Because of parylene's strength, for example, many of the methods used to remove or repair other conformal coatings won't work. You can't simply submerge parylene in a solvent like you might with an acrylic-coated component, for instance. Parylene isn't completely impervious to removal tactics, though. Focused heat, mechanical, and microabrasion methods can all be effective means of parylene removal.
Essential Procedures
Parylene's benefits as a conformal coating are well known. It resists heat, cold, moisture, and pressure; salt spray, electricity, and solvents can't permeate it. And while these attributes of parylene contribute to the conformal coating's appeal, they also present distinct challenges, particularly in regards to parylene removal, rework, and repair.
Because of parylene's strength, for example, many of the methods used to remove or repair other conformal coatings won't work. You can't simply submerge parylene in a solvent like you might with an acrylic-coated component, for instance. Parylene isn't completely impervious to removal tactics, though. Focused heat, mechanical, and microabrasion methods can all be effective means of parylene removal.
Thermal Removal
Although parylene is heat-resistant, it can't always take the heat, so to speak. Localized application of extreme heat, such as with a soldering iron, can, in fact, melt or burn through a parylene conformal coating. However, this parylene removal method is often a poor or risky choice for many applications; the associated extreme heat and prolonged exposure can damage PCBs and temperature-sensitive substrates, for example. After all, parylene has a higher melting point than many plastics.
Parylene removal using thermal means can also cause discoloration and yield residues. Furthermore, it is important to perform thermal parylene removal in a well-ventilated area. For these reasons, thermal methods are often regarded as the least-recommended means of parylene removal.
Mechanical Removal
Parylene also isn't impervious to a dedicated physical attack, and a little elbow grease can go a long way. Mechanical parylene-removal methods such as scraping, sanding, picking, and cutting can be effective. The drawback to mechanical parylene-removal approaches, however, is that parylene conformal coatings can be very thin—sometimes as little as 0.2 mils. Although usually an advantage, parylene's ability to be applied in extremely thin layers renders the coating vulnerable to inadvertent removal of too much material that can consequently damage the underlying substrate or components.
Microabrasion Removal
For most applications, the best parylene removal method is microabrasion. Related to the mechanical process, micro-abrasion is much more controlled, leading to better results. These systems use a stylus with a tiny nozzle to direct a stream of pressurized air and abrasive media at the parylene-coated component. The system can be handheld or automated for finer control. As the abradant gradually wears down the parylene coating, a vacuum captures both the abradant and the removed parylene. This method provides markedly better results than mechanical and thermal parylene-removal methods with less potential for damage.
Alternative Removal Methods
With none of the aforementioned parylene-removal methods providing a perfect solution, researchers have explored additional approaches. Researchers in Germany, for example, have used 193-nm excimer lasers and plasma jets together to not only remove parylene but also to remove the debris generated by the removal process. The combination of the two technologies reduces the need for an additional cleaning process after the removal as well. Another patented process uses a solvent called tetrahydrofuran to soften the parylene coating. While the tetrahydrofuran can't remove the parylene, it does make it easier to remove through mechanical means, although it does carry the risk of spilling over and compromising the coating on other areas of the component.
Avoiding Parylene Rework
Because parylene-removal is such a challenging process, the best plan of action is to avoid having to do it in the first place. Properly preparing the substrate prior to coating yields better adhesion; preparation includes both cleaning the substrate and, if necessary, employing an adhesion promoter such as A-174 silane. In addition to making sure that the parylene coating is applied where required, it is equally as important to prevent "no-coating zones" from being coated. Properly masking your substrate before placing it in the deposition chamber can also help to reduce or eliminate rework.
Removal of Conformal Coatings
Conformal coatings provide reliable protection for substrate surfaces of printed circuit boards (PCBs) and related electrical components. However, these coatings sometimes require removal for repair or other purposes. Many coating materials can be removed by abrasive, mechanical, plasma and thermal techniques, but chemical methods are typically the most popular for removing acrylic, epoxy, silicon, or urethane conformal coatings. Chemical removal does the least damage to PCB components.
However, no single chemical material/process is equally successful for all uses. In some cases, no chemical removal solution is applicable. One might think chemical substances are suitable for removal of superior parylene coatings, since its substrate-application employs a unique chemical vapor deposition (CVD) process. However, this is not so. Chemical removal of parylene is not recommended in the vast majority of cases; other techniques remove the parylene more reliably, with less damage to underlying components.
Are Any Chemical Removal Solutions Available for Parylene?
CVD-applied parylene coatings generate excellent conformal resistance to abrasion, chemicals, humidity, moisture, and extreme changes in temperature. However, chemical inertness is a basic property of parylene, complicating its chemical removal. Under these circumstances, the liquid stripping techniques customarily used for most other conformal coatings are generally ineffective for parylene, a consequence of its exceptional chemical resistance and otherwise inert nature.
Tetrahydrofuran (THF) is a colorless organic compound whose chemical formula is (CH2)40. It demonstrates:
Because of these factors, THF has been successfully used for removal of parylene coatings from substrates. If, for instance, the parylene coating has been applied at a thickness of .001 mm, the coated assembly can be immersed in the THF-based solvent for approximately two to four hours to affect removal. During immersion, the parylene coating begins to separate from the assembly's surface. Physical removal of the parylene with tweezers is enacted after the assembly has been rinsed in alcohol and allowed to dry.
THF is essentially the only reliable chemical base for removing parylene conformal coatings below the melting point. Some success has been achieved using either chloronaphthelene or benzolyl benzoate to affect parylene removal from substrates at temperatures above 150 degrees Centigrade. However, these chemicals are incompatible with most commonly applied parylene processes, offering only very limited use and minimal recommendation. Parylene's chemical inertness restricts chemical removal in virtually all cases.
Recommended Removal Techniques for Parylene Conformal Coatings
Mechanical: Care must be taken using any of the most prominent mechanical removal techniques. These involve cutting, picking, sanding or scraping the precise surface-expanse of coating to be removed. Because parylene's conformal coatings are exceptionally uniform, durable and resist manipulation, careless or inept application of any of these methods can result in damage to the assembly. Nevertheless, mechanical methods can generate very good results for spot-removal, areas where parylene needs to be removed due to:
Masking the surrounding area is always recommended.
Although little clean-up is required, cutting and related mechanical techniques are not reliable options for removing parylene from the entire surface area of any related parylene-coated assembly.
Plasma: Parylene can be readily removed through application of oxygen-based plasmas. For Parylene N, plasma removal is generated by opening of the substance's benzene ring; for Parylene C a similar reaction occurs, differentiated only by the absence of a chlorine atom. In either case the ring opens with introduction of an oxygen radical, causing:
Further plasma manipulation on the radical site expands the ring-opening, accelerating parylene removal.
Other Dependable Methods of Removing Parylene
Abrasion: In many cases, this is the easiest and fastest method for removing parylene conformal coatings uniformly applied to substrate surfaces. Masking the area to be stripped is recommended for spot removal; the objective is to ensure a dependably clean surface-edge when parylene is reapplied.
Thermal: Thermal methods generate high quality deletion of parylene coatings, but their use should be restricted to spot-removal. Application to a whole PCB or similar appliance is not suggested, since their use beyond spot-removal can lead to much diminished control of the process and ruined coatings outside the target area.
Conclusion
Although highly popular for removal of most conformal coatings, chemical removal of parylene is not recommended. Parylene is resistant to dissolution by solvents. Only THF provides a reliable chemical basis for removing parylene coatings from assembly substrates; other options are highly specialized and seldom applied. Of more value are dependable mechanical and plasma-based techniques; these are particularly useful for spot-removal assignments. Abrasion techniques represent a popular removal option; thermal methods have also been successful spot-removing parylene.
Solutions for Removal
Despite conformal coatings’ ability to dependably protect substrate surfaces of printed circuit boards (PCBs) and related electrical components, problems can sometimes occur which compel their removal. Chemical removal, which does the least damage to PCBs, is fine for wet coating substances like acrylic, epoxy, silicon and urethane. Chemical removal methods are far less successful for parylene, despite the use of a chemical vapor deposition (CVD) process for its film application.
CVD processes generate many of the property advantages that distinguish parylene from wet coatings. Parylene offers significant application advantages in comparison to the liquid methods – immersion, spray, etc. – used by acrylic, epoxy, silicone and urethane. Surface tension and gravitational influences effect wet coating methodologies, limiting the capacity to evenly cover all component surfaces. CVD generates uniform, pinhole-free, hermetic and homogeneous coverage of all surfaces with the gaseous parylene, including the smallest corners or crevasses, pointed edges or surface ripples. These properties position parylene as an ideal conformal coating for critical uses in aerospace, medical and microelectricalmechanical systems (MEMS) applications.
In addition to excellent film uniformity, parylene also provides:
But CVD-applied parylene is far less amenable to chemical removal than its wet competitors. There is one major exception.
Tetrahydrofuran (THF), a Chemical Removal Solution for Parylene?
Parylene is chemically inert. This property effectively negates the usefulness of the liquid chemical removal methods that work for the majority of conformal coatings. The one chemical that has been successfully used to strip parylene from substrates is tetrahydrofuran (THF), a colorless organic compound whose chemical formula is (CH2)40. It demonstrates:
Duration of use of THF for removal is largely dependent on the thickness of the parylene film. For example, a parylene thickness of .001 mm requires immersion between 2 – 4 hours in a THF-based solvent. The parylene coating begins to separate from the assembly's surface during immersion. After rinsing the assembly in alcohol and subsequent thorough drying, the parylene film is then removed from the assembly’s surface, physically, with tweezers.
Other than THF, the only other chemicals that have successfully removed parylene coatings are benzolyl benzoate and chloronapthelene, at temperatures above 150 degrees Centigrade. However, these chemicals offer only very limited use for removal of parylene films, since they are essentially incompatible with the majority of parylene processes; except for highly specialized cases, their use is not recommended.
Parylene's chemical inertness restricts chemical removal in virtually all cases. Thus, other removal processes should be employed to ensure complete coating-removal and the security of components underlying the parylene film.
Reliable Methods of Removing Parylene Conformal Coating
Abrasion: Expeditious and cost-effective, micro abrasive removal of parylene films are easy to implement and environmentally friendly. Micro abrasive blasting propels explicit formulas of inert gas/dry air and abrasive media at the parylene-coated component, via a tiny nozzle attached to a stylus; either a handheld human or automated systems can be used to pinpoint the targeted removal area. Conducted within an enclosed anti-static chamber, a vacuum system persistently removes the parylene debris from the substrate, with disposal implemented by filtration processes. Grounding devices dissipate electrostatic potential. Abrasion removes parylene coatings from a single test node, an axial-leaded component, a through-hole integrated circuit (IC), a surface mount component (SMC) or an entire PCB. Abrasion is often the easiest and fastest method for removing parylene conformal coatings uniformly applied to substrate surfaces.
Laser: Typically utilizing pulsed laser sources, laser ablation converts parylene to gas or plasma. Control must be exercised, since each laser pulse separates only a tiny proportion of the film’s material thickness. Nevertheless, ablation is cost-effective for complex removal jobs, since processing can be enacted in a single step. Better quality removal results, with 100% parylene-free areas; photo-ablation particularly delivers excellent outcomes for these purposes. Design compromise is lower than with other removal processes, since laser application can be controlled to a single micron. 3-D devices can also be effectively serviced.
Mechanical: Most mechanical removal techniques -- cutting, picking, sanding or scraping the precise surface-expanse of coating to be removed – require considerable care and attention. The exceptional uniformity of parylene coatings combines with their capacity to withstand manipulation and overall strength to accelerate damage if mechanical processes are imprecisely applied. While appropriate masking can lead to good parylene spot-removal, mechanical techniques are undependable for larger-scale surfaces.
Plasma: Application of oxygen-based plasmas can remove parylene films. For Parylene C and N, plasma removal begins by opening the benzene ring through introduction of an oxygen radical, causing generation of a hydroxyl radical between the polymer chain’s benzene rings. Oxygen absorption at the atomic/molecular level follows, causing development of an unstable peroxy radical, subsequently rearranged into either volatile carbon monoxide or carbon dioxide. Parylene removal proceeds more quickly with additional plasma manipulation on the radical site, growing the opening in the substance’s benzene ring.
Thermal: The thermal parylene coating removal technique (including using a soldering iron to burn through the conformal coating) is the least recommended technique of coating removal. Thermal is difficult to manage. Its use should be restricted to spot-removal; larger-scale removal application can rapidly generate ruined coatings outside the target area, emanating from much diminished process control and emission of toxic vapors.
Summary
THF is the only chemical solvent that consistently provides reliable parylene removal from assembly substrates; the limited chemical options remaining are highly specialized and seldom applied. Abrasion techniques represent a popular removal option; laser methods are expected to develop further as a major removal process for parylene films. Mechanical and plasma-based techniques are useful for spot-removal assignments. Thermal methods also have some use for spot-removing parylene, but are difficult to control.
Points to Consider
Despite parylene’s numerous benefits as a conformal coating, it has several disadvantages that should be recognized before it is used. Failure mechanisms that can emerge from parylene coatings have limited its wider scale application in comparison to liquid conformal films such as acrylic, epoxy, silicon, and urethane. In many situations, wet coatings can provide better performance and lower cost (or both) for many applications.
The Cost of the Parylene Process
The cost to apply parylene conformal coatings generally exceeds that of such liquid coatings as acrylic, epoxy, silicone and urethane. For one thing, the price of parylene dimer, the essential raw material of parylene conformal film, is rather expensive, ranging from $200-$10,000+ per pound, a factor that adds significantly to production expense before the process begins: The average parylene production-run requires a pound of dimer, generating high material costs from the outset, particularly if only a limited number of items are being coated. Because parylene is applied through a chemical vapor deposition (CVD) process, everything is coated. This includes product components such as the inner diameter of a printed circuit board (PCB), which needs to be film-free to work properly. Masking and other corrective/protective procedures need to be implemented to assure unnecessary coating does not occur. The existence of these conditions make parylene an inherently inefficient process and wasteful with production materials, which escalates the end cost to the customer. High capital costs for new production equipment are also common.
Other Parylene Disadvantages
In addition to higher production cost, other disadvantages of parylene conformal films include:
Summary
Parylene’s reputation as a superior conformal coating is well-deserved, but in no way represents performance infallibility. Parylene’s disadvantages can be overcome but need to be recognized, so they can be offset by appropriate attention to such issues as cost, the effects of CVD processing, delamination/adherence, coating resilience, materials’ applicability, and tin whiskers, to ensure the coating’s many performance advantages are accessible and implemented to the optimal degree.
Enacting Parylene Coating Repair
Parylene’s CVD method of application generates exceptionally lightweight yet durable conformal coatings, with superior barrier properties. Compared to liquid processes, the effects of gravity and surface tension are negligible, so there is no bridging, thin-out, pinholes, puddling, run-off or sagging.
Typically, parylene films measure between 500 angstroms to 500 microns in thickness; these ultrathin films eliminate performance complications arising from excessive coating mass or viscosity, conditions that might interfere with components’ function. The resultant protective layers maintain the performance capabilities that distinguish parylene from liquid competitors; for instance, a 25 micron coating will have a dielectric capability in excess of 7,000 volts.
Because parylene provides unparalleled component protection at film thicknesses far thinner than competing wet coatings, it is the coating of choice for many MEMs/nano technologies. Offering unsurpassed protection for numerous current conformal coating applications – aerospace, LED, medical, military, and a range of ruggedized products -- parylene films ensure high reliability and longer life for customer-end products; they diminish total costs for maintenance, repair and replacement, for producers and end-users. However, this does not mean the coatings themselves are flawless; they occasionally require repair.
Enacting Parylene Coating Repair
The unique CVD process that provides parylene with many of its advantages as a conformal coating also serves as a barrier to reworking the coatings. Circuit repair of parylene coatings is characterized by unique issues, emanating from parylene’s divergence from more conventional wet coating types. Reliably strong and hard, parylene coatings are exceptionally difficult to chip away or otherwise mechanically compromise; all covered surfaces are typically resistant when reworking is necessary.
Whereas acrylic- or silicone-based coatings use selective spot- or overall dip-exposure solvent-application methods for ready removal from PCBs, parylene’s specialized CVD coating technique results in film thickness and surface qualities that do not respond to these treatments. Similarly, parylene rework/removal cannot be accomplished as reliably through the thermal/physical deletion techniques typically applied for epoxy or urethane.
Resisting organic solvents, parylene’s removal is further complicated by melting or burning temperatures in excess of 350°C (higher in a vacuum), which generally exceed those of the plastic substances composing at least part of the PCB's structure. Just as specialized vacuum chamber equipment is required to apply parylene conformal films to assembly substrates, equally specialized methods – such as plasma etching or micro-blast abrasion – are necessary to safely remove parylene from the same surfaces. In addition,
Where rework is necessary, incision and removal techniques can also treat the parylene surface. This is a mechanical process involving cutting into the treatment surface to initiate separation of the film from the substrate, followed by lifting the damaged parylene with tweezers or a similar device. Although incision removal is often manual, it is suggested operators wear protective gloves. Human flesh contacting the surface to-be-reworked will invariably leave a residue of bodily fluid (oil, perspiration, etc.), which will require additional specialized removal, further complicating the rework process.
It should be noted that, in many cases, repair does NOT entail stripping the board entirely. It is more likely that spot removal of the parylene coating will be required for localized repair of a minute assembly component; typically, the item needing repair is so small that stripping the entire board is unnecessary, costlier and time-consuming. Under these circumstances removal is readily accomplished by using a soldering iron to burn through the parylene surface, affecting the repair, and touching-up the area with a liquid conformal coating; urethane is used most frequently under these circumstances. Of course, care must be taken: Localized application of extreme heat, such as with a soldering iron, can, in fact, melt or burn through a parylene conformal coating.
Whatever technique is employed, the objective is to strip the damaged parylene from the substrate and efficiently replace it to a state where assembly function can be maintained. Reworked/removed parylene cam frequently be recoated to original specifications.
Summary
Durable and heat/solvent-resistant, parylene is difficult to remove or rework. CVD generated substrate penetration significantly increases problems simply peeling the parylene film from the surface. Parylene can be removed for PCB rework by one of several methods. Among the most common removal techniques are, incision/deletion, laser ablation, mechanical or micro-blast abrasion, plasma etching, and thermal softening.
The optimum process of removal is determined by the nature of the device and the particular needs of the client or end-user. Perhaps the best approach is avoiding the need to rework, by appropriately:
Furthermore, after parylene removal, it is essential to develop machining parameters that will not cause damage to the specific PCBs being reworked. In this respect, thoroughly reviewing data sheets and related information for all assembly components requiring rework is recommended; equally important is establishing consistent performance standards for electrostatic discharge (ESD, a source of electrical shorts or dielectric breakdown), process management, and working temperatures. Rework that proceeds without adhering to these basic guidelines can become the source for future rework. Every case is different, so the rework process will require controlled customization. However, in many cases, areas where parylene has been removed can be recoated to conform to original design parameters.