As we all know, silver electrode NTC thermistor chip is widely used in infrared thermopile, thermal print head, integrated modules, etc. because of its good reliability and high cost performance, but its silver migration phenomenon is also a problem that major manufacturers need to overcome. The phenomenon of silver migration was raised in 1954 by D.E. YOST of the Bell Institute in the United States as a problem for printed circuit board. In the report, the Ag of the terminals used in telephone exchanges or electronic computers is dissolved and precipitated on the insulation board, and the insulation is destroyed by ionic conductance.
The applications of Ag on printed circuit board assembly (PCBA) are as follows:
First, the printed wires and graphics on PCB is coated with Ag layer by electroplating or reduction plating process;
Second, components which the lead and terminal are made of Ag coated;
Third, Ag coating for the purpose of improving the appearance, solderability and electrical conductivity of PCB conductors;
Fourth, brazing filler metal with Ag.
Especially in the case of occasion three, Ag not only covers the surface of the conductor, but also adheres to the side wall of the conductor, which leaves a dangerous hidden danger of silver migration on the PCB in the future.
The silver migration process can be divided into the following three types:
First, anode reaction (metal dissolution);
Second, cathodic reaction (precipitation of metals or metal oxides);
Third, the reaction between the electrodes (precipitation of metal oxide).
Silver migration is caused by electrochemical phenomena related to solution and potential, and is related to metal ion movement and precipitation from metal dissolution, diffusion, and electrophoresis. Especially in the electronic equipment with high density assembly, the interaction between materials and surrounding environment leads to ion migration and the change of electrical characteristics, which has become an important cause of faults. In the migration of metal ions, the most typical is migration of Ag ions.
Gold electrode NTC thermistor chip developed and produced by EXSENSE Electronics Technology Co., Ltd. has more stable electrical performance and better consistency than the silver electrode NTC thermistor chip, so it is widely used in optical communications, IGBT, medical equipment and other fields with high temperature measurement requirements.