Full automated packaging of high-power diode laser bars

Abstract

Full automated packaging of high power diode laser bars on passive or micro channel heat sinks requires a high precision measurement and handling technology. The metallurgic structure of the solder and intrinsic stress of the laser bar are largely influenced by the conditions of the mounting process. To avoid thermal deterioration the tolerance for the overhang between laser bar and heat sink is about a few microns maximum. Due to an increase of growing applications and growing number of systems there is a need for automatic manufacturing not just for cost efficiency but also for yield and product quality reasons. In this paper we describe the demands on fully automated packaging, the realized design and finally the test results of bonded devices. The design of the automated bonding systems includes an air cushioned, 8 axes system on a granite frame. Each laser bar is picked up by a vacuum tool from a special tray or directly out of the gel pak. The reflow oven contains a ceramic heater with low thermal capacity and reaches a maximum of 400degC with a heating rate up to 100 K/s and a cooling rate up to 20 K/s. It is suitable for all common types of heat sinks and submounts which are fixed onto the heater by vacuum. The soldering process is performed under atmospheric pressure, during the oven is filled up with inert gas. Additionally, reactive gases can be used to proceed the reduction of the solder. Three high precision optical sensors for distance measurement detect the relative position of laser bar and heat sink. The high precision alignment uses a special algorithm for final positioning. For the alignment of the tilt and roll angles between the laser bar and the heat sink two optical distance sensors and the two goniometers below the oven are used. To detect the angular orientation of the heat sinks upper surface a downwards looking optical sensor system is used. The upwards pointing optical sensor mounted is used to measure the orientation of the laser bars lower side. These measurements provide the data needed to calculate the angles that the heat sink needs to be tilted and rolled by the two goniometers, in order to get its upper surface parallel to the lower surface of the laser bar. For the measurement of the laser bar overhang and yaw an optical distance sensor is mounted in front of the oven. Overhang and yaw are aligned by using high precision rotary and translation stages. A software tool calculates the displacement necessary to get a parallel orientation and a desired overhang of the laser bar relative to the heat sink. A post bonding accuracy of +/- 1 micron and of +/- 0,2 mrad respectively is achieved. To demonstrate the performance and reliability of the bonding system the bonded devices were characterized by tests like smile test, shear test, burn in test. The results will be presented as well as additional aspects of automated manufacturing like part identification and part tracking.