Thermal profiling is an important part of the electronics manufacturing process. The following Tech Tips will provide some useful information to be used in the development of a profile.

There are several factors that affect the development of a thermal profile. With new materials and tighter temperature constraints, the profiling process has become more of a challenge for process engineers. Things to keep in mind are the type of substrate, the type of plating and the type of solder being used.

1) Thermal mass should be taken into consideration as the first factor in developing a thermal profile. Most profiling systems come with at least five thermal input locations available. This number of inputs allows one to maintain strict control by measuring a large mix of component types. More reliable yield data is attained by using this large mix. This will also provide the ability to read several types of components and control the process.

2) The component location and properties of the PWB assembly are important. If the components are on or near a large thermal plane or board edge, the thermal characteristics change significantly. PWBs with large thermal ground planes imbedded in the substrate act as heatsinks and absorb thermal energy from surrounding components (figure 4-1). This will result in insufficient reflow. As a result of the heat sink, you should notice the difference in the profile readout from one component to another.

3) Pay specific attention to the components on the edges of the board. These components will be riding on or near the chain in the reflow oven. The chain, although external to the board, will also act as a heat sink and draw heat away from the component. The components may reflow correctly in the center of the board, but may cause poor wetting on those near the board edges.

4) When attaching thermocouples to the board for reflow oven profiling, it is important to route them along the bottom of the board. This will reduce the risk of the thermocouples interfering with downward projected convection heat.

5) A unique challenge arises when dealing with Ball Grid Array (BGA) devices. The package body does not heat evenly due to the configuration of the solder balls, and the outer edge of the body will reach reflow before the center. Therefore, the thermocouple should be placed in the center of the BGA on the bottom side. This should ensure that the entire BGA reaches reflow.

6) When using ultra-thin thermocouple wire to monitor low profile devices, ensure that the uninsulated wire sections do not cross. This can be prevented by using kapton tape to hold the wires in place. The only point at which the two dissimilar metals should touch is at the weld bead. If the wires do touch you may get a false reading.

7) When mounting the thermocouples to a component such as a BGA, the weld bead should be attached using a high temperature solder. The thermocouple wires can be attached to the board using thermally conductive component epoxy.

8) Ensure ample time and temperature during the soak portion of the reflow cycle. Following the paste manufacturer's recommendations will ensure that the flux chemistry in the solder paste has sufficient time and temperature to chemically scrub oxidation from the solderable surfaces.

9) Time above liquidous state for eutectic solders that melt at 183oC may be reached between thirty and ninety seconds. This allows all of the solder spheres to form a good joint.

10) Keeping profile information from previous runs saved will allow you to judge an oven’s performance by comparing earlier runs to current profiles. If there is a significant change of more than ten degrees, you may want to adjust your profile accordingly.

These tech tips will provide the basics needed to accurately profile a given board. For additional information contact the EMPF helpline at 610-362-1320.