Laser beam soldering (LBS) is a non-standard manufacturing process for electronic packaging and interconnection technology today. Due to the actual trend towards complex and cost intensive products, LBS gains more attention for certain applications in this field. For mass production in automotive applications a fully automated and temperature controlled LBS process was developed. The achieved results are discussed with respect to quality, reliability and process efficiency and compared to established micro flame (hydrogen) soldering technology. The development of the LBS process is presented. The process window is optimized using High Speed Video Imaging. Temperature signals are logged by means of pyrometry. The processed parts are evaluated with metallographical assessment of solder joint quality. Especially cross sections reveal the fine grained structure and the shape of the meniscus of the solder joints. The reliability is proven using shear strength tests and thermally induced strain cycles. Conclusively, LBS is a stable, reproducible process for applications requiring controlled and locally restricted heat input. The thermal and mechanical stress is reduced in comparison to conventional techniques.
The increasing demands in MEMS fabrication in the past led to new requirements in production technology for these devices. Especially the packaging and assembly of optical devices like high power diode lasers require high accuracy in positioning and high reproducibility in combination with low production costs. Conventional assembly technology and mechanical adjustment methods are time consuming and exspensive. Therefore a need of new assembly procedures arose. Each component of the system has to be positioned and fixed. The equipment for manipulating is very expensive. Also adjustment of the parts after joining requires additional mechanical devices that need to be accessible after joining. The decreasing dimensions of the microsystems cause problems in assembly and increase dramatically the tolerance conditions
The laser beam micro forming as a non-contact tool offers the possibility of active adjusment after the assembly is completed.
This paper describes this new technology pointing out the used mechanisms of laser induces deformation:
From the basic mechanisms dedicated structures are derived to achieve the desired degrees of freedom for the adjustment process. Based on the upsetting mechanism the positioning of a cylindrical lens in front of a high power diode laser is shown. The adjustment in two degrees of freedom can be realized by irradiating the actuating structure from one direction. The achieved accuracy is in the micron range.
The knowledge transfer to a tube-like actuator will be demonstrated. The tilting of a mirror in two rotational movements will be shown.
The results to be presented have partially been achieved within the Collaborative Research Center (SFB) 440 "Assembly of Hybrid Microsystems" which was financially supported by the Deutsche Forschungsgemeinschaft DFG.
Today laser beam soldering (LBS) is a non-standard manufacturing process for electronic packaging and interconnection technology. However due to the onwardly going miniaturization and the world-wide trend towards lead-free solders LBS gains more attention for certain applications in this field. In mass-production, e.g. in interconnection technology of electronic devices, it is state-of-the-art to use solder alloys in the form of paste. Today it is a well- known process to handle or to deposit solder paste. Furthermore the composition of solder systems, consisting of metal alloys and fluxes, can be optimized for individual products and production conditions. Yet this optimization process is only in part performed for LBS with solder paste. Therefore the application of standard reflow solder pastes for LBS poses problems like gas eruptions, solder balling and overheating. In order to overcome these quality problems an adapted time-power-profile for LBS with solder paste has been developed, using synchronized high-speed photography and detection of secondary emissions from the joining area. The evaluation of the experiments allows the generation of high quality solder joints with standard diode laser systems and solder pastes. In addition it is possible to realize an online-process control via detecting the secondary emissions from the subsequent transformation stages of the solder paste. Conclusively it can be said LBS is a stable, reproducible process for applications requiring a controlled locally limited heat input and reduced thermal and mechanical stress compared to conventional techniques.
The production of microsystems and miniaturized devices often requires joining technologies, which meet the demands of ultra clean manufacturing. Especially for optical and medical products low pollution and distortion joining processes are necessary to guarantee the quality and the function of the device. For this applications laser welding with fiber lasers at laser powers of up to P = 10 W and focus dimensions < 30 micrometers have been used for welding micro mechanical devices. At intensities I > 10<SUP>6</SUP>W/cm<SUP>2</SUP> welding depths of 100 micrometers can be achieved with minimized pollution of the parts and smooth and clean appearance of the surface of the welds. For joining polymers and dissimilar materials high power diode lasers have been used providing even better conditions regarding pollution of the joining partners. By using material adapted laser wavelengths the heating of the material can be concentrated to the inner joining area of an overlap material joint. With this technique, thermoplastic polymer compounds and silicon glass compounds have been joined with low temperature and no influence on the quality of the parts with joining widths of less that 100 micrometers .