PCB board failure mechanisms of environmental tests

- Aug 05, 2016-

PCB board temperature and humidity test

Comprehensive test chamber by the cooling system, heating system, control system, system humidity, air circulation system and sensor system components, system block diagram shown below, the system belong to two major aspects of the electrical and mechanical refrigeration. Following is a brief working principle and process of several major systems.

1 refrigeration system is a key part of an integrated system of the chamber. In general, the refrigeration chamber comprehensive way all mechanical refrigeration and the auxiliary liquid nitrogen cooling. Using mechanical refrigeration vapor compression refrigeration, the cooling mode is but one of a wide range of economic cooling in the way of artificial refrigeration applications.

A vapor compression refrigeration types are: (1) single-stage refrigeration; (2) multi-stage refrigeration; (3) combined cooling. A vapor compression refrigeration system consists of a compressor, a condenser, throttle mechanism and evaporator components. Since the temperature to reach the low temperature test at -50 ℃, single-stage refrigeration is difficult to meet the requirements, so the comprehensive test chamber refrigeration generally use combined cooling.

2 heating system in terms of comprehensive test chamber heating system is relatively refrigeration system is relatively simple. It mainly consists of high-power resistance wire. Since the heating rate required comprehensive test chamber is large (generally require increased 2 ℃ / min), and therefore the heating chamber integrated power systems are relatively large.

3 control system controls the core part of the comprehensive test chamber, which determines an important indicator of heating and cooling rates, accuracy test chamber. Most of the existing comprehensive test chamber controller with PID control, but also a small part of PID and fuzzy control combining control.

4 humidity systems humidity system is divided into two sub-humidification and dehumidification.

Integrated humidification chamber generally use wet and steam, that the test chamber with a resistance wire to heat water directly, so that the water vapor to humidification. This humidification humidification ability, speed, humidity control sensitive, especially when cooling easy to implement mandatory humidification. However, care must be taken to keep the test chamber clean, or likely to cause pollution of the humidifying tray. Comprehensive chamber desiccant dehumidification methods are two kinds of mechanical cooling and dehumidifying dryer:

(1) Principle of mechanical refrigeration dehumidification dehumidified air is cooled to a temperature below the dew point, so that the moisture content is greater than the saturated water vapor condensation precipitation, thus lowering the humidity.

(2) the use of a dehumidifying dryer to pump air out of the test chamber, and the dried air injection, while the humid air is recycled into a drier, and after drying into the test chamber, so repeated dehumidification cycles.

Now most comprehensive test chamber using the former method of dehumidification. The latter method of dehumidification, the dew point temperature can achieve the following 0 ℃, applicable to the special requirements of the occasion, but expensive.

5 sensor system integrated in the sensor chamber is mainly temperature and humidity sensors. Temperature Sensor platinum resistance is more, there is also a thermocouple. Humidity measurements are psychrometer method and solid-state electronic sensors directly measure two kinds of methods. Wherein the dry and wet bulb measurement accuracy is not high, and now the comprehensive test chamber is gradually replaced by a solid state sensor for measuring the wet and dry bulb humidity.

6 air circulation system air circulation system in general by the centrifugal fan and drive motor constitutes its operation, which has a fixed speed and automatic adjustment of the two speeds. If you want to adjust the temperature of the test chamber fixed speed, by adjusting the heating resistor wire to control, it can control the relative automatic fan control is relatively simple.

7 temperature and humidity control temperature and humidity control is mainly manifested in the accuracy of temperature and humidity, temperature and humidity chamber substantially ordinary temperature can be up and down 2 ℃, humidity can be up and down 10 percentage points. Relatively good temperature and humidity chamber temperature can be achieved and down 0.5 ℃, humidity or minus 5 percentage points. For ordinary test, or in terms of international standards to meet the requirements in front of the chamber.

8 PCB board temperature and humidity testing and failure in temperature and humidity testing will encounter the problem of the rate of temperature change, the temperature changes quickly if it turns into thermal shock, and temperature changes caused by slow and test time is too long, not cost-effective. Generally, the temperature change rate is set at 20 ℃ / h. Under low temperature conditions can not be taken directly to the PCB board, to prevent water condensation leads to short-circuit occurred.

Usually low humidity conditions can not be considered, mainly due to expire in less than under this condition occurs, there is the low temperature and humidity of the equipment can fail relatively much more expensive, if not necessary, do not do.

Then there is each time with temperature and humidity control box must be to do before and after the test is the test temperature and humidity control temperature and humidity box tuned to a fixed setpoint, temperature and humidity control box to ensure that the working and extend the life of the specific temperature and humidity values are 23 ℃ and 50%.

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