Hermetic packaging poses particular challenges for the production technology, which can no longer be met by conventional methods such as gluing and soldering. The Fraunhofer Institute for Laser Technology ILT has developed an innovative packaging process for microcomponents and electronic parts based on laser glass soldering, which is suitable for use in mass production and fulfils the stringent environmental regulations of the EU's RoHS Directive.
Precision products such as semiconductors, sensors or optical and medical system components contain highly sensitive electronic elements. In most cases they must not come into contact with water, oxygen and other elements and therefore have to be hermetically sealed. Gas-tight packaging of the complex interior poses a great challenge for the joining process in microcomponents.
High-temperature processes such as anodic bonding and glass frit bonding are widely used methods for hermetically sealing components made of silicon and glass. The heat needed for joining is introduced into the component by a kiln process at temperatures of 300 to 600°C. As the most temperature sensitive component determines the maximum temperature of the entire system, these two processes cannot be used for temperature-sensitive functional elements. They are, for example, unsuitable for encapsulating OLEDs because the functional organic layers would be destroyed at a temperature of even 100 °C.
At present temperature-labile components are usually glued, but long-time tests on semiconductors and OLEDs have shown that the durability of the glued connection is limited. Oxygen and moisture gradually penetrate the interior of the component and affect its function. The limited durability and the temperature sensitivity of glued connections are a problem, especially for components used in the medical sector, as they cannot withstand, for example, sterilization processes in autoclaves. Electronic components such as sensors in implants can often only be replaced by performing a surgical operation on the patient. The manufacturers of these and other precision components are therefore seeking a way of prolonging the durability of their products. As high-temperature and gluing processes do not meet the requirements for joining microelectronic components to various materials, manufacturers are looking for a reliable low-temperature process.
Laser-based soldering with glass solder materials offers a suitable solution. This is a relatively new joining technique which subjects the total component to only minimal thermal loading. Research scientists at the Fraunhofer ILT are currently developing the technique with the aim of introducing it soon into series production. In this joining method the solder consisting of a glass particle paste is first applied precisely to the cover of the component using a print mask. The solder is then pre-vitrified in a kiln at a temperature of 350 - 500 °C depending on the type of glass paste used, so that the binders in the paste evaporate. After the solder has cooled the electronic component is joined to the cover. A defined and locally limited temperature increase is achieved by scanning the solder seam with a laser beam. The rest of the component is not affected by this application of heat. Owing to the high scanning speed of up to 10,000 mm per second, the joining process is quasi-simultaneously controlled. The entire solder contour is evenly heated, the cover can sink into the liquid solder bath and is thus hermetically connected to the component. Compared with gluing, the laser-based method achieves a considerable increase in the durability of the entire microcomponent, and the permeability of liquids and gases is practically zero. What's more, the solder seam is completely free of bubbles and cracks. For the medical sector in particular this means a significant increase in safety. "A further advantage of laser-based glass soldering is that the solder seam is very narrow, measuring just 300-500 µm, whereas glued seams have a width of several millimeters," explains Heidrun Kind, project manager at the Fraunhofer ILT. "This fact becomes increasingly important with the advancing miniaturization of precision components. Wide glued seams on OLEDs for example are regarded as visual defects. On sensors used in implants they can change the entire component geometry detrimentally. In environmental terms, too, the technique has a bright future. We are now able to use completely lead-free solder, which means that our method meets the requirements of the EU's RoHS Directive for the minimization of hazardous substances in electrical and electronic components."
Thanks to the maximum flexibility provided with regard to component size and shape, the process is highly suitable for industrial series production. It can be used to seal microsystem components as well as to join large components measuring 200 x 200 mm2. In addition to glass/glass components, substrates with MAM or ITO layers as well as glass/silicon components can be hermetically connected to each other.
Axel Bauer | Fraunhofer Gesellschaft
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