What You Can Takeaway
Electromigration can occur in two ways in PCBs and ICs.
The physical and chemical processes governing electromigration in each type of system are different.
The result of electromigration is a change in the resistance of the interconnect, depending on the exact changes that occur in an interconnect.
Electrochemistry occurs at the microscopic level in PCBs and ICs.
We’d all love for our circuits to be perfectly conductive and stable, but this is not the case in reality. The real world does not consist of perfect conductors and insulators surrounded by a vacuum, and the electric field will interact with the conductors and substrates in a real system. Whether you are designing ICs or PCBs, you’ll need to consider an important effect that arises in imperfect electronics: electromigration.
What is electromigration, and why does it occur? More importantly, how can it be prevented? A simple round of electromigration analysis for PCBs and ICs. The goal is to prevent short circuits and open circuits in these devices in different conditions. Some industry standards have been developed for this purpose. Here’s what you need to know about these standards and how electromigration can lead to failure in new devices.
Electromigration in Electronics
As more components get packed into smaller spaces, the electric field between two conductors with a specified potential difference becomes larger. This leads to some safety concerns in high voltage electronics, specifically electrostatic discharge (ESD). The high electric field between two conductors separated by air can cause air to experience a dielectric breakdown, creating an arc and current pulse in the surrounding circuits. Preventing these discharges in a PCB or in other devices requires separating conductors by some minimum spacing, and the spacing depends on the potential difference between the conductors.
Clearance distances described above are important for safety and preventing device failure, but the distance across the substrate is also important. Another point to consider is the distance between conductors across a dielectric. In PCBs, this is known as the creepage distance, the requirements for which (as well as clearance distance) are defined in the IPC 2221 standards. When the separation between conductors is small, the electric field can be large, which drives electromigration.
The mechanism that drives electromigration can be described as exponential growth when the current density in the conductor is large (in ICs), or when the electric field between two conductors is large (in PCBs). In order to prevent electromigration, you have three levers you can pull in your design:
Increase the separation between conductors (in PCBs)
Decrease the voltage between conductors (in PCBs)
Run the device at lower current (in ICs)
Electromigration in ICs: Opens and Shorts
In IC interconnects, the dominant force is not the field between two conductors and subsequent ionization. Instead, solid-state electromigration is the motion of metal along a conductive path (in this case, the metal interconnect itself) due to electron momentum transfer (scattering) at high current densities (usually >10,000 A/cm2). Electromigration follows an Ahrrenius process, so the rate of migration increases as the interconnect temperature increases.
The forces involved in the electromigration of copper are shown below. The wind force refers to the force exerted on a metal ion due to the scattering of electrons from metal atoms in the crystal lattice. This repeated ionization and momentum transfer to free metal ions causes them to diffuse towards the anode. There is an activation energy associated with this migration process; when energy transferred to a metal atom exceeds the Ahrrenius activation process, directed diffusion begins, which is guided by a concentration gradient (Fick’s law).
Schematic showing forces involved in electromigration. [Source]
As metal gets pulled to the surface of the conductor, it starts to build up structures that can bridge two conductors, creating a short circuit. It can also deplete the metal at the anode side of an interconnect, leading to an open circuit. The SEM image below shows the results from extended electromigration between two conductors. As metal migrates along the surface, it can leave behind voids (open circuits) or create whiskers that connect to a neighboring conductor (short circuits). In extreme cases with vias, electromigration can even deplete the conductor beneath the capping layer.
SEM image showing extreme electromigration in IC interconnects. [Source]
Electromigration in PCBs: Dendritic Growth
Similar effects occur in PCBs, resulting in two possible forms of electromigration:
Electromigration along the surface, as described above
Formation of semiconducting salts, which cause electrochemical growth of tree-like dendritic structures
These effects are governed by different physical processes. The current density between two conductors can be rather low because the dimensions in metal traces are quite large compared to the cross-sectional area of IC interconnects. In the first case, migration can occur at high current density, which causes the same type of stub growth over time. On the surface layer, subsequent oxidation can occur as the conductor is exposed to air.
In the second case, electromigration is an electrolytic process; the field drives an electrochemical reaction in the presence of moisture and dissolved salts. Electrolytic electromigration requires moisture on the surface and a high DC electric between two conductors, which drives the electrochemical reaction and growth of dendritic structures. Migrating metal ions are dissolved into an aqueous solution and diffuse across the insulating substrate. This is where IPC 2221 comes into play, as increasing the distance between neighboring conductors decreases the electric field between them, which suppresses the reaction that drives electrolytic electromigration.
SEM images comparing electromigration in PCBs and ICs. [Source]
Electromigration analysis in a new layout requires examining a design to ensure trace clearances do not violate design rules or industry standards. If you have access to some basic PCB or IC layout tools, you can check your layout against these rules and identify any violations. As ICs and PCBs continue to scale down, electromigration analysis will only become more important for ensuring reliability.
Electromigration analysis and design for reliability are easier when you use the right IC and PCB design and analysis utilities. The PCB design tools in Allegro PCB Designer and Cadence’s IC design software give you everything you need for electronics design in a single platform. You’ll also have access to a set of powerful analysis and simulation features.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.