Solder interconnect degradation with irregular joint shape

Solderjoints of LEDs mounted by SMD reflow on Cu wire assemblies can exhibit irregular shapes that are very different from the interconnects of components on normal printed circuit boards. In order to understand the effect of the irregularities on the solder interconnect reliability and the intermetallic layer growth, the solder degradation was studied by setting up a series of accelerating tests and finite element modelling. In addition, the sensitivity of design parameters on the aging behavior was analyzed. It turns out that the cyclic fatigue of the solder interconnects due to thermal expansion mismatch is not as critical as the one from assemblies on traditional circuit boards. However, the decreasing interconnect strength due to the embrittlement of the intermetallic compound (IMC) is more critical and some optimization must be considered in the design.