Silicone Conformal Coatings for Solventless Curing and Excessive-Throughput Meeting

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By
Brian Chislea & Erica J. Everett, Dow Shopper Options 

11.15.2020

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From smartphones and pill computer systems to battery chargers and car electronics, printed circuit boards (PCBs) include digital parts which are turning into more and more smaller, extra delicate and positioned extra tightly collectively. But it’s extra than simply PCB design guidelines which are altering. The excessive humidity, thermal biking, airborne contaminants and bodily stresses that had been as soon as related primarily with navy and industrial electronics are actually challenges for client and automotive electronics as properly. Whether or not it’s a cellphone inside a backpack or a PCB meeting inside a automobile, electronics might be in danger.

Pneumatic spray gun is used to use ultraviolet-cure conformal coating (3-6714 UV Conformal Coating or 3-6748 UV Conformal Coating) to an digital circuitboard.

To offer environmental and mechanical safety, conformal coatings are utilized to PCBs and different digital substrates. These skinny, polymeric movies are used with inflexible and versatile circuit boards, delicate digital parts resembling built-in circuits (ICs), and fantastic pitch designs with tight fabrication tolerances. Usually, conformal coatings aren’t utilized to radio frequency (RF) antennas and different board-level parts which are delicate to the extra capacitance of an added protecting layer.

Some conformal coatings are 1-part techniques that require the usage of solvents, chemical substances which will include risky natural compounds (VOCs) that pose potential environmental well being and security (EHS) considerations. There are additionally 2-part solvent-borne techniques, however they require mixing. At this time, many producers favor 1-part solventless silicone conformal coatings that velocity manufacturing and assist sustainability initiatives. By combining the optimum coating chemistry with the correct utility technique and curing course of, it’s potential to realize greater throughputs and better vitality effectivity.

Design and manufacturing

PCBs can fail for quite a lot of causes, however moisture and contamination are two of the commonest causes. If moisture accumulates on a board’s floor or inside a void within the conformal coating, corrosion and electrochemical-mitigation (ECM) between conductors might happen. Tin whiskers and dendrite development are much like ECM. Creep corrosion, the migration of corrosive merchandise on the floor of a PCB, also can trigger electrical shorting and present leakage. Inadequate cleansing or the improper utility of conformal coatings that ends in inadequately protected PCB areas are sometimes guilty. As well as, airborne contamination from mud or particles can penetrate insufficiently coated areas.

At this time, many client gadgets are anticipated to supply improved water resistance, a longtime requirement in automotive functions the place there may be routine publicity to climate circumstances. Within the automotive trade, vital techniques resembling steering and braking should present reliable efficiency. Silicone conformal coatings have been validated by the automotive trade to exchange dam and fill or PCB encapsulation, a protecting approach that encloses a whole machine to type a “brick”. By utilizing conformal coatings as a substitute of PCB encapsulants, automotive engineers can allow modular designs and assist car mild weighting with out sacrificing efficiency.

Mild weighting is very necessary to producers of electrical autos (EVs) and hybrids due to considerations about vary or mileage. Decreasing or eliminating heavy PCB housings will help, however not if PCB safety suffers. Silicone conformal coatings which are utilized in a number of layers can function the first protection towards environmental contamination. They’re additionally utilized in client home equipment resembling dryers, the place warmth and steam are elements. Transportable gadgets resembling smartphones and family gadgets resembling televisions aren’t utilized in similar environments, but each might have to withstand the excessive humidity of the tropics or the dusty circumstances related to desert environments.

Correct conformal coating choice and utility can mitigate these and different dangers, however it is very important use merchandise that assist each present applied sciences and rising designs. For instance, as extra fifth technology cell networks are deployed, extra digital designs will characteristic 5G electronics for quicker connections, greater throughput, and better capability. But, the better energy densities related to 5G electronics produce greater ranges of warmth, which may scale back the lifetime of PCBs, solder joints, and parts. Silicone conformal coatings present dependable safety towards excessive temperatures, assist high-throughput meeting, and supply many different benefits.

Getting began with conformal coating choice

When choosing conformal coatings for digital gadgets, it is very important begin with a well-balanced chemistry. Acrylic, urethane, polyurethane, epoxy, and parylene conformal coatings all supply benefits however include important trade-offs. For instance, acrylic resins present good humidity resistance however have a slender service temperature vary. Urethane and polyurethane resins are arduous and abrasion resistant however can increase well being and security considerations throughout processing. Epoxy resins supply good resistance to humidity however are tougher and extra inclined to cracking and delamination. Parylene is chemically inert however helps restricted throughput as a result of its deposition system is proscribed by chamber measurement.

In contrast, silicones are well-balanced. They resist chemical substances, moisture and contamination and are comfortable, versatile, stress relieving  and appropriate for high-throughput manufacturing. Their hydrolytic stability is vital as a result of 5G would require extra towers, base stations and transmitters, costly belongings which are uncovered to outside climate circumstances resembling excessive humidity and rain. In comparison with different conformal coating supplies, silicones enable water vapor to pass-through extra rapidly; nevertheless, it’s liquid water – not water vapor – that causes corrosion. Over time, different forms of conformal coating supplies have a tendency to soak up extra water than silicones, which may improve the danger of corrosion.

Importantly, silicone conformal coatings resist greater temperatures and supply longer-lasting warmth resistance and not using a important loss in properties. With their low modulus, silicones also can take up among the stress that outcomes from the fabric enlargement and contraction that happens at totally different charges when supplies have totally different coefficients of thermal enlargement (CTE). In contrast, urethanes and epoxies have a better modulus, greater glass transition temperature and are much less stress-relieving. Silicone-based coatings additionally present good adhesion to many substrates, typically with out the usage of a primer for better manufacturing effectivity.

Formulations and automation

Traditionally, many conformal coatings have been formulated with solvents resembling benzene, toluene, ethylbenzene or xylene (BTEX) that change into volatilized and produce doubtlessly dangerous airborne emissions. At this time, solvent-less silicone conformal coatings are serving to the electronics trade to handle each EHS and environmental sustainability considerations. By lowering or eliminating VOC emissions, these solventless merchandise additionally scale back the danger of fireside since many solvents are flammable or flamable. Solvents are additionally intently regulated and could also be topic to greenhouse gasoline reporting necessities.

The usage of solvents in conformal coatings also can complicate waste administration and disposal for amenities managers. In keeping with the U.S. Environmental Safety Company (EPA), solvent wastes are sometimes categorised as flammable supplies. Due to this fact, stable absorbent materials such because the wipes that come into contact with flammable solvents have to be disposed of as hazardous flammable stable wastes. Used applicators, unused options and supplies leftover from cleansing also can increase waste-related considerations. These challenges aren’t simply restricted to North America however are of accelerating concern globally.

Meeting line for motherboard manufacturing

Within the European Union, Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Waste (WEEE) are simply two of the directives that apply to digital content material. Relying on the character of an digital machine or product, the EU’s Finish of Life Automobiles (ELV), Power-using Merchandise (EuP), or Registration, Analysis, Authorization and Restriction of Chemical substances (REACH) directives may apply. As extra nations, together with China and South Korea, limit the usage of BTEX solvents, world producers are in search of to reduce the dangers related to prohibited supplies. Solventless conformal coatings will help with materials content material declaration necessities and assist design for atmosphere (DfE) concerns.

Solventless conformal coatings which are manufactured from silicone assist sustainability initiatives and may enhance manufacturing effectivity. As a result of there aren’t any solvents to evaporate, solventless silicone conformal coatings keep their utilized thickness with solely minimal shrinkage and might be utilized in a single move. In contrast, different forms of conformal coatings with a excessive solvent content material require considerably thicker layers. Solventless silicone coatings are additionally out there in decrease viscosity supplies for quicker stream charges throughout automated spraying or needle allotting. These options handy spraying assist the upper volumes of electronics related to client gadgets and automotive manufacturing. Importantly, automated allotting additionally gives quick, constant, and dependable outcomes throughout shifts or manufacturing places.

Curing strategies

Curing gives the electronics trade with a further alternative for price financial savings and effectivity good points. Conformal coatings use 4 important curing strategies: evaporative, moisture, warmth and ultraviolet (UV). Evaporative curing and moisture curing sometimes happen below ambient circumstances and don’t require the usage of specialised tools. Thermal curing makes use of infrared lamps or thermal ovens to provoke or speed up curing. Ultraviolet curing makes use of a UV mild supply and, sometimes, a secondary curing mechanism for areas of the meeting that had been shadowed and didn’t obtain direct UV publicity.

Evaporative drying or curing happens at room or near-room temperatures and is a prolonged course of which will require important house for work-in-process. Sometimes, PCBs are dipped at the least twice to realize the specified thickness on part edges. When the liquid service evaporates, the coating resin is left behind. With solvent-borne conformal coatings, air flow and fume extraction could also be required for evaporative curing. Services may want particular allowing and a red-label room for making use of solvent-borne merchandise.

Moisture curing makes use of the moisture within the air to provoke the curing course of. These formulations might be solventless or solvent-borne. For solvent-borne formulations, service solvents evaporate whereas atmospheric moisture reacts with the coating. Since moisture curing requires a minimal of 30 p.c relative humidity, amenities in chilly or dry climates sometimes require humidification, a further working expense. Relying on the character of a conformal coating, moisture curing can be utilized alone or together with one other main curing technique, resembling UV mild.

Warmth curing topics conformal coatings to excessive temperatures in order that the utilized warmth initiates the chemical crosslinking of the polymer. As a result of warmth curing requires your complete meeting to succeed in the prescribed treatment temperature earlier than the treatment time begins, important time and vitality are required. Remedy temperatures and durations differ, and a few conformal coatings now supply fast curing at greater temperatures of 150°C. Though warmth curing improves wetting and lowers viscosity, it is very important keep in mind that excessive temperatures can injury some digital parts. Warmth curing is commonly used alone however it may also be used together with one other curing mechanism, sometimes to speed up curing.

Ultraviolet curing requires irradiation at a particular wavelength and vitality depth. This course of could be very quick and happens at room or near-room temperature. In comparison with thermal ovens, UV curing tools has a smaller footprint and makes use of much less vitality. To succeed in shadowed areas the place UV mild can’t penetrate, or UV curing is in any other case incomplete, a secondary moisture treatment is commonly used. Producers have a selection of solventless silicone conformal coatings that use this twin treatment mechanism, however most of those merchandise don’t have a low sufficient viscosity to assist spray coating for high-volume manufacturing.

Printed circuit board

A more recent, solventless conformal coating that’s manufactured from one-hundred p.c silicone combines a low viscosity for automated spraying with a quick, main UV treatment that happens in below a minute with broad-spectrum UV mild. This product’s secondary moisture treatment avoids potential points with areas which are shadowed, or hidden, from the UV mild throughout curing. The low modulus that characterizes this solventless silicone coating protects delicate parts and its elastomeric formulation improves reliability towards stress. This modern merchandise additionally has a UV indicator for automated inspection throughout high-throughput manufacturing.

Extra concerns

Electronics producers typically want conformal coatings that meet UL-746E, a security customary from Underwriters Laboratories (UL) for supplies utilized in PCBs. They might additionally want conformal coatings that meet the UL-94 flammability ranking for plastic supplies, a class that features silicones. As well as, the trade needs merchandise that assist meet the necessities of IPC CC-830, a worldwide customary for coating efficiency capabilities, high quality and consistency. This self-certifying customary permits an electronics producer to verify {that a} conformal coating meets baseline electrical efficiency ranges for defense from moisture and contamination.

Additionally necessary is a conformal coating’s means to move environmental testing and environmental biking, humidity testing and water immersion testing. If a standardized check technique doesn’t exist, electronics producers might modify automotive and even aerospace requirements to fulfill the wants of client gadgets. Deliberate obsolescence is a factor of the previous, and smartphone customers and automobile patrons alike need security, efficiency, and reliability that lasts longer than simply the guarantee interval.

At this time’s electronics producers have a bunch of considerations. The supplies which are used to handle these challenges should present safety towards moisture and contamination whereas supporting high-throughput meeting and environmental sustainability. As a result of warmth is the enemy of dependable electronics, 5G’s better energy densities and elevated temperatures are of rising concern. Power-efficiency, employee security, and assist for automated inspection are additionally high priorities.

Conformal coatings will help. When choosing these merchandise, digital designers and engineers have a selection of chemistries, formulations, utility strategies and curing techniques. An modern solvent-less silicone conformal coating with UV and moisture twin treatment will help high-volume producers to fulfill a number of necessities. Many different merchandise are additionally out there. The best accomplice will help with product choice and utility improvement whereas persevering with to supply modern applied sciences like solventless silicone conformal coatings for high-throughput meeting.

— Brian Chislea is senior technical service and improvement scientist, and Erica J. Everett is advertising supervisor for transportation and mobility at Dow Shopper Options

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