is the process of evaluating the effect of heat on the solder being used on a printed circuit board. In the reflow soldering process, the board with solder should be heated in a way to ensure that the solder melts and solidifies at particular temperatures over specific periods of time to ensure proper solder joints are formed.

Reflow profiling is used to optimize the soldering process by minimizing the drastic change in temperatures which might cause cold solder joints, charred boards, tombstoning and uneven wetting.

The reflow profiles are divided into four zones:

• Preheat Zone 
• Soak zone
• Reflow zone       
• Cooling zone

Preheat Zone: Initially the PCB is at room temperature. It cannot be subjected to high heat suddenly as there is the risk of thermal shock which can cause the components on the board to explode, melt or become nonfunctional. Hence to prevent this, the PCB is subjected to preheating where the temperature is increased by 3 every 1 second.

Soak Zone: In the Soak zone, the temperature achieved through preheat zone is maintained so that the FLUX applied on the board is activated and constant temperatures can be maintained for all components present on the board.

Reflow Zone: The reflow zone allows the temperature to rise again (by 3 every 1 second) till it reaches the spike value so that the applied solder begins to melt and start wetting with the copper surface which helps in creating better solder joints.

Cooling Zone:  The cooling zone is the profile where no additional heat is applied to the PCB and air is supplied to supplement the cooling process such that the temperature is decreased at a rate of 4 at every 2 seconds so that the freshly created solder joint is solidified.

Different types of solder have different reflow profiles. So it is important to evaluate the reflow profile of the solder being used.

There are two types of Reflow profiles:

Ramp-Soak Spike (RSS) Profile

Ramp soak spike profile indicates the temperature that is subjected to a given PCB is allowed to rest in the Soak zone (constant temperature value) before the temperature is increased again to hit the spike (maximum value).

Ramp To Spike (RTS)

For the Ramp to Spike Time-Temperature profile, the temperature is increased gradually without to reach its maximum value (Spike).

Ramp Soak Spikes are primarily used for Leaded Solder and Ramp-to Spike profiles are suggested for Lead-Free solder.

As mentioned in the table above, for Leaded solder the preheat stage usually starts at 150 and is increased to 180. The temperature of 180 is kept constant (Soak zone) before it is increased again to reach its spike value.

As Observed for Lead-free solder, the temperature keeps rising gradually to reach its maximum (spike) value.