Authored By:
Domingo Vazquez, Alfredo Garcia, Ricardo Macias, Humberto Ramirez,
Cristina Amador, Iulia Muntele, Shane Lewis, Mulugeta Abtew
Sanmina Corporation
Jeff Chinn
Integrated Surface Technologies, Inc.
Summary
Conformal coating - partial or full application using immersion, spray, vapor deposition or other methods, as the coating material formulation requires, has as ultimate goal protection of the assembled board or specific components from the effects of the working environment. Typical environmental testing applied to qualify conformal coating material involves humidity and temperature cycling, and exposure to corrosive environments. The limited scope experiment presented here has the intent to compare the behavior of a typical acrylic coating used on large scale in various applications and of a specific super-hydrophobic nano conformal coating formulation. Review of the fundamental properties of the coating materials place the nano coating in a more favorable position than the typical acrylic coating.
Additionally, as an invisible nanometer thick coating that is a dielectric, repels water, adds negligible mass to a printed circuit card, can be applied without masking, does not require removal at rework, and does not impede heat transfer, nano coating could be an attractive alternative to existing conformal coating materials. For practical verification, a batch of IPC B-52 boards were assembled in a no clean lead free process. Subsequent processes steps were applied and once coated, the boards were submitted for environmental exposure and SIR (surface insulation resistance) measurements.
Conclusions
Physical properties of nano conformal coatings present them as an attractive alternative to the more traditional conformal coatings (acrylic, epoxy, and urethane). The current experiment was formulated to gather an initial set of measurements and side by side comparisons of test boards produced in the same conditions typical for lead free no clean SMT assembly.
Plasma treatment of the boards before application of the conformal coating had a positive influence only in the case of the corrosive gas testing. The plasma treated boards subjected to salt spray or temperature/humidity/bias testing did not show improved SIR values at the end of testing as compared to the not treated boards. Washing the boards after SMT assembly did not influence the coating application or the SIR values for the corrosive gas or the salt spray test. Although it cannot be generalized, among the boards subjected to humidity and temperature testing, regardless of the coating type, the boards that measured close to the high threshold value were often from the batch that was washed after boards' assembly.
The SIR values were referenced to two different threshold values: 5 G omega / IPC-CC-830 and 100 M omega as is usually reported for humidity and temperature testing / IPC-9202. Reworked boards coated with acrylic were not included in the testing due to the size and scope of the experiment. Overall low SIR values measured on nano coated reworked boards during the humidity and temperature test are an indication of the possible influence of rework process. The 8 boards were reworked at selected locations (BGA, QFP, 1206 capacitors); all SIR patterns on the reworked nano coated boards measured lower than the equivalent patterns on the as assembled boards.
The acrylic coated boards presented the more consistent behavior for two of the tests, with the data running close together for groups of boards. Selected SIR patterns on some of the nano coated boards did show very promising values, however, repeatability was not accomplished in this experimental run. More data is needed to identify the reason behind the low repeatability, as well as to why the reworked and recoated boards have shown the lowest SIR values at the end of humidity and temperature test.
The test for which nano coating outperformed the acrylic coating is the salt spray, where moisture was present as liquid water; on the THB test where moisture was in vapor form, the nano coating presented the least protection to the boards. Although acrylic coating performs better than the nano coating in two out of the three tests, it does not offer the desired level of protection in all harsh environmental testing conditions applied either.
It is preferable that the experiment be repeated in the future with a different nano coating material formulation, and include an additional conformal coating material (parylene - for example). As the nano coating material properties and application process evolve, it is desirable to compare its performance with the most used coatings as well as the better performing coatings (all other processes considered equal).
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