Copper ion migration due to circuit board failure
There are two main failure modes of general high-density layered circuit boards. The first type is shown in Figure 7.15 for copper ion migration between conductor layers. For copper ions with a radius of only 0.87A, epoxy resin can be regarded as a sponge-like structure, so if copper ions are affected by the voltage between conductive layers It is easy to form dendritic copper precipitates from the cathode to the anode through the epoxy resin.
Copper ion migration is more likely to occur in the upper and lower conductive layers and less likely to occur in the same layer of wires. The main reason is that the thickness of the insulating layer between adjacent conductive layers is usually only 40um, and the distance between the wires in the same conductive layer Generally, it is 100um. From the viewpoint of the electric field strength, the electric field strength in the direction of the vertical wire is larger than the electric field strength in the plane. Therefore, the conductive layer between the upper and lower phases is more likely to generate copper ions.
If copper ion migration occurs in the same conductor layer, the possible reason is the result of factors other than resin characteristics. For example, Figure 7.16 shows the surface of the joint part of the circuit board with the chip removed. From the photo, you can see that there are three circuits The phenomenon of copper ion migration occurs. The minimum line distance between these wires is 65um. Therefore, the phenomenon of copper ion migration not only occurs between adjacent wire layers with a distance of only 40um, but also between adjacent wires with a distance of 65um.
The main reason for the lateral copper ion migration is the weak bonding strength between the copper foil on the surface of the printed circuit board and the resin. The same problem also occurs in the position of the anti-tin paint, and there will be some undercut between the copper wire and the anti-tin paint. Therefore, processes such as gold plating after the formation of the solder resist paint will cause copper migration problems in the undercut if the residual chemicals are not cleaned.