Hidden Solder Joints Inspection used in Surface-mount Technology

In this work, the capabilities of three (standard convection reflow, vacuum reflow and vapor phase reflow) soldering methods of surface mounting technology (SMT) are studied for high quality and reliability hidden solder joint formation. Comparative evaluations of the voids percent, which are formed in the hidden solder joints, were carried using X-ray inspection and testing the mechanical strength of the joints. To determine the contents of the hidden voids in the solder joints, X-ray inspection is used as the calculation of the area percentage occupied by the available voids is carried out by specialized software. Thermography measurements were also performed and a correlation has been established between the temperature distribution in the tested boards and the presence of voids within the hidden solder joints. A method for digital processing of infrared images to determine the percentage of voids in soldered joints is proposed. Three groups of 20 boards were studied and the percentage of voids within the hidden solder joints of power MOS transistors are evaluated. The use of thermography approach allows automate the control process of the voids percentage in hidden solder joints with a large area.