Thick film technology
Thick film technology is used to produce electronic devices such as surface mount devices, hybrid integrated circuits and sensors.
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:
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 miniaturization 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.
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