Research on Encapsulation of Fuel Vapor Pressure Sensor
Time:2022-12-12 Read:785
Reading guide: In order to realize the packaging of piezoresistive micro pressure sensors for automobile fuel vapor pressure detection, and cope with the use environment of oil contamination and vibration, reduce the impact of packaging structure stress on the pressure sensor chip, the packaging method of using the chip back pressure bearing, integrated with the circuit on a ceramic substrate, and assembled with the plastic shell to form a complete sensor is adopted.

Introduction

In the automotive field, due to the extremely high volatility of the fuel, the steam generated by fuel is one of the main sources of environmental pollutants. With the increasingly strict laws and regulations prohibiting the emission of fuel vapor, it is more important to use the fuel evaporation control system to monitor and treat the fuel vapor. The pressure sensor that can detect the micro range pressure is a key component of the system. 

By monitoring the pressure difference between the fuel tank and the system and the atmosphere, the opening and closing of the corresponding valves of the system can be controlled, and at the same time, whether the vapor leaks can be monitored. The pressure threshold value detected by the fuel vapor pressure sensor is usually less than 5 kPa. 

Therefore, the sensor used for fuel vapor pressure detection should first have high sensitivity, be able to accurately detect micro pressure, and then be able to deal with fuel vapor medium, so as to achieve reliable operation in a bad environment. In the related research, the performance parameters of the sensor chip continue to improve to meet the needs of micro pressure measurement, but if you want to play the role of the chip, you need to design the packaging of the chip. In the packaging process, we must consider the impact of packaging components on the pressure-sensitive chip. The packaging stress generated by the packaging components seriously affects the reliability of the sensor, which is one of the key difficulties of the sensor packaging technology. 

As fuel vapor pressure detection belongs to micro-pressure measurement, the sensor chip needs to have higher sensitivity. Very small interference will have a great impact on the output signal of the sensor, such as temperature change, vibration, stress and external force deformation, electromagnetic interference, etc. Reducing or avoiding the interference of these factors while protecting the chip from reliable operation in harsh environments is the key to packaging design.

Aiming at the automobile fuel vapor pressure detection environment, according to the customer's requirements, the pressure detection range is - 3.75 ~ 1.25 kPa, and the output error is less than ± 3.5% FS, the micro pressure sensor packaging is designed. The impact of packaging elements on the sensor output is analyzed by finite element simulation. Based on this, the packaging structure and process parameters are optimized. By integrating the pressure-sensitive chip and the compensation amplifier circuit on a PCB substrate, A complete fuel vapor pressure sensor is packaged together with the designed sensor housing, which has been tested to meet the use requirements. This packaging method reduces the size and packaging cost of the sensor and meets the vehicle standard.

Packaging design and simulation

The core of the pressure sensing chip as the pressure sensing device of the fuel vapor pressure sensor is the Wheatstone bridge composed of four equivalent semiconductor resistors. When the external pressure acts on the sensor's pressure-sensitive diaphragm, the stress in the area of the pressure-sensitive resistor will change, resulting in a change in the resistance value of the pressure-sensitive resistor and the output voltage signal. The sensor output is directly related to the stress in the varistor area, so the package stress and package structure deformation should be minimized in the package design process.

Due to the need to work in an environment of oil contamination, vibration, automobile level ambient temperature, and micro pressure, the packaging of fuel vapor pressure sensor should meet the following conditions: sealing protection, chip-sensitive film device layer should not direct contact with oil contamination, water vapor, etc; The thermal expansion coefficient of each component of the packaging structure shall be close to reducing the thermal stress caused by the mismatch of thermal expansion coefficient; The packaging structure should have good vibration resistance to meet the reliability under the vibration environment. At the same time, the cost of automotive electronics should be strictly controlled.

The designed packaging structure of the fuel vapor pressure sensors mainly includes pressure-sensitive chips, ceramic substrates, sensor shells, etc. In order to prevent the fuel vapor from damaging the structural layer on the front of the chip, the back of the chip is used to contact the fuel vapor for pressure bearing. At the same time, the air on the front of the chip is isolated from water vapor through a waterproof and breathable membrane. 

The pressure-sensitive chip is directly pasted on the ceramic circuit board to form a complete sensor output circuit with the peripheral circuit, and the ceramic circuit board is stuck to the shell through the bracket with sealant to form a sealing chamber and strengthen the local stiffness of the circuit board, so as to prevent excessive deformation at the circuit board where the chip is bonded due to vibration, thus affecting the chip output. Because of the packaging method that the chip is directly bonded to the substrate, the material of the substrate and the process parameters of the sensor chip is the most important for the chip. The thermal expansion coefficient of the chip and the substrate should be close, and only the ceramic circuit board meets the requirements.

Thermodynamic analysis

The shell material and the thickness of the ceramic circuit board are the key factors affecting the packaging stress. Common plastic shell materials include PA6, ABS, etc. In order to measure its influence on the output of the designed fuel vapor pressure sensor, a finite element model of the sensor was established. The transient thermal structure was analyzed using ANSYS, and the cyclic temperature load was applied. The initial temperature was set at 22 ℃, and it returned to 22 ℃ after three temperature cycles. The stress distribution of the sensor and chip under alternating temperature load is obtained. The smaller the thermal stress is, the better the packaging reliability is.

In the stress distribution program of the sensor, the stress is mainly concentrated on the shell and the circuit board, the chip surface stress is small, and the packaging structure plays a certain role in protection. The relationship between the maximum stress on the chip surface of different simulation groups and time is extracted. It can be seen that the sensor housing made of PA6 material has less stress effect on the chip under thermal cycle load than ABS material. At the same time, as the thickness of the ceramic PCB increases, the chip surface stress decreases gradually, which may be because the thicker PCB insulates the stress transmission between the shell and other packaging components. In the six groups of simulations, the maximum stress value of the chip surface in group 3 is the minimum (1.32 MPa) under the alternating temperature load, and the residual stress is 0.0052 MPa. After comprehensive consideration, the shell made of PA6-GF35 material and a thickness of 1200 are finally selected μ M PCB substrate.


Vibration analysis

The vibration environment during the vehicle driving process will cause the vibration of the sensor structure, which will further cause the pressure-sensitive chip to bear the dynamic acceleration load and affect the sensor output. The dynamic performance of sensors with different chip structures is also different. Generally, the structure with higher first-order modal frequency has better dynamic performance. In this paper, the chip of flat membrane structure and island membrane structure has been modal analyzed, and the first-order modal frequencies are 59 kHz and 35.3 kHz respectively. On this basis, the harmonic response analysis of the mode superposition method is used to apply acceleration loads of different frequencies to the chip. Since the acceleration load perpendicular to the membrane has the greatest impact on the chip output, the direction of the load applied in the simulation is perpendicular to the membrane, and the size is 10g.

When the vibration frequency is near the first mode frequency of the chip, the chip surface stress will increase significantly due to resonance. The maximum stress of the flat membrane structure and island membrane structure is 0.47 MPa and 3.1 MPa respectively, and the output effect of the island membrane structure under vibration load is much greater than that of other structures. Therefore, in order to deal with the vibration environment, the fuel vapor pressure sensor should avoid the use of the central mass block structure similar to the island membrane in the chip selection, and can choose the front beam membrane island structure or flat membrane structure chip.

Chip stacking process

As a key material used in the packaging process, the chip adhesive can directly bond the chip with the substrate. However, due to the difference in thermal expansion coefficient between different materials and the residual stress and thermal stress generated by the patch adhesive during the temperature change process, its performance parameters and usage will affect the output of the chip.

Elastic modulus

A three-layer system was designed by using ANSYS simulation. A batch of paster with the elastic modulus of 50-400 MPa was selected, and a temperature load from 150 ℃ to 22 ℃ was applied. The relationship between the maximum stress on the chip surface and the elastic modulus of the paster was obtained. It can be seen that the maximum stress on the chip surface increases with the increase of the elastic modulus of the poster.

Glue thickness

In the process of chip placement, in addition to the elastic modulus of the adhesive, the thickness of the adhesive is also one of the key factors affecting the chip surface stress. The maximum stress on the surface of the pressure-sensitive chip under the different thicknesses of the patch adhesive is obtained by applying a temperature load that increases from room temperature to 125 ℃. The thermal stress of the chip decreases first and then increases with the increase of the thickness of the patch adhesive. This may be because when the patch adhesive is thin, the stress generated by the expansion of the substrate can be absorbed with the increase of the thickness of the adhesive, thereby reducing the thermal stress transmitted to the chip. However, when the patch adhesive reaches a certain thickness, Its excessive volume expansion will affect the chip.

At the same time, the thickness of the patch adhesive will also affect the dynamic performance of the sensor, 50, 100, 150, 200 μ The first-order modal frequencies of the chip with m glue thickness are 81.7, 59.2, 47.0, and 40.3 kHz respectively. Through the simulation, it can be seen that the thicker the patch adhesive is, the lower its first-order modal frequency is, and the influence of vibration on the sensitive diaphragm of the sensor can be amplified. In addition, the high thickness of the paster will allow greater chip displacement, which may seriously affect the connection between the bonding line and the chip.

Therefore, under the condition of ensuring the chip bonding strength, the patch adhesive should choose the soft adhesive with a small elastic modulus, and the adhesive layer should not be too thick. Generally, chips can be bonded with glass to increase the bonding area and improve the bonding strength. In order to reduce the cost, chips are directly bonded. Although the adhesive strength of the soft adhesive is low and the way of pressure bearing on the back of the chip puts forward higher requirements for the adhesive strength, because the fuel vapor pressure is a small pressure, the use of soft adhesive can meet the requirements for adhesive strength, and will not lead to overloading and failure of the patch adhesive. At the same time, in order to face the oil pollution environment, considering comprehensively, fluorosilicone with oil resistance is selected as the patch adhesive in this paper, and the adhesive thickness is 100 μ m.

Test results

According to the above packaging design, the pressure chip is directly pasted on the ceramic PCB to form a complete sensor circuit substrate with the temperature compensation chip and peripheral circuits. Fix the circuit board with the housing through sealant to ensure air tightness.

The calibration data is collected, the correction coefficient is calculated, and stored in the EEPROM of the compensation chip to calibrate the output signal of the sensor. Place the calibrated sensor in the constant temperature box for the performance test, use a constant voltage source to provide a 5V power supply voltage, set the initial temperature as - 10 ℃, boost the pressure from - 3.75 kPa to 1.25 kPa, 1 kPa per step, record after the data is stable, change the temperature, and repeat the steps to test the sensor output at different temperatures. It can be seen that the test samples can be output proportionally in the whole temperature zone and the output is basically coincident, with good linearity and low-temperature drift. The maximum error in the whole temperature zone is about 1.2% FS, which can meet the use requirements.

Conclusion

In this paper, a micro-pressure chip packaging method is proposed for automobile fuel vapor pressure detection. The back of the chip is used to bear pressure. The pressure-sensitive chip and temperature compensation chip are directly pasted on the ceramic PCB to form a complete fuel vapor pressure sensor with the shell. The influence of key packaging parameters on the pressure-sensitive chip was analyzed through thermodynamic and dynamic simulation. It was found that the residual stress of the chip caused by the paster with lower elastic modulus was smaller, and the thin adhesive thickness would have better dynamic performance. The reliability of the packaging structure was verified, and the sensor samples after fabrication and calibration were tested to meet the use requirements.

Compared with traditional chip packaging, this packaging method uses the chip back pressure bearing to avoid the damage caused by the device layer on the chip surface contacting the fuel volatile gas. The chip is integrated on a circuit board with the peripheral circuit and the compensation chip by pasting, without the need for bonding glass, which reduces the size and significantly reduces the packaging cost. At the same time, using finite element simulation software to assist packaging design can optimize packaging structure parameters, and reduce workload and design costs.