Under a certain temperature and humidity environment and potential difference, electrochemical migration is easy to occur between adjacent lines and solder joints, resulting in insulation failure. In the case of serious dust pollution, the deposition of dust particles in electronic equipment changes the critical humidity on the surface of circuit board, thus changing the failure mechanism and time of electrochemical migration.
Zhou Yilin and Lu Wenrui, researchers from the school of automation of Beijing University of Posts and telecommunications, wrote an article in the 12th issue of Journal of Electrotechnics in 2020, using temperature humidity bias experiment to study 13 ~ 18? The influence of dust particle coverage density with m particle size on the electrochemical migration failure time of circuit board under the interaction of ambient temperature, humidity and electric field intensity. It is found that the electrochemical migration failure time caused by particle coverage density changes non monotonically.
The particle coverage density is less than 350? At g / cm2, the failure time and particle coverage density have a negative exponential function; Above 350? G / cm2, positive exponential function. The mechanism of particle distribution in high and low density areas on electrochemical migration failure is analyzed from the two aspects of particle adsorption of water and changing the growth path of crystal branches, which lays a foundation for establishing the reliability detection method of high-density circuit board in dust polluted environment.
Electrochemical migration is an electrochemical phenomenon. In high-density circuit board packaging, under certain temperature and humidity conditions, water film condenses on the surface of insulating materials, and the anode metal of lines or solder joints is hydrolyzed to form metal ions. Under the action of electric field force, dendritic metal deposits are gradually formed by migration to the cathode and reduction, which is called “dendrite”, which grows from cathode to anode, The failure phenomenon that leads to the significant reduction of surface insulation resistance (SIR) between adjacent poles is called electrochemical migration.
The main factors affecting electrochemical migration are temperature, relative humidity, bias voltage, line spacing, electrode material and so on. G. Digiacomo obtained an empirical formula that the surface electrochemical migration failure time and temperature of high-density printed circuit board obey Arrhenius equation, but have a negative exponential function with electric field strength and relative humidity. Yang Shuang and a. christou studied the relationship between electrochemical migration failure time on the surface of silver impregnated circuit board and temperature, relative humidity and voltage, and carried out failure physical modeling. The decrease of wire spacing increases the electric field strength and accelerates the occurrence of electrochemical migration. R. Ambat et al. Found that the retention of flux on the surface of circuit board will hinder the electrochemical migration reaction, and dust will promote the electrochemical migration, but did not explain its reaction mechanism.
China’s air pollution is serious. Dust particles can flow with the air into the interior of electronic equipment. They adhere to the surface of circuit boards and electronic components by gravity and electrostatic force, causing various electrical contact faults, and the fault of electronic components will further affect the reliability of the whole system. Dust can be divided into soluble salt and insoluble particles. The greater the solubility of soluble salt, the lower the critical humidity of the covered circuit board, which is more likely to reduce the insulation resistance and accelerate the electrochemical migration failure.
Experiments show that with the increase of ion concentration in salt solution, the electrochemical migration of circuit board failure mechanism changes to ionic conductivity. The dust insoluble particles on the surface of the circuit board adsorb water under the capillary force and slow down the desorption of water. The moisturizing effect of flake mica particles in the high temperature stage is stronger than that of granular SiO2, and the delay of water desorption in the dehumidification stage is more significant.
The coverage of dust on the circuit board will also increase the local temperature and accelerate the electrochemical migration failure. The dielectric properties of dust particles lead to the uneven distribution of electric field between wires, which changes the growth path of crystal branches formed after the migration of anode metal ions and delays the failure. Dust pollution can also cause the temperature rise on the surface of the circuit board, which intensifies the electrochemical migration.
In conclusion, dust will affect the surface humidity, temperature and electric field distribution of circuit board, and then change the electrochemical migration mechanism and failure time.
The indoor natural dust deposition experiment in Beijing shows that the surface density of dust deposition basically increases linearly, and can reach 170? g/cm2。 It is found that inorganic matter accounts for 70% of the indoor natural dust in Beijing, and the rest are organic matter and carbon black. The soluble salt in inorganic matter is about 4%, and the rest are insoluble particles. The researchers of Beijing University of Posts and Telecommunications took insoluble particles in dust as the main environmental pollutants and the particle coverage density of dust as the influencing factor to study the failure characteristics, failure mechanism and failure time of electrochemical migration of circuit board under the interaction with temperature, relative humidity and bias voltage, It lays a foundation for establishing the detection method of the influence of dust pollution environment on the reliability of circuit board.
Fig. 2 block diagram of multi-channel insulation resistance test system
The researchers selected 13 ~ 18? As a representative of dust insoluble substances, SiO2 particles with a particle size of m were tested on a standard comb circuit board to study the mechanism and characteristics of the effect of dust particle coverage density on electrochemical migration failure. Through the experiment, it is found that with 350? G / cm2 as the boundary, the failure time in the low-density area and high-density area covered by particles shows different variation rules.
Fig. 3 meniscus formed by particles adsorbing water on the surface at low density
In the low density range of particle distribution (less than 350? G / cm2), the particles adsorb water under the capillary force. With the increase of particle coverage density, the water film on the surface of the circuit board increases, which promotes the electrochemical migration of metal ions. The intermittent failure frequency of insulation resistance on the surface of the circuit board increases, the number of crystal branches increases, the growth range increases, and permanent failure occurs. The failure time has a negative exponential function with the particle coverage density.