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Substrate Cleaning - Imperative for Effective Parylene Coating
by Gustavo Arredondo, Technical Manager at Para Tech Coating
Complex electronic circuit assemblies that are destined to operate under rigorous conditions require effective electrical, chemical, vapor and moisture isolation. Parylene coating provides these protective properties as a very thin, low-mass encapsulating film. However, the durable protective value of Parylene depends on the film adhesion, which in turn depends strictly on the cleanliness of surfaces to be coated.
Cleaning-related issues may not be immediately apparent in assemblies ready for coating, but the long-term consequences of contamination may degrade the mission of the systems in which they are used, and are costly to resolve. The failure of critical assemblies can also seriously damage the reputation of an equipment supplier, and have a negative impact on future business.
Unlike typical liquid coatings that are applied by dipping or spraying, Parylene coating is accomplished in a vapor phase chemical vacuum deposition process. This involves cross-link polymerization at the molecular level, with a powdered raw material converted to gaseous form and then directly polymerized as a transparent film on assembly surfaces. There is no intermediate liquid phase. The finished film is absolutely conformal, since it develops with equal thickness and at a fixed rate on flat areas as well as around edges and in crevices.
Organic or non-organic production-related contaminants remaining on assemblies can disrupt the Parylene film-to-surface bond, causing de-lamination and compromising coating protection. Additionally, substrates with outgassing components can leach out residue during the deposition process, cross-contaminating surfaces and preventing a direct bond with the reactive monomer.
Following Industry Standards
Visual inspection alone is not sufficient to confirm the readiness of an assembly for Parylene coating. It is crucial to consistently measure, monitor and inspect every production run in order to avoid costly cleaning issues. CWST follows a sampling process of 10% of assemblies in a production run to confirm the readiness of a customer’s assemblies for Parylene coating, per IPC-J-STD-001.
This in-process circuit assembly handling seeks quality and reliability levels that support uninterrupted operation in the harshest environments. The IPC-J-STD-001 target for surface cleanliness is 10µgm NaCl/in2 or less. Surprisingly, this standard of performance is as important to reliability in demanding commercial applications as it is for military and aerospace assemblies. A compromise of this standard puts assemblies at risk and is not advised, even for commercial applications. If contamination problems are indicated during sampling, the customer is so advised, and CWST can assist with failure analysis, testing, and defining optimal cleaning techniques.
The IPC-J-STD-001 target for surface cleanliness is 10µgm NaCl/in2 or less. This in-process inspection for circuit assembly handling seeks quality and reliability levels that support uninterrupted operation in the harshest environments. The IPC-J-STD-001 target for surface cleanliness is 10µgm NaCl/in2 or less. Surprisingly, this standard of performance is as important to reliability in demanding commercial applications as it is for military and aerospace assemblies. A compromise of this standard puts assemblies at risk and is not advised, even for commercial applications. If contamination problems are indicated during sampling, the customer is so advised, and CWST can assist with failure analysis, testing, and defining optimal cleaning techniques.
Assemblies produced with a so-called ‘no clean’ flux may appear to be contaminant-free, but residues generally remain that will compromise results. The term “no-clean” is a misnomer as it applies to the standard of performance required for Parylene. Remnants of no-clean flux are difficult to inspect visually, but will turn white in an alcohol bath used to prepare substrates for Parylene deposition. Dissolved residues will contaminate subsequent assemblies and create rework issues and production delays.
Why Cleanliness is Important
There are two primary classifications of printed circuit board contaminants: organic and non-organic residues. Trace organic remnants act as natural release agents, disrupting the bond between Parylene film and underlying surfaces. This compromised adhesion may not be immediately apparent, but the film can eventually delaminate as a result of thermal cycling or mechanical stress, thus neutralizing the protective benefits of the coating. Non-organic residues are electrically conductive, and can form current leakage paths under the coating that will disturb the functions of an electronic assembly.
The most common challenge to Parylene film adhesion on circuit assemblies is flux residue. Other, less common but serious contaminants include various chemical residues, halides, leached plastics, waxes, light hydrocarbon and silicone oils, adhesive residues, dust and fingerprints. CWST routinely conducts a rudimentary alcohol and de-ionized water wash sequence on all assemblies to be Parylene coated to remove minor oils and fingerprints. However, sophisticated chemical analysis is required to detect and identify other contaminants.
Ionic Exchange Chromatography is an effective analytical technique for qualifying inorganic contaminating residues such as chloride, fluoride, potassium and sodium. This technique separates ions and polar molecules based on electrical charge, identifying specific contaminants so that an appropriate solvent and cleaning system combination can be selected.
Organic contaminants such as silicon oils and mold release agents require a separate technology for detection and identification. Fourier Transform Infrared Spectroscopy (FTIR), a complex analytical instrument, can differentiate between organic and inorganic contaminants, and then identify specific organic chemical compounds. FTIR testing is based on comparing spectrum results to a database of known materials.
Gas Chromatography testing is also used to detect and identify organic contaminants. This process is capable of separating an unknown liquid mixture into its individual components and specifying each component. The process involves volatilizing a test sample in a carrier gas stream. A variation of gas chromatography combines this regimen with mass spectroscopy, offering the additional ability to characterize unknown, complex organic chemical mixtures.
Cleaning Options - Methods and Materials
There are numerous cleaning agents that can be used once contaminants have been precisely identified. The appropriate solvent can be selected for use in a spray, solvent immersion, ultrasonic, vapor-degreasing, media blasting, tumbling, or even hand-wipe cleaning process, depending on the nature of the cleaning challenge.
Simple detergent cleaning is generally adequate for soluble contaminants, while less soluble materials require more complex solvents. The objective is to use cleaning materials that are non-hazardous, non-toxic and VOC free. The cleaning technique can be solvent-based, semi-aqueous based, aqueous based, or solvent/aqueous based.
In the past, solvent options were limited to environmentally objectionable fluids such as chlorofluorocarbon or chlorinated hydrocarbon fluids. Fortunately a number of advanced cleaning agents and recycling methods have been developed in recent years that safely and effectively address contamination challenges in electronic manufacturing. Solvent alternatives include non-chelating, non-phosphate, biodegradable cleaners, non-chlorinated solvent cleaners, non-glycol based solvent cleaners, hydrocarbon based formulations, methyl ethyl, isopropyl, aqueous-based alkaline cleaners and deionized water.
Circuit cleaning is a multi-step process, and may be required prior to stenciling and solder reflow, following reflow, and after any post-reflow activities, including any unscheduled rework. At every step, it is imperative to confirm solvent compatibility with all devices and components in the electronic assembly to avoid solvents that are conductive or corrosive.
Designing for Success
Conventional SMT circuit assemblies present fewer cleaning challenges than more complex devices with soldered assemblies added after the SMT reflow and cleaning steps, although even basic surface-mount components have the potential to trap residues. Items such as protective cages or heat sinks may include small gaps, crevices or other physical features that entrap flux residues.
Attention given to the potential for hidden residue entrapment during the assembly design phase will help minimize contamination issues in production. Careful design can save downstream costs and reduce potential warranty issues. Some manufacturers find it useful to conduct detailed cleaning and contamination review at the prototype level, well before an assembly reaches full production.
Careful post-production cleaning is critical for every assembly, especially complex devices. Manufacturers need to determine through experience and measurement how best to orient assemblies in the cleaning chamber, and where to locate and direct spray nozzles so as to dependably remove contaminating residues.
Reference: A study titled “Solvent Substitution For Electronic Assembly Cleaning” has been conducted by Sandia National Laboratories (M.C. Oborny, E.P. Lopez, D. E. Peebles and N. R. Sorensen) to analyze alternate solvents for a range of circuit assembly contaminants.