Achieving high reliability in service
As packaging densities increase and electronic assemblies move to ever-harsher environments, electrochemical reliability becomes more critical. The combination of contaminants, moisture and electrical bias facilitate electrochemical corrosion processes that result in loss of continuity or short circuits. Moisture plays an important role for circuit failures, and condensation can significantly accelerate failures.
Surface insulation resistance (SIR) measurements are based on the measurement of a leakage current associated with this electrochemical corrosion process, which is extremely sensitive to moisture. Very thin water films (~100 nm) naturally occur in high humidity environments. Assembly or environmental temperature changes can lead to much thicker and visible liquid water layers, causing accelerated corrosion on circuit assemblies. Insulation resistance can drop rapidly, and failures can occur within a few minutes.
Conformal coatings are being increasingly utilised to mitigate against electrochemical failures and tin whiskers. However, it is challenging to achieve even coverage over electronic assemblies, particularly over sharp corners. Humidity testing in combination with SIR testing is used to characterise coating coverage.
We have developed new condensation test aimed at achieving condensing conditions where a liquid layer is formed. This is a more aggressive test for a coating, yet is representative of typical failure modes. It indicates where coating coverage is poor, allowing end-users to improve coverage or develop alternative strategies.
For this new approach, the condensation level on test samples can be accurately controlled, and the condensation period and cycles programmed independently of the humidity conditions set in the chamber. By depressing the temperature of the board below the ambient in the humidity chamber, controlled levels of condensation can be achieved on the test board. This test procedure is now available to the electronics industry and custom thermal interposers can be developed to allow testing on production assemblies.