Aachen, Germany--Precision products such as semiconductors, sensors, or optical and medical system components may contain highly sensitive electronic elements that 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 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 oven joining can be 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.
Currently temperature-sensitive 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 that may have to be sterilized 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, and they are looking for a reliable low-temperature process.
Laser-based soldering with glass solder materials offers a suitable solution, subjecting the total component to only minimal thermal loading. Research scientists at the Fraunhofer ILT (www.ilt.fraunhofer.de) are currently developing the technique with a goal of introducing it into production.
In this joining method glass particle solder paste is first applied precisely to the cover of the component using a print mask. The solder is then pre-vitrified, to drive off the binders, in a kiln at a temperature of 350–500 °C. 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. The laser solder seam is very narrow, measuring just 300–500 μm, whereas glued seams have a width of several millimeters, a fact becoming increasingly important with the advancing miniaturization of precision components. Wide glued seams on OLEDs can be regarded as visual defects and on sensors used in implants they can change the entire component geometry detrimentally.
Environmentally the technique is attractive because it uses completely lead-free solder, which complies with requirements for the minimization of hazardous substances in electrical and electronic components. Process flexibility with regard to component size and shape, makes it suitable for industrial production. Where it can be used to seal microsystem components as well as joining large components measuring 200 x 200 mm2 and glass/silicon components can be hermetically connected to each other.
For more information contact Axel Bauer at Fraunhofer Institute for Laser Technology, Aachen, [email protected]