May_EDFA_Digital

edfas.org 23 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 2 The stack is made of seven chips slightly shifted in emis- sion wavelength allowing the operability of the LIBS over the wide requested temperature range. Each chip, commonly called a laser bar, is made of 62 single emit- ters and supplies a total instantaneous optical power of 260 W, which makes up 1800 W of optical power at the stack level. Themaximumpulsed current exceeding 225 A stresses the component with respect to both electrical and thermal aspects. These two points may negatively affect the component’s reliability and were investigated during the construction analysis. CONSTRUCTION ANALYSIS In describing the critical factors of the laser diode stack, the design focus is on an efficient electrical path through the laser bars and on the heat dissipation toward the stack case. The seven AsGa chips are assembled on eight thermal dissipators. This stack is then soldered on an AlN submount substrate and connected by two large electrodes soldered on alumina substrates. The total assembly process is highly complex and requires six dif- ferent soldering alloys and requires six different sets of process parameters (Fig. 3). Such a complex process flow requires perfect control of each step. Due to its criticality, the design and assembly process were both scrutinized. Excessive growth of intermetallics due to unmatched tem- peratures, a high rate of porosity, poor melting of solder micro balls, and low wetting were some of the potential defects that were searched for in every solder type of the stack assembly. After visual external inspection, a microsection of the component was prepared, followed by SEM/EDX analysis to detect any process deviation that could endanger reliability. The technological analysis revealed a very mature manufacturing process, as none of the expected defects were observed except for a few non-melted micro balls. However, aligned porosities were found in the chip/ heat dissipator solders of the crosscut sample (Fig. 5). This aligned pattern is the result of the material deposition Fig. 2 Tilted and top views of the laser diode stack. Fig. 3 SEM view of the six solder types.