Our technology makes the difference

Metallux AG: Through innovative measures, long term stability and moisture resistance of pressure sensors can be increased. The new improved generation of pressure sensors benefits on the one side from results of modern measurements- and calculation tools as well as through marked improvements in the screen print technology and better algorithm during the laser trimming process. Metallux AG, a leading manufacturer of piezo resistive pressure sensors was thus able to improve the quality of their pressure sensors considerably.

Current situation:
Conventional manufacturing process of piezo resistive ceramic pressure sensors

Piezo resistive pressure sensors are manufactured through a screen print process. A Wheatstone bridge is printed onto a thin membrane and burned in at 850°C. After the firing process the offset of the bridge needs to be compensated. For this resistors are integrated in the bridge, which are aligned through laser trimming. These laser trimmed resistors are however a reason for instabilities. At the edge of the laser trims, micro cracks are forming, which under thermal- and / or mechanical stress are expanding, thereby leading to a change of the resistance value. Also the compensating resistors are integrated on the Wheatstone bridge. Even small deviations of the resistance values lead to a noticeable measurable change of the signal. Generally the compensation of the temperature alignment of the bridge (TC-0) is achieved with laser trimmed PCT resistors. Noticeable measurable instabilities are the results.

1. Objective: Long term stability to be improved significantly

Action: redesign of layout

With the redesigned layout, resistor compensation network is no longer trimmed with long analog cuts, as generally done. The network instead consists of a digital part, which through cutting of conductive tracks resistors are switched to the bridge. Only a small rest of the bridge is compensated by a short analog trim. This results in a network which, after laser compensation, consists of untrimmed –therefore very stable- resistors and only a small part of trimmed resistors. Therefore the number of micro cracks which occur over time and contribute to instability, are significantly reduced.

Optimise space requirement

A radial alignment of the resistors coupled with a reduction of the structural width, equalises the increased space requirement of the digital/analog resistor network. The thickness of the printed layers is continuously checked by optical thickness measurements, ensuring immediate pro-active action during a change of print-parameters. This enables the optimisation of the screen print process, resulting in markedly less variation of the untrimmed offset and the temperature coefficient. As an additional benefit, the number of trim-cuts decreases significantly.

2. Objective: optimise moisture resistance

Implementation: Double layered protective coating consisting of glass and polymer

Materials used in screen printing technique ( Ag, AgPd, Au, etc. ) tend to migrate with existing moisture and voltage potential. Should water be released ( e.g. condensation), onto the structure, high ohmic connections, can establish themselves on the printed boards. Such shortcuts are resulting in unavoidable offset shifts. A double layered protection coating improves the moisture protection of the sensor substantially. This is made up of a dense glass layer and an additional polymer coating By treating the glass layer before application of the polymer coating, a sealing effect of the glass layer is achieved. Thus the resulting diffusion barrier against moisture is considerable higher than the sum of moisture barrier effects of the two single layers. In combination with a lead-free, noclean solder a moisture- temperature test according to DIN EN 60068-2-30 shows only negligible chances.

Optimise glass solder and joining process

With flush mounted pressure sensors a thin membrane is glass-soldered to the massive ceramic main body. Adhesion of the membranes on the baseplate often presents a weak point. Many of the glass solders under thermal and / or mechanical stress tend to form cracks. The membranes remain firmly and reliably connected to the baseplate, however those tiniest cracks cause often more or less strong characteristically formed offset drifts which, especially with absolute pressure sensors are very noticeable.

Extensive investigations and series of measurements were required to determine a suitable glass solder as well as the appropriate optimal soldering parameters. Apart from the selection of the correct glass solder and an optimised temperature profile during the soldering process, also the distribution of tension on the joining area is responsible for the linearity and hysteresis. Finite-element-calculations (FEM) are ensuring to optimise the geometry of the base body especially in this area.

Result: quality improvement through better long term stability and moisture resistance

Extensive layout improvements reduce instabilities significantly. Well thought out sealing processes improve the moisture resistance. The sum of all described exercises together with the vast screen printing experience of our Metallux AG team members enabled us to considerably improve the quality of the new generation pressure sensors. The marginal offsetchanges during the 1.000 hr. test at 125°C underline the improved quality impressively.

About Metallux

Metallux is a high performance manufacturer of electronic components in Tick-Film Technology. The comprehensive product portfolio consists apart from Standard Pressure-,Linear-,Rotary- as well as Foil Sensors, High Voltage- and Power Resistors to custom specific complete solutions as well as Potentiometers and Joysticks. Customer orientated solutions and the right ideas result in satisfied customers.

Innovative products together with long term experience - numerous well known customers ranging from the automotive, electrostatic, medical, industrial and sensoric industry rely for more than 22 years on the company situated near Stuttgart, Germany.