Study On Electrochemical Migration Failure And Detection Of Printed Circuit Boards With Different Materials Under Soluble Salt Contamination

With the rapid development of science and technology,all kinds of electronic equipment tend to miniaturization and high-density design.The wiring density of printed circuit board(PCB)is increasing,and the spacing between wires,pads and contacts is becoming smaller and smaller.Under certain temperature,humidity and bias voltage(THB)conditions,the insulation failure of circuit board caused by electrochemical migration(ECM)between adjacent electrodes,wires and contacts is becoming more and more frequent.In recent years,domestic air pollution is serious,resulting in a large amount of dust deposition on the surface and inside of electronic equipment.The coverage of natural dust particles will change the local temperature and electric field intensity of the circuit board surface,and the soluble salt will reduce the critical humidity,making the ECM mechanism of the circuit board more complex.Therefore,the study of ECM failure mechanism,reliability life modeling and detection methods of different electrode materials of circuit board caused by soluble salt pollution in dust has important guiding significance for the design,material selection and reliability evaluation of circuit board in electronic products.In this paper,the failure mechanism and influencing factors of soluble salt on the ECM of the circuit board in dust pollution were studied,the ECM characteristics of common electrode materials were analyzed,and the reliability modeling of circuit board under soluble salt pollution and the anti-ECM detection methods of different materials were studied.Firstly,the immersion silver plated high-density standard PCB was taken as the research object,the action characteristics of the concentration of the soluble salt solution,the temperature,the relative humidity and the bias voltage on ECM were studied through orthogonal experiments,and the effect of salt concentration on the failure mechanism of ECM of circuit board was analyzed.Then,the electrochemical reaction mechanism of common circuit board electrode materials(bare copper,immersion tin,and electroless nickel immersion gold)polluted by two kinds of soluble salts was experimentally studied respectively.Finally,the life modeling of ECM of different electrode materials of circuit board was carried out,and the anti-ECM ability of circuit board of different materials was analyzed.By comparing the characteristics of water drop(WD)experiment and THB experiment,a salt solution drop detection method of anti-ECM of different electrode materials was proposed.The results of orthogonal experiment and range analysis show that the effect of soluble salt in the dust on the ECM of circuit board cannot be ignored,which is only weaker than that of the relative humidity.Under the THB tests,with the increase of salt concentration,there are two insulation failure mechanisms-ECM,and first ionic conduction then ECM.In the low concentration stage,the insulation failure is caused by ECM,and the ECM reaction becomes more intense with the increase of ion concentration.When the ion concentration is higher than a critical concentration,the insulation failure mechanism changes into two stages:first ion conduction and then ECM.Two representative salt solutions of NaCl and Na2SO4 were selected for WD experiments under different concentrations.The time to the insulation failure of circuit board was obtained through surface insulation resistance monitoring,and the compositions of the reactive products were detected and studied.The results show that bare copper and immersion tin plated circuit boards have similar failure characteristics.In the low concentration range only ECM occurs,bare copper material migrated with copper,and tin-plated material migrated with tin and a small amount of copper,while in the high concentration range,ionic conduction first occurred first and then ECM.Electroless nickel immersion gold has occasional ECM when the voltage rises to 5V,showing good resistance to ECM.The main migrating components are nickel and copper.The anions in the salt solution can stimulate the anodic reaction.The influence degree of different soluble salt anions mainly depends on the number of charges carried by themselves.A large number of charges will promote the anode to produce more metal ions and accelerate the formation of the dendrites.Based on Faraday’s theorem,the life modeling of ECM of bare copper material and immersion tin plated circuit board was carried out.Combined with the ECM mechanism,the ECM resistance of three materials can be obtained from weak to strong:immersion tin,bare copper and electroless nickel immersion gold materials.The characteristics of WD experiment and THB experiment are compared and analyzed,and a salt drop detection method for anti-ECM of circuit boards with different electrode materials based on ECM failure mechanism under dust soluble salt pollution is proposed.