Patent Application
20050103105
The present invention relates to an acceleration sensor system, which is able to be used especially for measuring accelerations having tight tolerances of the sensor parameters with respect to temperature and service life.
Acceleration sensor systems for low accelerations (low G acceleration sensors), which are used, for example, for measuring a braking deceleration or a slope inclination of a motor vehicle, are generally constructed in hybrid technology using hermetically sealed housings, in order to minimize environmental and temperature influences. However, such modules are generally not easy to combine using joining techniques suitable for mass production, such as printed circuit board assembly and press-in methods.
Such sensor systems have great sensitivity with regard to mechanical strains, since the sensor signals are generally read out capacitatively, and evaluated at high amplification. In cost-effective, printed circuit board-assembling assembly techniques, such as PLCC or SOIC, the sensor or the evaluation electronics is injection-molded around with plastic or molded. In sensors produced by microstructuring and integrated electronic circuits, because of different coefficients of expansion of the plastic and the semiconductor material of the sensors and the circuits, as well as on account of relaxation phenomena in the plastic, this leads to disadvantageous effects in the offset stability of the output voltage and the sensitivity. Furthermore, temperature variations, a nonlinear variation with respect to temperature, hystereses and long-term drift (particularly of the temperature variation) make their appearance. Moreover, the manufacturing tolerances of various samples, i.e. the different form of the effects, is considerable. That is why these cost-effectively employable construction techniques are hardly ever used in the case of acceleration sensors, especially low G acceleration sensors if there are safety-related requirements, e.g. in the motor vehicle field.
By comparison, the acceleration sensor system according to the present invention particularly has the advantage that a sensor system having good measuring properties is achievable, using low production expenditure and advantageously low costs. In this connection, especially good electrical characteristic data, which correspond to those of known ceramic-hybrid constructions may be achieved, together with a good assembling capability on printed circuit boards in standard assembly processes (SMD). Consequently, for example, the sensors in control units and separately built sensors may be combined on one printed circuit board.
The sensor system according to the present invention is also adaptable to the respective requirements for the electrical sensor characteristic quantities and environmental influences. In this connection, as a function of the requirements, a requisite electrical performance and robustness with respect to environmental influences may be achieved in that, based on the flexible construction within the premold housing, components may be added in order to respond to higher requirements or components may be omitted appropriately for cost reduction.
A surprisingly good stress decoupling is achieved by using an adhesive layer having a uniform thickness greater than 50 μm, preferably greater than 100 μm, a soft adhesive material being used which is, in particular, softer than the chip material of the acceleration sensor chip.
Furthermore, according to the present invention, the sensor element and the evaluation chip may be stress-decoupled, so that tight electrical tolerances with respect to offset and receptivity, especially also low temperature variations, small nonlinearities with respect to temperature, hystereses, long-term drifts and lower manufacturing tolerances of the electrical characteristic values may be achieved.
The assembling of printed circuit boards may be achieved using standard machines in control units or separately built sensors. According to the present invention, since the electrical wiring configuration takes shape first on the printed circuit board, various functions, such as dielectric strength, electromagnetic compatibility, sensing direction may be implemented by simple changes in the printed circuit board layout and possibly a different assembly program of the printed circuit board assembler, without one's having to resort, for this, to costly, inflexible and expensive hybrids and metal modules. Furthermore, a mechanical stress decoupling of the acceleration sensor chip and the evaluation chip may be achieved.
Advantageously, an acceleration sensor chip is used having service life-stable and temperature-stable sensor parameters, so that a system is created which is stable over a long period of time even at the high loads in the automotive field.
The influence of environmental influences may be reduced, alternatively, by individual passivating layers, for instance, made of gel, on the surfaces of the sensor chip, evaluation chip and of bonding connections, or by applying a large-surface passivating area which encompasses the sensor chip, the evaluation chip and the line bonds. Such passivating by a larger gel mass may be realized at relatively low production costs and great long term stability. In this connection, a gel may advantageously be used that is stable to temperature and service life.
According to the present invention, a sensor chip and an additional evaluation chip, such as an ASIC, or, alternatively an acceleration sensor chip having an integrated evaluation circuit may be situated in the housing.
The chip or chips may, on the one hand, be cost-effectively adhered directly in the housing. Alternatively, the chip or chips may be affixed in the housing by using an intermediate layer. As the intermediate layer, one may use a substrate made, for instance, of silicon, glass, ceramic or metal, possibly even several platelets, to which the chip or chips are adhered; moreover, instead of a substrate, a conductive or a non-conductive adhesive layer may be used. Using such an intermediate layer, one may achieve a combination of mechanical stresses, for example, by different thermal coefficients of expansion, and thus a great stability of the electrical characteristic values. In addition, the substrates of the evaluation circuit and of the sensor chip may be put at any electrical potential desired. This improves the EMV or the electromagnetic compatibility.
According to the present invention, the acceleration sensor chip and the evaluation chip developed, for example, as an ASIC may be applied over one another, so that a decoupling of mechanical stresses is possible.
A sensor system 1 has a premold housing 2, 3 having housing lower part 2 and cover 3, which are bonded to each other in a connecting region 4, for instance, by laser welding or by adhesion, and which surround a housing inner chamber 5.
According to
Chips 12, 13 are connected to each other and to lead frame 6 via line bonds 14. Chip surfaces 15, 16 of sensor chip 12 and evaluation chip 13 have been passivated using passivating layers 17 made of a gel. Furthermore, contact regions 19 of lead frame 6 are also provided together with line bonds 14 in housing inner chamber 5 with passivating layers 20 made of a gel.
In the specific embodiment of
In the specific embodiment of
Acceleration sensor chip 12 has elastic regions generated by microstructuring, e.g. vertical plates or reeds which are elastically deformed as a function of an acceleration or rotary speed acting on them, the measuring signal being read out capacitatively by evaluation chip 13.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10347418.8 | Oct 2003 | DE | national |
