Editor's Note: This column originally appeared in the November 2012 issue of SMT Magazine.

In this day and age, the technologies and know-how going into PCB assembly are dramatically escalating and, for some assemblers, it’s difficult to keep up with those advancements. Take, for example, the thermal profile, one of the most important yet constantly changing elements in the PCB assembly process. Unfortunately, there’s still a belief in our industry that one size fits all when it comes to the thermal profile. We have to remember, for the sake of our OEMs and their products, that all thermal profiles are not created equal. 

However, the significance of a good thermal profile goes far beyond that fact. Many critical aspects of the thermal profile should be embraced by OEMs as a way to effectively track the reliability of their products. To start, let’s define a thermal profile. An effective thermal profile is created to make solder joints the perfect connections they need to be and to ensure they comply with associated solder paste manufacturer specifications, the types of components on the PCB, the type of PCB, and the thermal mass a particular board carries.

For each new assembly, thermocouples are attached to each PCB and then it’s run through the reflow oven to determine the hot and cold spots on the board. The temperature is measured to ensure the board is not overheated, which will damage the components, or too cold, to create perfect solder joints. 

A Keen Eye on Critical Aspects

At least a half-dozen key and critical aspects are associated with creating an effective thermal profile. These include:

• Reflow oven calibration;
• The number of zones in the reflow oven;
• The oven design itself;
• The thermal profiler;
• Whether or not it is a panelized or individual board;
• Component material mixture; and
• Thermocouple attachment to components.

Top-notch calibration of the reflow oven means complying with the specifications of the reflow oven manufacturer. Compliance also includes performing regular preventive maintenance on the reflow oven and keeping a disciplined record of maintenance.

The number of heating zones in a reflow oven differs. In a small tabletop reflow oven, there can be as few as four to six zones. In a full-size oven, there can be 12 to 16. As shown in Figure 1, generally, the more zones the better to permit incremental temperature increases or temperature rise. Thus, it’s always better to have 12 zones versus six to eight because the objective is to find the coolest spot on the board versus the hottest and assure all components are properly assembled.