Effect of passive components stand-off height on their solder joint mechanical strength

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Dr. Krammer Olivér
Department of Electronics Technology

The aim of the thesis work is to investigate the effect of surface mounted components stand-off height on the solder joint quality and mechanical strength.

An experiment was planned for examining the mechanical strength of the solder joints as a function of the height of the solder joint. A special test-board was designed with equipotential wires between the components soldering surfaces. By electroplating, the thickness of these wires can be increased selectively, so that they serve as a spacer for the placed components on them. Thus, the stand-off height under the components can be individually adjusted.

I designed the stencil for the solder printing, and performed the reflow soldering of the experimental components (0603 size SMD resistors). The solder thickness under the components was determined by cross-sectional examination on the minor part of the prepared samples. The half of other part of the samples were subjected to a 24-hour heat treatment for comparison, and then the shearing strength of both the treated and the untreated (as-reflowed) components was measured regarding the different cases of standoff height. By scanning electron microscope, the composition of the broken surfaces was examined, of which I drew conclusions to the location of fractures, and to the differences between the heat-treated, and the untreated cases.

As the results of my experiments, the occurrence of tombstone-defect increases by increasing the solder thickness. Furthermore, due to the higher spacer the soldered components often tilt, which significantly reduce the joints’ quality.

As the results of the measurement of the mechanical strength, by increasing the solder thickness (20 185 µm) the solder joints shear strength increases steadily (19,5 N 22,9 N). The obtained results averages occurred within the limits of deviation, thus the growing trend was sustained by Student’s t-test.

In addition, the effects of high-temperature-storage on the mechanical strength were examined. The result is that the shear strength of samples exposed to a short heat treatment was greater with 3 N than the original (not heat-treated) samples strength in cases of every stand-off heights.

As the results of the fracture surface investigations, the cracks usually were not formed in the IMC layer while measuring the shear strength, instead typically either a solder joint or the component metallization have broken. Among these two cases, the fracture surface formed in the solder joint was the most common.


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