TWI Industrial Member Report Summary 1052/2014
By Helen Goddin and Andrew Whitaker
The medical industry requires new developments in technology for micro-devices. In particular, there is an increase in the use of electronics and sensors for implants, and a need for a change in the materials used for encapsulation. There would be many benefits in using only a polymeric encapsulant rather than the traditional metallic box solution. Challenges in this area include understanding and monitoring the failure mechanisms in a biological environment, and designing a suitable accelerated test. In this project, testing is considered for silicon devices and thick film sensors in a salt water solution. Voltage readings were monitored, to detect early failure of the components.
A novel test has been developed successfully to determine the effectiveness of coatings to protect electronic assemblies from aqueous solutions.
Test equipment allows the key variables of voltage, temperature and aqueous environment to be controlled and varied using a standardised specimen geometry so that different coatings can be compared.
One established epoxy and one silicone were tested. The epoxy was found to give a longer lifetime.
Dendrite growth was identified on the interdigitated pattern test board, indicating a significant film of water developed at the interface, capable of supporting metal ion transport.
On the MOSFET test cell, corrosion was identified on aluminium pads, and a red phase formed on gold thick film stepping stones. This indicates at least local areas of moisture penetration through the coating.
Early results show that failure mechanisms can be linked to voltage measurements in active devices and sensors.