As the additive-electronics industry advances, the miniaturisation of electronics necessitates complex circuit designs and demanding performance requirements in harsh environments. Polymer thick film (PTF) provides reliable and effective silver-ink solutions to meet these circuit needs. However, such printed electronics are known to be afflicted by silver migration. The ionic migration of silver occurs when direct current (DC) is applied between two electrode traces in the presence of an electrolyte, such as ionised water, sweat or detergent. Under the electric potential, silver ions become mobilised and deposit silver dendrites between the anode and cathode, which ultimately short circuits the device. While this problem has been well known to the electronics industry, silver migration has become more prevalent in intricate modern-circuit geometries. By understanding the mechanisms of silver migration, short circuiting can be more effectively reduced and failures prevented in PTF technology.
SILVER MIGRATION – TRACING THE PATH OF FAILURE
The metal composition, moisture and current are the key factors that cause the ionic migration of metal ions – migration will not occur without all three. PTF inks are often formulated with silver flakes which congregate on the surface of the film when dried, providing a medium for the ionic migration of silver to take place. Under an applied-electric potential, the silver flakes at the anode – the positive trace – oxidise to positively charged silver ions. These ions are free to move throughout the material to the cathode – the negative trace – via the aqueous electrolyte. Under the electrochemical gradient, the silver ions travel towards the electron-rich environment of the cathode where reduction of the ions occurs. This results in the dendritic deposition of silver metal. As the silver dendrites grow across the electrodes, shorting of the circuit inevitably occurs and the device fails.
“By understanding the mechanisms of silver migration, short circuiting can be more effectively reduced”
Elimination of moisture infiltration is not an option for current printed-electronics applications. Therefore, solutions for preventing silver migration lie in the management of secondary factors which determine the severity of failure. Material selection and processing, such as ink composition and drying/curing conditions, will affect the stability of the material. Drying or curing printed films at higher temperatures for longer periods of time ensures that any present moisture is sufficiently evaporated from the substrate and ink, resulting in a more durable circuit. Carbon, gold, aluminium and copper are known to be migration resistant, while the addition of palladium or platinum to a silver ink will minimise silver migration. However, these substitutes and additions will affect the printed films’ electrical performance, may require specialised-processing parameters, or alter cost effectiveness. Additionally, the use of reactive thermoset-polymer systems that are heavily cross-linked, crystalline or hydrophobic, may reduce the penetration of moisture and electrolytes into the silver ink. Nonetheless, there are trade-offs in mechanical properties, chemical compatibility and processing requirements.
In addition to material selection, circuit design can be engineered to lower silver migration risks. To reduce the surface area of silver ink exposed to moisture and other contaminants, traces should be printed as thin as the application allows. Spacing between traces should be greater than the width of the trace, as increased distance between the electrode traces will require higher degrees of silver migration to cause short-circuit failures. Additionally, the encapsulation of the conductor with carbon or dielectrics offers increased circuit protection. Application parameters will also dictate the degree of silver migration that occurs, as ionic migration can occur at low currents at near ambient temperatures. Thus, high-operating voltages will increase the strength of the electric potential, while high-operating temperatures increase the kinetic energy and diffusion of the silver ions.
“Solutions for preventing silver migration lie in the management of secondary factors”
Finally, the environmental conditions of the printed-electronic device must be considered. Exposure to harsh, wet environments will introduce not only moisture, but contamination to the circuit. The presence of these minerals, salts and other electrolytes induces the corrosion of silver and increases the conductivity of water. As a result, there is an increase in the quantity and mobility of silver ions. These harsh environments will ultimately increase aging over a short period of time and affect the lifetime of the device. Consideration must be taken to limit exposure to these environments, protect the circuit and reduce the likelihood of silver migration in the circuit design.
Heraeus Electronics goes beyond supplying materials by offering expertise in PTF technology. Heraeus Electronics’ engineers are available for circuit-design collaboration as well as advice on best practices and material-processing conditions to aid in silver migration mitigation and other material needs.