What is the electromigration failure mechanism in high current RF traces and how do I prevent it?

Electromigration is a gradual failure mechanism where high current density causes metal atoms to migrate along the direction of electron flow, eventually thinning the conductor until it opens or voids form: (1) Mechanism: when current flows through a conductor, the electrons transfer momentum to the metal atoms (electron wind force). At high current densities: the momentum transfer is sufficient to displace metal atoms. Atoms accumulate at the anode end (forming hillocks) and deplete at the cathode end (forming voids). Over time: voids grow until the conductor cross-section is reduced to zero (open circuit) or the hillocks create shorts to adjacent conductors. (2) Where it occurs in RF systems: power amplifier drain feed lines: high DC current (1-10A) flows through narrow traces to the PA. The current density can exceed 10^6 A/cm² in narrow traces. Wire bonds: bond wires carry high current in small cross-sections. The wire-pad interface is particularly vulnerable (current crowding at the bond foot). Bias lines in MMICs: internal metallization lines on GaN, GaAs, or SiGe ICs carry both DC bias and RF current. The thin metal films (typically 2-4 μm thick) are susceptible at high current densities. (3) Black equation (empirical lifetime model): MTTF = A × (J)^(-n) × exp(E_a / (k_B × T)). Where J = current density (A/cm²), n = current density exponent (typically 1-2), E_a = activation energy (0.7-0.9 eV for Al, 0.9-1.1 eV for Cu), and T = temperature (K). Higher current density and higher temperature accelerate the failure. (4) Prevention: design rules: keep current density below the safe limit. For copper traces on PCB: J < 10^5 A/cm² (at 100°C). For aluminum metallization on MMICs: J < 2 × 10^5 A/cm² (at 125°C). For gold metallization: J < 5 × 10^5 A/cm² (gold is more resistant than aluminum). Trace sizing: for a DC current of 5A on a PCB: minimum trace width = 5A / (10^5 A/cm² × 35 μm × 10^-4 cm/μm) = 5 / (10^5 × 3.5 × 10^-3) = 14.3 mm. This is very wide. In practice: use a copper pour (ground plane) or thick copper (2-4 oz) for high-current PA supply lines. Temperature control: keep the trace temperature as low as possible (the exponential term in Black equation means small temperature increases significantly accelerate electromigration).