SIR testing and the performance of conformal settings
SIR testing and the performance of conformal settings
Keywords Conformal coatings, NPL, Publication
Measuring the SIR of conformally coated boards is commonly used to assess the efficacy of different coating materials and formulations to protect circuitry from environmental exposure. Typically these measurements are made under constant temperature/humidity conditions, with relatively long intervals between successive measurements. This approach to testing generates information that is of limited use. An ongoing DTI supported project, between industry and the National Physical Laboratory, aims to improve this approach so that users of conformal coatings can compare the performance of different coating chemistries and formulations on a range of different finishes on substrates as diverse as ceramic and polyimide (Flex).
The new procedure involves repeatedly measuring the SIR of coated boards at intervals of 10 minutes over a period of 48 hours. During the test the boards are exposed to alternating dry and damp conditions (10 per cent and 90 per cent R.H. respectively) whilst the temperature is progressively increased from 40°C to 80°C. This programme of environmental conditioning gives an indication of how the circuit will perform in service and provides valuable information on the performance of coatings as a function of temperature and humidity. The results show that the commonly used conformal coatings e.g., acrylics, silicones and polyurethanes are all moisture permeable. Despite this the coatings do protect against corrosion, either in humid environments or during immersion in salt water.
Two reports are currently available from the project, one describing the results of the test programme (CMMT (A) 267) and the other describing the new method for collecting and interpreting SIR (CMMT (A) 271).
Further information: Dr Chris Hunt, National Physical Laboratory. Tel: +44(0)20 8943 6065; E-mail: chris.hunt@npl.co.uk;WWW: http//:npl.co.uk/npl/ei/index.html
