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The process of hot air reflow soldering is essentially a heat transfer process. Before starting to “cook” the target board, the reflow oven zone temperature needs to be set up.

Reflow oven zone temperature is a set point where the heat element will be heated to reach this temperature set point. This is a closed loop control process using a modern PID control concept. The data of hot air temperature around this particular heat element will be fed back to controller, which decide to turn on or off the heat energy.

There are many factors that affect the board’s ability to warm up accurately. The major factors are:

• Initial PCB temperature
  In most of situations, the initial PCB temperature is the same as room temperature. The greater the difference between PCB temperature and oven chamber temperature, the faster the PCB board will get heat.
• Reflow oven chamber temperature
  Reflow oven chamber temperature is the hot air temperature. It may be related to the oven setup temperature directly; however, it is not the same as the value of set up point.
• Thermal resistance of heat transfer
  Every material has thermal resistance. Metals have less thermal resistance than non-metal materials, so the number of PCB layers and cooper thickness will affect heat transfer.
• PCB thermal capacitance
  PCB thermal capacitance impacts the thermal stability of the target board. It is also the key parameter in obtaining quality soldering. The PCB thickness and the components’ thermal capacitance will affect heat transfer.

The conclusion is:

Oven setup temperature is not exactly the same as PCB temperature. When you need to optimize the reflow profile, you need to analyze the board parameters such as board thickness, copper thickness, and components as well as be familiar with your reflow oven’s capability.

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• How to optimize the reflow profile?
• How to Maintain a Reflow Oven?
• Under what conditions do you set different temperatures for the TOP and Bottom heating elements of a reflow oven?