How to Use a Reflow Oven

Reflow ovens are used during Surface Mount Technology (SMT) manufacturing or in Semiconductor Packaging processes. Typically, the reflow oven is part of an electronics assembly manufacturing line and is preceded by printing and placement machines. The printing machine prints solder paste on the board and the placement machine places components onto the printed solder paste.

Setting Up a Reflow Oven

Setting up a reflow oven requires knowledge of the solder paste being used in the assembly. Does the paste require a Nitrogen (low Oxygen) atmosphere during heating? What are the reflow specifications including peak temperature, Time Above Liquidus (TAL), etc? Once these process characteristics are known the Process Engineer can endeavor to set up the reflow oven recipe with the goal of achieving a certain reflow profile. The reflow oven recipe refers to the oven settings, including the zone temperatures, convection rates and gas flow rates. The reflow profile is the temperature that the board “sees” during the reflow process. A number of factors need to be considered when developing a reflow process. How big/massive is the board? Are there very small components on the board that could be dislodged by high convection rates? What is the maximum component temperature limit?  Will fast ramp rates be problematic? What is the desired profile shape (traditional cash register or a straight ramp)?

Reflow Oven Features and Characteristics

Many reflow ovens feature automatic recipe set up software allowing the oven to create a starting point recipe based on the board characteristics and solder paste specification. This starting point recipe can be further refined to center the profile in the process window by profiling the oven using a thermal recorder or trailing thermocouple wires. Oven set points can be adjusted up/down based on the actual thermal profile versus the solder paste specification and the board/component temperature limits. If automatic recipe set up is not available, the process engineer can use a default reflow profile and adjust the recipe to center the process through profiling. Automatic set-up usually provides a better starting point and reduces the iterations/adjustments required.

Once a centered profile has been achieved the process can be further qualified by running multiple profiles and calculating a process Cpk. This Cpk value will allow the reflow oven process engineer to determine whether the process is centered and repeatable and thus ready for production.