Packaging and System Integration
For the proof of functionality as well as for the characterization of produced sensors and actuators, the IMPT studies, develops, and applies several aspects of AVT (packaging and system integration).
Generally, an electrical contact between a component (sensor or actuator) and a circuit board is created through ultrasonic wire bonding. An ultrasonic wire bonder is a welding tool that can produce discrete electrical connections with chips on conductor plate substrates. The components to be joined must exhibit sufficient surface contact (“pads” / landing spaces) and the wires that are usually used are 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 to form a permanent joint. For the ultrasonic wire bonding, a manual and a semi-automatic bonder are available at the IMPT. Here, 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 the two parts to be joined and the intermediate adhesive layer. The wetting of the to-be-joined surfaces plays an important role for the final bonding strength. In order to influence the wetting, the surfaces can be treated (processed or coated) or an adhesive with a suitable viscosity can be chosen. The adhesive yields internal strength (cohesion) through physical setting processes or through chemical reactions. This process is called curing. The activation energy for curing may be met with a certain degree of 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 used 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 is created through a filler metal (solder). 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 can be processed in many different ways, 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 oven (up to 800°C) is carried out.
Supported by temperature and electric fields, anodic bonding is a process where metals or semiconductors with ion-containing glass substrates are connected. By anodic bonding, very stable, hermetic, and cost-efficient joints can be created. However, the process poses very high requirements for the to-be-joined surfaces with regard to evenness, quality (low roughness), and cleanliness. 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 SiO2 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.