Technology and Research of Oil Pressure Sensor

The pressure sensor developed by MEMS technology has small size, light weight, fast response, high sensitivity, easy mass production and low cost. The advantages, they have begun to gradually replace the pressure sensor based on traditional electromechanical technology. A variety of MEMS pressure sensors have been applied to automotive electronic systems, such as engine common rail pressure, oil pressure, manifold air intake pressure, and vehicle tire pressure. Among them, the Oil Pressure Sensor is an important sensor for measuring the oil pressure of the automobile engine, and its reliability is directly related to the safety of the automobile and the human. This paper selects MEMS pressure chip, successfully develops automobile engine oil pressure sensor, and studies the packaging process and reliability of oil pressure sensor. In the development process of the sensor, the sensor packaging and assembly process was systematically analyzed and tested strictly according to the quality requirements of automotive electronic products, and the reliability of the sensor was greatly improved by process optimization.

The MEMS pressure sensor utilizes the principle of piezoresistive effect, adopts integrated process technology to dope and diffuse, and forms a strain resistance along a specific crystal orientation on the single crystal silicon wafer to form a Wheatstone bridge, which utilizes the elastic mechanical properties of the silicon material. Anisotropic micromachining is performed on the same silicon material to form a diffused silicon sensor integrating force sensing and force-electric conversion detection. Usually, four polysilicon resistors are fabricated on the sensor chip, and the resistor is formed at the edge of the silicon film because at the edge of the film, when the film is subjected to a force, the strain-induced resistance change is maximized. The four piezoresistive resistors R1, R2, R3, and R4 form a Wheatstone bridge to form a pressure detection circuit. When the input voltage in the bridge is Vin, and the four piezoresistors on the diaphragm are equal (ie, R1=R3=R3=R4) =R), when the film is deformed by force, the two resistances become larger, the two resistances become smaller, and ΔR1=-ΔR2=ΔR3=-ΔR4=ΔR, then the output voltage Vout can be expressed as

Where Voffset is the output of the sensor at zero stress and zero strain. It can be seen from equation (1) that the piezoresistive pressure sensor has two working modes, one is a constant voltage working mode and the other is a constant current working mode.

One of the important packaging forms of MEMS pressure sensors is an oil-filled stainless steel structure called an oil-filled pressure-sensitive core. The basic manufacturing process includes patches, leads, package housings, oil-filled and secondary assembly. 1 is a schematic view showing the structure of an oil-filled pressure-sensitive core, and FIG. 2 is a second package sample of a pressure sensor.

3.1 chip mounting process

The sensor's placement process has a great impact on the performance of the sensor. Generally, it requires sufficient patch strength, as small a patch stress as possible, and can meet the operating temperature of the sensor. The patch materials used for pressure chips are mainly solder and glue, and different patch materials have great influence on sensor performance. Since the solder chip requires metallization on the back side of the chip, the process is relatively complicated, and the process of using the glue for the patch is simpler and the cost is lower, so the pressure sensor uses the patch glue process for the patch. Since the softness and hardness of the rubber have a great influence on the performance of the sensor after curing, the influence of the soft and hard rubber on the zero output of the pressure sensor was tested through experiments. For the same chip, no patch adhesive or soft patch adhesive (Young's die) was used. The amount is about 1 to 100 MPa, the glass transition temperature is lower than -40 °C), the hard patch rubber (Young's modulus is 3.56 GPa, the glass transition temperature is 85 °C), etc., in the range of -30 to 125 The zero output of the sensor was tested at °C. The test results are shown in Figure 3. The test results for the two sensor samples are given.

It can be seen from Figure 3 that the effect of the patch glue on the zero point of the sensor changes with temperature. At low temperatures, the zero point of the sensor using the hard rubber patch is significantly higher than that of the soft rubber and the non-glue. The rise in temperature is getting smaller and smaller. There are three main reasons for this: 1 The elastic modulus of the patch adhesive becomes smaller with the increase of temperature; 2 The patch adhesive is cured at a high temperature, which causes shrinkage residual stress at low temperature; 3 The thermal expansion coefficient of the patch adhesive and the chip material are different. Thermal stress. It is important to note that after 85 ° C, the effect of the hard gel suddenly becomes smaller, almost as small as in the absence of glue. This is because the glass transition temperature (Tg) of the hard rubber is 85 ° C, and the Young's modulus of the rubber is smaller than the Tg point, so that the influence of the zero drift of the sensor becomes small. Therefore, when using the patch adhesive, the Tg of the glue is required to be greater than the operating temperature of the sensor to ensure the stability of the sensor zero point and the reliability of the work.