Response Alignment
Temporal delays in temperature equalization between the heating air of a reflow oven and the high mass components on a printed circuit board must be quantified to achieve high quality solder joints. Through thermal lag calibration, process engineers establish the offset between the oven temperature and the actual board temperature during the heating cycle. This measurement prevents incomplete solder melting on large components while protecting smaller components from overheating.
The calibration results are used to adjust the conveyor belt speed and zone temperatures.
Profiling Routine
Measurement of the thermal delay is carried out using a profiling vehicle with thermocouples embedded under high-mass components like ball grid arrays or transformers. This thermal lag calibration measures the time difference between when the oven reaches a set point and when the component joint reaches the same temperature. High lag values require longer soak times or slower conveyor speeds to ensure the board heats evenly.
Low lag values permit faster throughput but require tight control to avoid thermal shock. Solder reflow profiles are optimized by adjusting these dwell times to match the calculated lag values.
Solder Uniformity
Failure to account for this thermal delay results in cold solder joints on large components or thermal damage to fragile sensors. This difference in heating rates can also cause tombstoning of small chip components due to unequal solder wetting forces. Dynamic adjustment of the oven profiles based on lag calculations prevents these defects and increases overall production yield.
The calibration must be repeated whenever a new board layout is introduced to the production line.
