Modeling of thermal phenomena in a high power diode laser package

High power solid state semiconductor laser arrays are widely used in today´s industrial, military and bio-medical systems, as well as in various material processing technologies (welding, cutting, surface treatment, etc.). The current analysis has been motivated by the following major factors associated with the design and use of high power semiconductor laser packages: (1) adequate functional (optical) and mechanical (structural, ldquophysicalrdquo) reliability is certainly a must for the successful operation of the packages in question; (2) elevated temperatures, thermal stresses, strains and displacements are the major contributor to both functional and structural (ldquophysicalrdquo) laser package failures; examples are: output spectrum broadening, increase in the near-field nonlinearity-ldquosmile effectrdquo, fatigue and brittle cohesive and adhesive cracking in the materials, and others; (3) predictive modeling, both numerical (finite-element-analysis (FEA) based) and analytical (ldquomathematicalrdquo), have proven themselves as an effective and cost-effective means to understand, analyze, predict and prevent thermal failures in packaging engineering in the most time- and cost-effective fashion. Accordingly, the objective of our study is to demonstrate the use of predictive modeling in the analysis of the thermal phenomena in, and design-for-reliability of, high power laser packages. We employ FEA-based simulations to analyze the steady-state and the transient thermal behavior of a conduction cooled packaged semiconductor laser operated in a continuous wave (CW) mode. We show, as an example of the application of analytical modeling approach, how one could assess the size of the inelastic zones in a low-yield-stress solder at the interface between the semiconductor laser die and a copper heat sink (ldquosub-mountrdquo). It is these zones that are responsible for the finite fatigue lifetime of the solder bond. It has been established that solder interfaces ar- e the weakest link, as far as laser package reliability is concerned. This is due primarily to the low yield point of, and, as the consequence of that, high level of inelastic strains in, the solder materials. The solder material works therefore in a low-cycle-fatigue mode. At the same time, owing to its low yield stress, the interfacial solder material is able to provide a reasonably effective strain buffer between the low expansion semiconductor and the high expansion substrate. This ldquobufferrdquo relieves the thermal stress in the semiconductor die, whose functional (optical) performance is absolutely crucial and cannot be compromised. The developed FEA and analytical models enable one to analyze the thermal phenomena in high power laser devices and conduction-cooled packages, as well as to predict and prevent thermal failures.