We all hope that the PCB board can be perfectly conductive and stable, but the actual situation is not so. The real world is not composed of perfect conductors and insulators surrounded by vacuum, and the electric field will interact with the conductors and substrates in the real system. Whether designing IC or PCB, it is necessary to consider the important impact of imperfect electronic products: electromigration. What is electromigration and why does it happen? More importantly, how to prevent it? A simple round of PCB and IC electromigration analysis. The purpose is to prevent short circuit and open circuit of these equipment under different conditions. Some industry standards have been developed for this purpose. You need to understand these standards and how electromigration can cause new equipment to fail
Electromigration in electrons
As more components are stacked in a smaller space, the electric field between two conductors with a specified potential difference becomes larger. This leads to some safety problems in high-voltage electronic equipment, especially electrostatic discharge (ESD). The high electric field between two conductors separated by air will cause the air to undergo dielectric breakdown, thereby generating an arc and a current pulse in the surrounding circuit. To prevent these discharges in PCB boards or other equipment, conductors need to be separated by a certain spacing, which depends on the potential difference between the conductors. The above gap distance is important for safety and preventing equipment failure, but the distance across the substrate is also important. Another point to consider is the distance between conductors across the dielectric. In PCB, this is called creepage distance. When the distance between conductors is small, the electric field may be large, resulting in electromigration. When the current density in a conductor is large (in IC), or when the electric field between two conductors is large (in PCB), the mechanism of driving electromigration can be described as exponential growth. To prevent electromigration, you can use three levers to pull in your design:
Increase the spacing between conductors (in PCB board); Reduce the voltage between conductors (in PCB board); Operate the device at a lower current (in the IC).
Electromigration in IC: open circuit and short circuit
In IC interconnects, the main force is not the electric field between two conductors and subsequent ionization. In contrast, solid-state electromigration is due to electron momentum transfer (scattering) at high current density, which causes the metal to move along the conductive path (in this case, the metal interconnect itself). The migration speed increases with the increase of interconnect temperature. The forces involved in copper electromigration are as follows. The wind force refers to the force exerted on the metal ions due to the scattering of electrons from the metal atoms in the lattice. This repeated ionization and momentum are transferred to the free metal ions, causing them to diffuse toward the anode. This migration process has activation energy. When the energy transferred to the metal atom exceeds the ahrenius activation process, the directional diffusion begins, which is carried out under the guidance of the concentration gradient (Fick's law). When the metal is pulled to the surface of the conductor, it begins to establish a structure that can bridge two conductors, thereby causing a short circuit. It also depletes the metal on the anode side of the interconnect, resulting in an open circuit. The SEM image below shows the results of extended electromigration between the two conductors. When the metal migrates along the surface, it will leave gaps (open circuits) or generate whiskers (short circuits) connected to adjacent conductors. In extreme cases with vias, electromigration may even deplete the conductors under the capping layer.
Electromigration in PCB: dendritic growth
Similar effects occur in PCB boards, resulting in two possible forms of electromigration:
As described above, electromigration along the surface, the formation of semiconductor salts, leads to the electrochemical growth of dendritic structures. These effects are controlled by different physical processes. The current density between the two conductors may be low because the size of the metal traces is very large compared to the cross section of the IC interconnect. In this case, migration will occur at high current density, resulting in the growth of the same type of stubs over time. On the surface layer, oxidation may subsequently occur as the conductor is exposed to air. In the second case, electromigration is an electrolytic process. This field drives electrochemical reactions in the presence of water and salt. Electrolytic electromigration requires water on the surface and high direct current between the two conductors, which will drive the electrochemical reaction and the growth of dendritic structures. The migrated metal ions are dissolved in the aqueous solution and diffused over the entire insulating substrate. Increasing the distance between adjacent conductors reduces the electric field between them, thereby suppressing the reaction of driving electrolytic electromigration. Electromigration analysis in the new layout needs to check the design to ensure that the trace gap does not violate design rules or industry standards. If you can use some basic PCB board or IC layout tools, you can check the layout against these rules and find any violations. With the shrinking of IC and
PCB board, electromigration analysis will only become more and more important to ensure reliability.
