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How Does Cleaning Affect Conformal Coating?
December 31, 1969 |Estimated reading time: 4 minutes
By Sini Aleksic Ph.D.; Helmut Schweigart, Ph.D.; and Harald Wack, Ph.D., Zestron
Providing electronic assemblies with protective coatings is an important and necessary measure to ensure their reliability. As conformal coating generally is the last step in the manufacturing process, errors may have a drastic effect on production costs and field failures. To ensure optimum adhesion of the protective coating, it is important to provide the highest cleanliness of the assemblies before coating. In fact, a number of materials created during the soldering stages significantly impact the cross-linkage and have to be removed. Specific analysis of such "toxins" is therefore recommended in addition to traditionally accepted cleanliness tests.
Assemblies exposed to critical and frequently alternating climatic influences require high surface cleanliness. Malfunctions due to impurity-related leakage current and electrochemical migration can otherwise arise. Contamination-induced failure due to insufficient cleaning is cited as 8% probability for conformally coated assemblies1 (Figure 1).
Standards related to cleanliness include but are not limited to: IPC J-STD-001, chapter 8; IPC-A-610, section 10.4; IPC-TM-65, section 2.3.25; IPC TM 650; IPC TP 1113; and IEC - 68-2.
The no-clean process has assemblers most conflicted about whether to clean or not prior to conformal coating. Despite its potential risks and shortcomings, the no-clean concept has established itself as a dominant production process for a majority of product segments. Unfortunately, many companies have learned a difficult lesson by adopting no-clean processes that include additional production steps, such as component underfill or applied conformal coatings. However, lessons were at the same time learned within the cleaning community that have promoted new innovations, such as broader process windows, improved economics, higher industrial safety, and full material compatibility. These improvements have assisted the return of cleaning as a value-added production step. It has been said that cleaning behaves similarly to coating in that, being introduced retroactively, it severely impacts the entire assembly process assemblies and becomes unnecessarily expensive.
As mentioned previously, failures under conformal coatings are related to remaining contamination. The later can be categorized in two categories: ionic and non-ionic in nature. They include but are not limited to salts, resin-based residues, and organo-metallic complexes.The most common measurement of ionic impurities does not permit any conclusion concerning the local distribution of these conductive contaminants, as it only measures the total level of ionic contamination across the entire surface area. However, the precise distribution of residue on assemblies is extremely important when assessing the short- or long-term potential hazard from these residues. Alternative test methods providing an analysis of local ionic distribution are, for example, ion chromatography or specifically designed flux test methods.
Non-ionic contamination includes materials like organic resin. While most companies take a close look at ionic contamination readings, little attention has been allocated to the amount of resin residue on an assembly. If a protective coating is applied on top of the resin residues, for example, wettability is impaired and adhesion becomes insufficient due to the different temperature-dependent expansion coefficients. The residual amount of resin is specified by IPC J-STD-001D, section 8.3.6.1. For class 3 assemblies, a level <257.95 µg/sq.in. is required, which corresponds to the typical amount of resin left around a single solder joint. In IPC-TM-650, section 2.3.27.1, an extraction method is used followed by high-performance liquid chromatography (HPLC) to determine the amount of resin.
Depending on the coating system used, organic tin salts, which manifest themselves through a chemical reaction of metallic solder and fluxes (i.e. tin organic salts), can even act as polymerization inhibitors. These impurities are generally identified by EDX using a scanning electron microscope. Table 1 summarizes the most common residues and their respective quantitative limits to marginalize their harmful impact on the conformal coating quality.
The higher solid content and aggressive activators within lead-free solder pastes have furthermore complicated this topic. They are responsible for an increase in malfunctions, and a reduction in the reliability and life of electronic assemblies, among other challenges. The presence of activators constitutes the basis for any potential electrochemical malfunction, due to its hygroscopic nature. During previous studies, the authors investigated the relative water permeability of different coating materials in relation to temperature. The objective was to simulate climatic stressors and understand their impact better. It could be concluded that, for polyurethane and acrylic coatings, the permeability of water increased with higher temperatures (Figure 2). This can be an important observation, indicating that hygroscopic contamination left under conformal coatings will begin to react with then-dissociated water, in turn initiating well-known electrochemical failure mechanisms, such as dendrite growth or leakage currents.
Figure 2: Coating permeability measurement.
Additional test protocols currently are underway to establish the actual adhesion of different conformal coatings in relation to the contamination. Initial results indicate that, for a commonly used polyurethane coating, the pull forces were 3.61 N/mm2 for a cleaned substrate. With an un-cleaned substrate on the other hand, the pull force was only 2.48 N/mm2. This translates into a difference in excess of 50%. Further studies will be conducted shortly to provide more extensive experimental data support data.
ConclusionCleaning prior to conformal coating remains a topic of contention and cost-effectiveness. The trend towards cleaning is clearly visible in the high-end product segments. Evidence of the permeability of water and the detrimental effects of remaining contamination underneath conformal coatings has been established and discussed. We set out to stress problems with protective coating applications and the importance of an integrated and stable cleaning process. Acknowledging their importance and implementing process changes will furthermore help with the justification of cleaning as a "true" value-added step.
REFERENCES:1. Modern Electronic Packaging Integrated Circuits Engineering Corporation
Sini Aleksic, Ph.D., is advanced research scientist, ZESTRON America. Helmut Schweigart, Ph.D., serves as head of application technology, ZESTRON Europe. Harald Wack, Ph.D., is an SMT Editorial Advisory Board member and VP and CEO, ZESTRON America. Contact them at h.wack@zestronusa.com.