Thermal processing of solder for electronics manufacturing takes place in a reflow oven. A reflow oven can be a small batch (box) style oven for very small lab scale operations. For larger manufacturers, an inline or conveyor belt reflow oven is the best choice. In the early days of electronics manufacturing, infrared (IR) heating technology was employed. Later on, the industry, led by companies like BTU International, switched to convection heat transfer. Convection reflow ovens had many advantages including vast improvements in heating uniformity. The most modern reflow ovens use closed-loop convection.
How are Reflow Ovens Configured?
Inline reflow ovens are configured with a number of heating zones followed by cooling section(s). Depending on the length and throughput of the oven there may be few zones, 8 or less, or many zones, 12 or more. During the reflow process the zones are programmed with a thermal set point. This set point corresponds to the temperature the circuit board should be exposed to as it passes through that zone. The program with the temperatures for all of the zones, as well as the belt
speed, is referred to as “the recipe”. The temperature that the board “sees” as it is processed through the oven is called “the profile”. The recipe is optimized to achieve the profile that the solder manufacturer has specified for the solder paste that is used on the board. A number of specifications are employed including a maximum and minimum peak temperature, Flux Activation Time (FAT), Time Above Liquidus (TAL), heating and cooling ramp rates, etc.
Reflow ovens can process circuit boards in air atmosphere or in a controlled Nitrogen or Forming Gas environment. The use of Nitrogen or Forming Gas is meant to reduce or eliminate oxidation of the assembly during the heating process. The Oxygen levels in a Nitrogen reflow process can be as low as less than 10ppm but may be higher than 500ppm for some less critical processes. Nitrogen capable reflow ovens are typically equipped with an automatic gas sampling system and an Oxygen Analyzer.