The concept of galvanic corrosion is often encountered,when discussing technology reliability or performing failure analysis,Understanding these concepts is very important for failure analysis and reliability design.
Galvanic corrosion, also known as galvanic effect,that is, the two metals that are connected and different in activity are dissolved in contact with the electrolyte to react with the galvanic cell. The more active metal atoms lose electrons and are oxidized (corroded). The essence is that the active metal is oxidized.
The conditions under which Galvanic corrosion occurs are as follows:
(1) Two electrodes with different activities (two metals or metals with different activities and inert electrodes).
(2) Electrolyte dissolution (or humid environment and corrosive atmosphere).
(3) Forming a closed loop (or the positive and negative electrodes are in contact during electrolyte dissolution).
Generally, a highly active metal is a negative electrode and is dissolved. The active sequence of the metal is shown in Figure 1-82.
In the surface finished of PCB, the common galvanic corrosion is:
In the immersion silver process, the copper is etched under the edge of the solder mask, as shown in Figure 1-83. In the immersion silver process, because the gap between the solder mask and the copper crack is very small, it limits the supply of immersion silver chemical liquid to the silver ions , but the copper here can be corroded into copper ions and then copper outside the crack. A silver reaction occurs on the surface. Because ion conversion is the motive force of the immersion silver reaction, the degree of attack on the copper surface under the crack is directly related to the thickness of the immersion silver.