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Effectiveness of Conformal Coatings on Surface Mount Components as Tin Whisker Mitigation
August 28, 2012 |Estimated reading time: 7 minutes
AbstractConformal coating is considered to be a mitigation strategy to address the failure risk presented by tin whiskers. While various conformal coatings have been shown to be effective on whiskering tin plated coupons, the effectiveness of conformal coating on actual assembly hardware has not been adequately examined. In this study, sets of assembled quad flat packages were conformally coated and evaluated for tin whisker mitigation as well as the coating coverage. Six coating materials were examined. Whisker growth was examined after specimens were subjected sequential to temperature cyclings, corrosive gases and temperature and humidity conditions. Whiskers were observed on coated surfaces that had relative thin layer of coating. Paralyne C coating was found to be the most effective coat-ing at whisker mitigation in terms of overall coating coverage and containing whiskers.IntroductionConformal coatings are thin polymeric layers, designed to protect printed circuit boards (PCBs) from the harsh environments, such as moisture and chemicals [1]. Tin whiskers are electrically conductive crystalline structures that can grow long enough to bridge adjacent conductive surfaces. The application of conformal coating on exposed electrical conductive surfaces is considered to be one of the most effective mitigation techniques for preventing electrical short failures caused by tin whisker [2, 3]. In terms of tin whisker mitigation, a conformal coating may prevent whiskers from contacting a coated surface or contain whiskers under the coated surface. Previous studies have been found that conformal coating served well in whisker containing [4]. However, Woodrow et al. [5, 6] reported on the effectiveness of several differences conformal coating materials to contain tin whiskers and found that tin whiskers penetrated all test coatings regardless of mechanical properties such as modulus, tensile strength and hardness of conformal coating. Most studies of conformal coating as a tin whisker mitigation strategy have used coupons with flat surfaces with uniform coating thickness. This study evaluated the effectiveness of six conformal coating materials for tin whisker mitigation on assembled quad flat packages (QFPs). Coating coverage was examined and the whisker growth after assemblies were subjected to environmental loading sequences with temperature cycling (TC), elevated temperature humidity (TH) and mixed flow-ing gases (MFG) was documented. ExperimentsA 184 IO QFP with Sn plated alloy 42 leads was used for this assessment. Parts were assembled with Sn-Pb solder paste to a custom designed printed wiring board that allowed individual part assemblies to be separated after assembly. Sets of part assemblies were coated by one of six conformal coating materials. Table 1 shows six conformal coating materials and their coating method. ALD-Cap O5TA200 coating is atomic layer deposition (ALD) with targeted thickness of 200 nm. All the conformal coating materials were applied by qualified commercial conformal coating services. One set of part assemblies were left un-coated to compare the whisker growth on the conformal coated and non-coated specimen. Table 1: Conformal coating materials under test.Table 2: Modified EIA MFG test method (Class IV).Coated and non-coated test specimens were separated into two groups and subjected to environmental loading which included TC, TH and exposure under the mixed flow gases (MFG). TC condition included cycle A: -55°C/20°C, cycle B: -15°C/60°C, and cycle C: 20°C/95°C. Each defined cycle was 2 hours in duration with 30 minute dwells. Assemblies were subjected to 100 of each defined cycles for a overall total of 300 cycles. After TC, specimens in Group 1 were stored at the TH conditions of 50°C/50%RH for 200 hours. Following TC exposure, the Group 2 specimens were exposed to a MFG conditions with four corrosive gases (H2S, Cl2, NO2, and SO2) for 48 hrs followed by 200 hours in the previously defined TH condition. The modified EIA-364-TP65A class IV was used for the MFG test and the specific conditions are presented in Table 2.
All specimens were subjected to inspection prior to the environmental exposure sequence and after completing each load sequence cycle. Inspections included examination of the soldered leads using optical and scanning electron microscope (SEM). Potential whisker locations where the coating was peeled off or mechanical deformed surface were noted. After optical inspection, parts were inspected on suspected whisker locations under a SEM. If no whiskers were detected under optical inspection, at least two terminals were inspected using the SEM. In addition, any other observed anomalies were documented.
Results and DiscussionFigure 1: Initial inspection by SEM prior to environmental loading sequences--a) AR1 coated; b) SR coated; c) UR coated; d) AR2 coated; e) Paralyene C coated; and f) ALD-Cap O5TA200 coated. Figure 1 shows the initial inspections of assembled parts prior to the environmental loading sequences. All spray-applied conformal coating had non-uniform coverage com-pared to those that underwent the vapor deposition method. Acrylic coating type 1 samples were found to have particularly poor coating coverage on pads and lead wire edges compare to the other spray coated samples. Other spray applied conformal coatings also did not uniformly cover the lead especially at the edges. On the other hand, the conformal coating applied by the vapor deposition process, parylene C and ALD showed uniform coating coverage on all surfaces, including lead edges and pads. Mechanical surface damage such as scratches was observed on all sam-ples. No whiskers were observed at the initial inspection.Figure 2: Image analysis process for coating coverage--a) Captured the image by BSE mode; b) Smooth the image using median filter; c) Divide the image into bright area; and d) dark area. After the environmental loading sequences, whiskers were observed on many of the samples. For the spray coated samples, whiskers were found in areas with relative thin conformal coating. No whiskers were observed on the parylene C coated samples. In terms of whisker mitigation, the coating coverage is very important. In order to evaluate the coating coverage, surfaces of two leads were examined on each side of the QFPs (eight total for each component) for each lead conformal coating material. For each lead, four areas at the same position relative to the top of lead were identified. The selected surface areas of selected leads were captured via SEM using back scatter electron (BSE) detec-tion mode. In BSE mode, lighter-colored areas indicate higher atomic number (Z) material--in our case it is the metal finish devoid of coating, or covered with thinner coating. Image software was used to calculate the areas with bright and dark pixels on images. The captured images were subjected to a smoothing process using median filter to reduce the noise within images as shown in Figure 2. From the processed images, areas of dark and bright regions were calculated using image software. Finally, coating coverage (%) on selected leads can be estimated using (1) The process for the quantitative image analysis for evaluating the coating coverage indicates that Acrylic type 1 (AR1) has the lowest coating coverage with 72.6% com-pared to other spray method coatings as shown in Figure 3. While the parylene C and ALD coating applied by vapor deposition had approximately 100% coating coverage.Figure 3: Estimated coating coverage for six types of conformal coatings.
In terms of non-coated specimens, more whiskers grew on the surface of specimens (Group 2) which was exposed to corrosive gases in MFG test than without MFG exposure (Group 1). After second accumulated loading cycles, the whisker density on specimens in Group 2 (1168/mm²) with MFG exposure has nearly two times higher density than that of Group 1 (601/mm²) as depicted in Figure 4. The statistical analysis using ANOVA showed that there is a significant difference in whisker density between specimens with and without MFG test. The longest whisker growth on Group 1 is 32 μm and 65 μm in Group 2.Figure 4: Whisker growth on non-coated specimen after second accumulated loading cycles: a) Without MFG exposure (Group1) and b) With MFG exposure (Group 2).In Figure 5, whiskers on the conformal coated specimens can be observed stretching the coating layer in order to escape. It can also be seen that the whiskers were contained under conformal coating layer or formed the dome shapes which may contain the whisker in the coating and some of whiskers had bent shape which might be caused by the ad-hesion strength of coating layer. Most of the escaped whiskers on conformal coated specimens were found on areas with relatively thinner coatings such as non-uniform coating surface in AR1 or edge of lead, or areas with mechanically damaged coating.Figure 5: Whisker growths on conformal coated specimens: a) Escaped whisker on AR1 and b) Contained whisker on UR.
Conclusion
Six types of conformal coating materials were evaluated for their effectiveness in containing whisker growth under temperature cycling and temperature humidity and exposure under corrosives gases. Among six selected conformal coatings, parylene C and ALD coating showed a uniform and near 100% coating coverage compared to other coatings which were applied by a spray method. Under the environ-mental loading sequences, only parylene C coating did not show any whisker penetration.
Acknowledgments
The authors would like to thank the Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland and the more than 100 companies and organizations that support its research annually.
References:
1. M. Pecht, "Contamination of electronic assemblies.,"Boca Raton, Florida: CRC Press, 2003.2. T. Fang, M. Osterman, S. Mathew, and M. Pecht, "Tin whisker risk assessment," Circuit World, vol. 32, no. 3, pp. 25-29, May ,2007.3. M. E. McDowell, "Tin whiskers: a case study," presented at the Aerospace Applications Conference, 1993.4. J. Kadesch and H. Leidecker, "Effects of Uralane Conformal Coating on Tin Whisker Growth," in 37th Nordic annual conference, ed. Helsingor, Denmark, 2000.5. T. Woodrow and E. Ledbury, "Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy," presented at the IPC/JEDEC 8th International Conference on Lead-Free Electronic Components and Assemblies, San Jose, California, 2005.6. T. Woodrow and E. Ledbury, "Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy, Part II," presented at the SMTA International Conference, Rosemont, Illinois, 2006.