With the push to Pb-free soldering there is an increased focus on profiling for solder pastes. Many people in assembly realize the importance of using this critical measurement when defining and monitoring a process.

What is often not known is the importance of profiling for adhesive curing. Looking at a typical data sheet for an underfill, surface mount adhesive, potting material, etc. — there are recommended cure conditions. This is the recommended cure time and temp for the adhesive in order to obtain optimum properties (the ones listed on the datasheet). This means the adhesive itself needs to see this time and temperature.

For example, an underfill has a recommended cure of 5 minutes at 150°C. This underfill is placed under a metal-lidded BGA on a 0.092” thick board (that contains a power plane and a ground plane) and then placed in an oven with 20 to 30 other boards of the same configuration, set at 150°C for 5 minutes. To the surprise of the manufacturer the underfill did not appear to have cured.

Subsequent to this, a profile was taken of the area under the components found that the underfill was attaining only up to 130°C as a peak. Using a standard profiler, we found that once loaded the boards needed to remain in the oven for 15 minutes in order for the underfill to attain 150°C for 5 minutes.

Therefore, it is important to perform thermal profiling for adhesives as well as for solder pastes in order to produce reliable parts.