Packaging and System Integration
As proof of the functionality as well as the characterization of produced sensors and actuators, the IMPT studies, develops and applies several aspects of the AVT.
Generally an electrical contact between a component (sensor or actuator) and a circuit board is produced by an ultrasonic wire bonding. An ultrasonic wire bonder is a welding technique for producing discrete electrical connections with chips on conductor plate substrates. The components to be joined must exhibit sufficient contact surface (“pads” / landing spaces), at which, the wires are mainly made of gold or aluminum and have a diameter of 10 μm. The wire is not melted, but rather reamed using pressure, heat and ultrasonic energy forming a permanent joint. For the ultrasonic wire bonding, a manual and semi-automatic bonder are available at the IMPT. For this process, Au or Al-wires are bonded on the Au, Cu, or Ni contact pads. Electrical contacts can also be produced by soldering.
Joining and Integrating
To connect micro components to each other, different techniques are used. At the IMPT, methods of gluing, soldering, and anodic bonding are employed. Gluing and soldering belong to the group of substance-to-substance joining methods.
An adhesive bond consists of two parts to be joined and the intermediate adhesive layer. The wetting of the joint surfaces plays an important role for the final bonding strength. In order to influence the wetting of the joining parts, the surface can be treated (processed or coated) or an adhesive with a suitable viscosity can be chosen. The adhesive yields internal strength (cohesion) by the physical setting process or by a chemical reaction. This process is called curing. The activation energy for the curing may be met with humidity, temperature or exposure to UV light. In addition to cyanoacrylate, epoxy resin or UV-curing adhesive types, different washable (easily removable) adhesives (Crystalbond™, Aquabond™, special waxes) are applied at the IMPT. Depending on the additive particles in the adhesive, one can create electrically-conductive or thermally-conductive bonds.
Soldering is a thermal process in which a substance-to-substance connection between two base materials with a filler metal (solder) is created. One can either join two parts with the solder (joint soldering) or create an electrical joint between two electronic components (soldering contacts). Clean surfaces of the joining components are required for soldering. When soldering, as the joining material (solder) fuses, the melting points of the base materials (joining components) will not be reached. The solder is processed in many different forms such as in the form of a paste, a bar, a preformed element, a rod or, in special cases, it can also be sputtered. Depending on the application, the solder is applied for the mechanical joining as well as for electrical contacting of micro components at the IMPT. For an extremely strong mechanical bond with a very thin adhesive layer, brazing of micro components with a glass layer in a high-temperature over (up to 800°C) is carried out.
Supported by temperature and electric fields, anodic bonding is a process for the joining of metals or semiconductors with ion-containing glass substrates. By anodic bonding, very stable, hermetic and cost-efficient joints can be created. However, the process faces very high requirements with regard to evenness, quality (low roughness) and cleanliness of the joining surfaces. With silicon micromechanics the construction of three-dimensional systems with integrated data processing is possible. At the IMPT, anodic bonding is used for bonding Si or Si02 chips depending on the application.
For years, the Institute of Micro Production Technology has been dealing with the function-oriented assembly of micro components used in research. In particular, dynamic systems of actuators require a precise and reliable guidance of active and passive parts to each other. In order to ensure the actuator function at a very small tolerance level, various handling concepts and self-alignment principles are developed and applied. The many years of experience of the IMPT with precision micro-machining of hard and brittle materials (Si, Al2O3-TiC, sapphire and SiC) were beneficial for the production of high-precision micro-assembly components. By using ultra-precise abrasive cutting (dicing), a wide variety of geometrically complex and high-precision micro components and profiles could be crafted. A profile accuracy would, in these cases, lie below 5 µm. An example for the self-aligned micro-assembly of actuators with a ball-bearing track, using 200 µm ruby balls, was established successfully on more than one occasion. In this case, an air gap of less than 8 µm between the active and passive part was made possible. A further development of this process is a system with a frictionless electromagnetic guide, which was constructed and tested in various designs. Before beginning any assembly, all requirement criteria, specifications and the choice of assembly type are selected and established.