Conformal Coating Cleaning Guide
Cleaning electronics prior to applying conformal coatings can sometimes be a cost-intensive and complicated procedure, so questioning its importance is not unreasonable. However, the following case study articulates the criticality of proper cleaning.
Example of Device Failure From Improper Cleaning
A multinational corporation with a medical device division had a contract manufacturer who cleaned board surfaces after reflow/soldering but used tap water. The contract manufacturer did not see this as problematic, but washing with tap water is not standard industry practice. Tap water composition depends on the municipal water utility provider, the water treatment plant’s pipe quality, the local water infrastructure, the pipe quality throughout a building, and more. If there’s ever a question of contagion or biological contamination, the municipal water provider often dumps large amounts of concentrated bleach into the system and, many times, without notice. Incidentally, the medical device manufacturer had optoisolator components on the assembly that were very sensitive to chloride ions. Chloride leached into the component package from the tap water and corroded a wire bond, leading to loss of functionality. Although the assembly appeared defect-free with a visual inspection, the damage was revealed under X-ray imaging:
In this case, improper cleaning had severe ramifications, causing a critical failure unnoticed by visual inspection. Ultimately, the quality and reliability of the hardware produced hinges upon knowing what residues are present, the impact those residues may have on electronics assemblies, and proper cleaning.
Poor coating cleanliness can lead to:
- Electronics corrosion (whether or not they are submerged in a liquid)
- Loss of adhesion/delamination of the coating
- Poor long-term reliability, especially in harsh environments
When safety and reliability are critical, cleaning is just as important. Contamination can come from many sources, and it typically occurs on boards before conformal coating, causing various failure-inducing problems.
Conformal Coating Contaminants
Sources of Contamination
During the assembly process, contamination can originate at any point. The most prominent sources of contamination are transport and handling of the devices to be coated, unmonitored manufacturing processes, and storage, particularly for prolonged periods.
Common sources of contamination include:
- Etching chemicals – These highly conductive chemicals can also be corrosive. They must be removed and rinsed, chemically neutralized, and are sources for current leakage.
- Fluxes can leave residues that can cause reliability defects if not removed. All flux residue contaminants must be cleaned from the substrate.
- Inter-layer residues can be spread during the drilling processes and through plating processes.
- Residues from the cleaning process chemistry itself must be removed.
View our webinar on proven methods for achieving protection from corrosive environments
Types of Contaminants
Although particulate residues, such as paper fibers, can absorb moisture in service and result in unintended electrical shorts, two broad categories of contaminants exist: non-ionic residues and ionic residues. Recognizing the differences between these two types is important because each type affects conformal coatings differently. Ionic residues can result in corrosion, electrical leakage, or electrochemical migration in the presence of moisture and electrical bias. Non-ionic residues, such as oils, can inhibit bonding mechanisms and lead to dewetting or delamination.
Ionic contaminants break down into separate molecules inside the conformal coating and become conductive, potentially leading to short-circuit or undesired changes in inductance. Ions can also lead to corrosion or dendrites. These ionic contaminants often originate during assembly processes while soldering components to the board. However, salts from worker perspiration can also be a source of ionic contamination.
Non-ionic contaminants prevent coatings from adhering correctly to an assembly. Non-ionic contaminants are typically organic compounds, such as skin oils, hand lotions, greases, and silicones.
Cleaning PCBAs before applying conformal coatings is a critical function of the coating process.
Testing for contaminants
Cleanliness testing is suggested in order to detect contaminants to eliminate before the coating process initiates because the type of contaminant will affect the method used for cleaning. Ionic and non-ionic contaminants are tested for separately. While cleaning limits harmful defects, testing ensures proper cleaning. Possible testing methods to locate contaminants are pictured below:
The Resistivity of Solvent Extract (ROSE) test method measures the solution resistance change over time.
Ion Chromatography checks for anions, cations, and weak organic acids (WOAs) in a solution extract.
FTIR Spectroscopy helps with the identification of pure organic compounds and less complex mixtures.
Conformal Coating Cleaning Process
Both ionic and non-ionic residues should be thoroughly cleaned off before applying coating materials for enhanced product reliability and coating performance. Ionic contaminants dissolved in water can exacerbate the problem by depositing salts onto the printed circuit board assembly. Solvents and surfactants can remove organic and non-ionic compounds.
The following materials and processes can be used:
Semi-aqueous wash with cleaning chemistry (surfactants, saponifiers, etc.), followed by a clean deionized (DI) water rinse
- Warm deionized (DI) water
- Ultrasonic agitation
- Vapor degreaser
- Vapor-phase soldering
- Aerosols defluxers
- Isopropanol (IPA) swabs
Conformal Coating Cleaning Benefits
Cleaning PCBAs before applying conformal coatings is a critical function of the coating process. Cleaning processes and subsequent quality testing support optimal coating adhesion when done correctly. Conversely, poor cleaning can introduce numerous defects that lead to coating failure.
Conformal Coating Cleaning Tips
Industry standards, such as IPC-A-610, provides recommended practices for safely handling electronic assemblies to minimize the amount of contamination:
- Keep workstations clean and neat. There must not be any eating, drinking, or use of tobacco products in the work area.
- Minimize the handling of electronic assemblies and components to prevent damage.
- When gloves are used, change as frequently as necessary to prevent contamination from dirty gloves.
- Do not handle solderable surfaces with bare hands or fingers. Body oils and salts reduce solderability, promote corrosion and dendritic growth. They can also cause poor adhesion of subsequent coatings or encapsulates.
- Do not use hand creams or lotions containing silicone since they can cause solderability and conformal coating adhesion problems.
- Never stack electronic assemblies, or physical damage may occur. Special racks may be provided in assembly areas for temporary storage.
- Always assume the items are ESDS (Electrostatic Discharge Sensitive) even if they are not marked.
- Personnel must be trained and follow appropriate ESD practices and procedures.
- Never transport ESDS devices unless proper packaging is applied.
In general, you should tailor the cleaning method to the contamination. Bear in mind that mixed contaminants might require multiple cleaning steps. Furthermore, sensitive substrates and components can’t handle all cleaning methods.
|Substrate Materials||Reactions to Cleaning Methods|
|Electronic Components||Generally tolerate all cleaning methods, but items such as displays, MEMS, and optical components can be sensitive.|
|Solder Mask||Very robust but can be discolored by plasma. A known point of delamination and contamination in surface energy.|
|Other Conformal Coatings||Some conformal coatings have very low surface energy, and some can inhibit cure of other coatings (acrylics and silicones, for instance).|
|Solder Joints||Combined with the standoff gap of components and chips, these are areas most likely to hold contaminants both before cleaning (flux) and after (the solvent or detergent).|
|Other Metallic Surfaces||Copper and silver can be discolored by aqueous/acidic/basic solutions in cleaning, as well as possible oxidation or tarnishing.|
The conformal coating cleaning process should be tailored to both the substrate and conformal coating. Chemistries should be matched to materials, and methods should be matched to substrates.
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Conformal Coating Cleaning Services With HZO
Because conformal coating applications are so diverse, there is no one-size-fits-all cleaning recommendation for every job – cleaning needs are unique. If you need specific, actionable advice from conformal coating experts, reach out to HZO today.
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