What is a Thermal Profile?
The thermal profile is an critical consideration in the solder reflow process. A thermal profile is a measurement of the temperature, of the product, during thermal processing. The recipe is the temperature settings of the reflow oven in each zone. The temperature settings combined with the belt speed, and the convection rate settings, will produce the thermal profile on the board or product. The difference between the reflow oven set points, or recipe, and the thermal profile is governed by the laws of heat transfer and is affected by mass (weight of the product), surface area, thickness, and the heat capacity (the ability of a material to absorb or give up heat).
How to Profile a Solder Reflow Oven
A solder reflow oven is profiled by attaching thermocouples (TC’s) to the board that is being processed. These thermocouples are attached using solder or epoxy. Aluminum or Kapton tape can be used to attach thermocouples, but is not suggested due to trapped air and loosening of the tape’s adhesive. Best practices for attaching thermocouples also suggest using strain relief to prevent the TC’s from being pulled off the board when the wires are tugged. Placing multiple thermocouples in differing locations and on different types of components on the board will give you the most comprehensive understanding of the thermal profile. The attached thermocouples can be measured by trailing the wires through the oven while processing or using a recording device that can be passed through the reflow oven by use of an insulating cover. The profiles can be measured in a loaded or unloaded state by either processing boards ahead of and behind the thermocoupled board to simulate the load or by sending the thermocouple board through alone for an unloaded test.
The measured reflow oven profile is often displayed visually, as a graph, but the raw temperature vs. time data can also be downloaded as a table and can be manipulated by MS Excel or other statistical programs. The reflow oven profile will be compared to solder specifications and board/component limitations including: ramp rates, peak temperature, TAL (time above liquidus) and other factors.
Controlling the Solder Temperature Profile
Solder reflow ovens control the temperature profile by adjustments to the zone temperature set points and the belt speed. Modern reflow ovens used for mass production also have the ability to control convection rate – the speed at which the hot gases hit the product. In a convection solder reflow oven convection is the dominant component of heat transfer. Convection rates are adjusted by setting the pressure in the heated air plenum in each zone. The pressure is adjusted by the control of the fan speed in each heated plenum. Many reflow ovens employ automated recipe generator software tools to facilitate the development of oven recipes. If using a reflow oven recipe generator check that it also considers the convection rate.