Temperature evaluation of solder joints for adjusting reflow profiles

In order to adjust the temperature profile of a reflow oven to a specific soldering task, thermocouples are placed on selected places on the printed circuit board. Since the thermocouples have a very small thermal mass and correspondingly short response times, compared to other types of temperature sensors, they are able to record the dynamic changes in temperature that occur during a reflow process. The goal is to monitor the rather rapid temperature changes of small components (e.g. CR, CC, LED) and the slower temperature changes at larger components (e.g. electrolytic capacitors, BGA). That way the temperature profile is optimised to securely solder all components within a limited total thermal load, to prevent damage of the components. In contrast to the established methodology to adjust temperature profiles the investigation presented in this paper focuses on measuring the solder temperature during the reflow process. A laboratory reflow oven is used, which is tailored for soldering lead-free solder prototypes and offers four channels for simultaneous temperature measurements through an integrated USB port, which also enables programmable control of the reflow profiles. In this work the temperatures of the solder are measured directly by making use of T-type thermocouples. Although having the comparative disadvantage of being less linear than their K-type counterparts, they have the advantage of being able to integrate better with solder, hence enabling a deeper study of specific solder temperatures, as opposed to merely the bulk component temperatures offered by K-type thermocouples.