Purpose - The purpose of this paper is to evaluate the effect of solder wettability on the thermal performance of a thermo-electric cooler (TEC) of a 980 nm pump laser module. Design/methodology/approach - In this paper, TEC thermal performance has been evaluated using a heat pump test. The results were compared with scanning acoustic microscopy (C-SAM) results in order to have a better understanding of the thermal behaviour of the TEC. In the C-SAM experiments, images were taken at the interfaces between the housing and TEC, as well as at the interfaces between the chip-on-carrier (CoC) and TEC. Findings - The heat pump test results indicate a strong correlation with the C-SAM test results. The C-SAM observations show good solder joint at the interface between the TEC and housing in the case of the device that yielded a good heat pump test result (11.5°C) and poor solder joints (gross de-lamination) at the interface between the TEC and housing in the case of the device that yielded a poor heat pump test result (24.4°C). The C-SAM observations did not show much difference at the interface between the CoC and TEC. The results from this study were used to qualify the post-vacuum soldered laser pump devices at JDS Uniphase, Plymouth, UK. Originality/value - The findings presented in this paper indicate that the level of solder wettability at the interfaces between the piece parts impacts the thermal performance of the TEC.