Flux activators can become a catalyst for electromigration and dendrite growth.

There is no magic bullet to address component reliability; however, conformal coating can go a long way to extending product reliability within the harsh environments they operate – provided it is applied on a residue-free substrate surface. To ensure optimum adhesion of the protective coating, it is critical that assembly surfaces are properly cleaned prior to coating. In essence, conformal coating is a thin polymer protective film that, when applied, conforms to the PCB surface. Although conformal coatings keep the substrate’s surface dry and free from potential contamination, they are semi-permeable against humidity, depending on temperature and material type. For example, water permeability for polyurethane and acrylate coatings can increase from near zero at 20ºC to 14,000 g/m2h at 80ºC, comparable to the vapor permeability of Gore-Tex fibers. And bond strength, a measure of adhesion, is greatly reduced when applying a coating to a contaminated surface.

Failures under conformal coatings are related to remaining contamination, typically classified as ionic or non-ionic in nature. They may include salts, acidic flux activators, resin- and rosin-based residues, as well as organo-metallic complexes. Flux is used to clean or deoxidize the metal surface to be soldered; otherwise intermetallic bonding will not take place. However, any corrosive material left on the surface must be cleaned after soldering, as this may lead to climatic failure mechanisms.

For example, if flux activators, which are hygroscopic and acidic in nature, are exposed to moisture in the presence of conductive electrolysis and voltage, they become a catalyst leading to electrochemical migration and dendrite growth. Additionally, flux residues can lead to creeping currents, which can cause electrical shorts and/or bit failure in RF connections. In all cases, these are undesirable results.

Once we understand the substrate failure mechanisms, it is important to recognize how conformal coating itself can fail. Typical failures due to unclean boards are poor coverage, dewetting, incomplete polymerization, loss of adhesion and cracking. These can have several measureable root causes.

For one, ionic contamination levels are critical, as they analyze the substrate’s surface purity. High ionic contamination indicates the presence of a large amount of hygroscopic and conductive impurities. Due to the absorption of humidity, these impurities will build a moisture layer between the surface of the assembly and the coating. This will lead to coating delamination and possible failure. Also, if a protective coating is applied over non-ionic contamination such as organic resin, wettability is impaired and adhesion is compromised due to the different temperature-dependent expansion coefficients. Net result: Coatings can peel.
Another critical factor in achieving optimal conformal coating adhesion is substrate surface energy or surface wetting capability. This quantifies the disruption of intermolecular bonds that occur when a surface is created. Since ionic and non-ionic contaminants will lower the substrate surface energy, it is vital to minimize all residues.

So, how clean is clean for substrates, not just those targeted for conformal coating? For many Class 2 and 3 applications, many of which incorporate conformal coating as part of their process, cleanliness targets are typically set by IPC standards. Other times, cleanliness goals are mandated by the customer or set forth by the cleaning agent supplier. Aggressive but achievable limits can be as follows:

• Ionic contamination: <0.4 Ωg/cm2
• Surface tension: >40 mN/m
• SIR: >1x10E8Ω2

A clean substrate is critical to component reliability, as well as conformal coating integrity. Many cleaning methods are available. However, for consistency, it is best to incorporate either batch or inline cleaning equipment in the production process. Depending on the flux system used, these machines can clean with DI water or an engineered cleaning agent, either solvent- or aqueous-based.

There are limitations with the use of DI water. It may seem logical to clean OA fluxes with deionized water. But, water-soluble fluxes are typically activated by halides. In their organic state, these are difficult to remove with DI water. If partially ionized, they form hypo-halide solutions that can be very corrosive and result in electrochemical migration and contamination-induced leakage currents.

Also, due to its poor organic solubilization, DI water is not capable of cleaning RMA, no-clean, synthetic and highly polymerized “3D” flux residues. A change from eutectic, water-soluble solder materials to any of the aforementioned products will inevitably result in the formation of white residues on the assembly. Pb-free formulations are even more difficult to clean. Some aqueous cleaning agents have been shown to effectively clean substrates (reduce ionic contamination), as well as improve bonding and coating. This has been demonstrated not only with OA fluxes, but also RMA and no-clean fluxes.

Many other types of contamination and residue must be removed prior to coating: fingerprint oils and salts, fume residues from rework, adhesive residues and solder balls. To be safe, always clean your substrates, and certainly before conformal coating.

Harald Wack, Ph.D., is president of Zestron (zestron.com); h.wack@zestronusa.com.