Tests prove coating credibility

23 June 2008

Figure 3: Final SIR drop for different coatings after surfactant contamination
Figure 3: Final SIR drop for different coatings after surfactant contamination

Ling Zou and Chris Hunt report on a test method for assessing conformal coating protection performance of electronic assemblies being used in harsh environments

NPL has successfully developed a novel test method that will enable engineers to characterise conformal coatings and evaluate the protection performance afforded underlying electronic circuitry.

Conformal coatings are being more widely applied to electronic assemblies to achieve high circuit reliability, even in environments previously thought to be hostile for electronic equipment. They must be robust, protective and removable for access/rework. Coatings are typically semi-permeable membranes and can prevent contaminants instantaneously reaching the circuit, but will not be completely impermeable to the wide range of contaminants. Permeation of contaminants is dependent on coating material (molecular chemistry), coating coverage, contaminant and concentration of contaminant.

To have confidence in coatings to maintain higher levels of reliability in hostile environments, manufacturers and users need to be able to test them under conditions representative of harsh field environments. However, the lack of a standard test method to evaluate the extent and nature of the protection coating afforded against a specific aggressive environment, has hindered wider use of the coatings.

The new method developed by NPL is significantly different from other methods, and reflects more realistic uses of conformal coating. A new test assembly with a wide range of components has many advantages in identifying coating coverage issue and protection problem. Contaminants are introduced in the method to represent the harsh environments in which electronic boards are most likely to operate.

Figure 4: Final SIR drop on different test patterns after surfactant contamination
Figure 4: Final SIR drop on different test patterns after surfactant contamination

The new method has been validated by measuring the protection performance of seven different coating types (inc water-based acrylics; solvent-based-acrylic fluoroacrylate; silicone; polyurethane; epoxy) against solvent-based fluxes, a surfactant and SO2 gas. The test boards were conformally coated using preferred industry processes, to which the chosen contaminants were applied. The boards were subsequently subjected to damp heat exposure, and Surface Insulation Resistance (SIR) are monitored.

Figure 1 shows a typical SIR response for polyurethane coating with (Figure1a) and without (Figure 1b) contaminant. The letters in the legend refer to different test patterns associated with different components, as shown in Figure 2. Comparing both figures the significant SIR drops in Figure 1b is due to contaminant penetration through the coating. On a contaminated board, a low SIR result does not necessarily mean the coating is permeable to the contaminants, as each coating has a different level of moisture permeability, which will affect its SIR values under the test condition. Therefore the Log SIR change before and after contamination is therefore more representative of contaminant permeation. The final Log SIR drop is presented in Figure 3 for the protection by different coatings against the surfactant contaminant. The greater SIR drop, the greater penetration of the contaminant, and the less protection of the coating. The final log SIR value is also a useful evaluation of the electrical reliability of the circuit underneath the coating.

The coating protection is not only dependent on coating chemistry, but also the component type, as seen in Figure 4. There is a significant difference on SIR drop between different test patterns. The largest SIR drop is seen on fine pitch leads components and BGA components.

Fig 1. SIR plot for polyurethane coating
Fig 1. SIR plot for polyurethane coating

Some SO2 exposure tests were undertaken, and different levels of corrosion were observed, which was found to be dependent on the coating type, as shown in Figure 5. Hence, visual inspection results can be useful in discriminating between the corrosion protections of coatings.

The work has shown that the new test method using recommended test parameters and the new test assembly design has been successful in characterising the conformal coating protection performance in harsh environments. The SIR technique is suitable discriminatory tool for assessing coating performance, and measuring the reliability of underlying circuitry. The combination of final SIR and SIR drop results provides an indication of the permeability of the coatings to moisture and contaminants from harsh environments.

Coating protection performance is dependent on coating chemistry, coverage and contaminants. Therefore, when assessing coating protection performance, the contaminants should be selected to represent the harsh environment in which the electronic assembly will be used. Using a contaminant solution to contaminate test assembly is a feasible method to simulate an assembly exposed to a harsh environment.

The test conditions are significant and to achieve the optimum discrimination it is recommended is to use the following test conditions of 40°C/93%RH with 5V bias test for contaminated assembly, and 40°C/93%RH with 50V bias test for uncontaminated assembly. The 85°C/85%RH test condition is too strong for measuring contaminated boards, and the SIR results are more dominated by water permeability of the coating rather than contaminants.

Fig 2. Test patterns
Fig 2. Test patterns

The new test board assembled with a wide range of components has shown advantages in identifying conformal coating protection problems on specific components. There is potential protection failure on fine pitch leads components and BGA components, where coating coverage is a particular issue.

Visual inspection of coated PCBs after SO2 exposure showed a reasonable good degree of correlation with the SIR results. This method can be used to determine coating corrosion protection performance from industry pollution gas. However, it should be emphasised that only the SIR technique is capable of providing information on the reliability of the underlying circuit.

The authors work for the NPL.

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