Thick film technology
Thick film circuits are widely used in the automotive industry, both in sensors, e.g. mixture of fuel/air, pressure sensors, engine and gearbox controls, sensor for releasing airbags, ignitors to airbags; common is that high reliability is required, often extended temperature range also along massive thermocycling of circuits without failure.
The manufacture of such devices is an additive process involving deposition of several successive layers of conductor, resistors and dielectric layers onto an electrically insulating substrate using a screen-printing process. A typical thick film process would consist of the following stages:
Lasering of substrates
Most used substrates are made of 96% Alumina Al2O3. Alumina is very hard and not very machinable, therefore lasering of the material is the most efficient way to machine it. The thick film process is also a process of minuazation where one substrates normally contain many units (final circuits), with the lasering it is possible to scribe, profile and drill holes. Scribing is a lasering process where a line of laser pulses are fire into the material and 30-50% of the material is removed, this weakens the substrate, after all other process are done to build the thick film circuit the substrates can easily be divided into single units. Profiling are for example used lot in the sensor, where a circuit need to fit round tubes or other different complex shapes. Drilling of holes, provide via between the two sides of the substrate, normally hole sizes are in the range 0.15-0.2 mm.
Lasering before processing the substrates has a cost advantage to lasering or dicing using diamond saw after processing.
Inks for electrodes, terminals, resistors, dielectric layers etc. are commonly prepared by mixing the metal or ceramic powders required with an organic vehicle to produce a paste for screen-printing. To achieve a homogeneous ink the mixed components of the ink may be passed through a three roll mill. Alternatively, ready made inks may be obtained from one of the many companies offering products for the thick film technologist.
Screen-printing is the process of transferring an ink through a patterned woven mesh screen or stencil using a squeegee.
After allowing a period of time after printing for settling of the ink to occur, each layer of ink that is deposited is usually dried at a moderately high temperature (50 to 200 °C) to evaporate the liquid component of the ink and fix the layer temporarily in position on the substrate so that it can be handled or stored before final processing. For inks based on polymers and some solder pastes that cure at these temperatures this may be the final step that is required. Some inks also require curing by exposure to UV light.
For many of the metal, ceramic and glass inks used in thick film processes a high temperature (usually greater than 300 °C) firing is required to fix the layers in position permanently on the substrate.
Laser trimming of resistors
After firing, the substrate resistors are trimmed to the correct value. This process is named Laser trimming. Many chip resistors are made using thick film technology. Large substrates are printed with resistors fired, divided into small chips and these are then terminated, so they can be soldered on the PCB board. With laser trimming two modes are used; either passive trimming, where each resistor is trimmed to a specific value and tolerance, or active trimming, where the feedback is used to adjust to a specific voltage, frequency or response by laser trimming the resistors on the circuit while powered up.
Mounting of capacitors semiconductors
The development of the SMD process actually evolves from the thick film process. Also mounting of naked dies (the actual silicon chip without encapsulation) and wire bonding is a standard process, this provides the basis for minituarization of the circuits as all the extra encapsulation is not necessary.
Separation of elements
This step is often necessary because many components are produced on one substrate at the same time. Thus, some means of separating the components from each other is required. This step may be achieved by wafer dicing.