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Adhesive Matters: The importance of surface preparation for effective adhesion

A surface free from contamination is essential to securing a strong adhesive bond – but how can this be achieved?
The answer, quite simply, is through effective surface preparation. Let us take a look at different surface preparation products and techniques; from cleaners and degreasers – to primers and flux removers.

 

Abrasion

Mechanical abrasion is one of the most commonly used surface preparation methods and is suited to most materials.

The main objective of mechanical abrasion is to remove weak boundary layers, however this approach will also change the topography of the adherent, consequently increasing the bondable surface area. In addition, mechanical abrasion will enhance the adhesive’s ability to fully wet the surface.

Popular abrasion methods include sandblasting, vapour honing and the use of abrasives or cleaning and finishing materials.

Sandblasting with fine sand can only be used on substrates sufficiently thick to prevent distortion while vapour honing is satisfactory when minimum reduction is desired in metal thickness.

 

Cleaners and Degreasers

As the product name would suggest, cleaners and degreasers have been created to remove greases, oils, lubrication fluids, metal cuttings and fines from the surface that is to be bonded.

Cleaners and degreasers are highly effective and are available in both aqueous and solvent-based formulations.

Loctite 7063 is an example of a solvent based formulation. As a final pre-assembly cleaning treatment, this hydrocarbon-based, non-CFC solvent is designed to be used as a spray or in immersion cleaning processes. On the other hand, Microcare MCC-BGA is an aqueous formulation. This stencil cleaner will remove all types of solder paste as well as uncured chip bonder and inks. Safe for use on webbing, frames and misprinted boards, MCC-BGA provides fast and thorough cleaning – even inside stencil apertures.

When choosing a cleaner or degreaser, the major factors to consider are dry time, residue, odour and substrate compatibility. Residue is a particularly important concern because if there is any secondary processing of the part, e.g., painting or bonding, a residue could interfere with that process. Substrate compatibility is another common concern when dealing with plastics and solvent-based cleaners.

For precious metals and jewels, degreasing will generally be entirely satisfactory, with the possible exception of silver where the tarnish should be removed with medium grit emery paper. A stabilised Trichlorethylene vapour phase degreaser is recommended.

 

Primers

Primers are designed to enhance the bonding quality of adhesives. When applied as a surface coating, they will change the base of a surface to enable more effective adhesion.

The requirement for a primer is dependent on the material – for example a primer will be required when adhering to any porous substrate. This is because a pourous substrate will eventually absorb the adhesive that would normally create a long lasting bond. By using a primer, the adhesive’s ability to wet out the surface is significantly increased, thus solving the problem.

Plastic is another material requiring a primer – though the type of primer needed for this material can be very specific. For example, polyolefin requires a special polyolefin primer. Amongst a range of other primers, Henkel manufactures Loctite 770, a primer that has been designed to prepare the surface of difficult to bond plastics just like polyolefin and other low energy surfaces.

Another key priming product is Dow Corning PR-1200. Dispersed in naptha, PR-1200 is a one part primer available in clear or red that can be used on most metals, glass, ceramics and masonry to enhance adhesion. Typically applied in a thin layer, PR-1200 and indeed other Dow Corning primers, are dilute solutions of silane coupling agents and other active ingredients. These products should be applied in a very light, even coat by dipping or spraying and excess material should be wiped off to avoid over application.

 

Accelerators and Activators

Both adhesive accelerators and activators are applied ahead of the adhesive act as catalysts. While both product types are designed to increase cure time, activators also improve the quality of adhesion.

One of the most commonly used adhesive activators is Loctite 7455 – used for applications requiring an increased cure speed. While this activator can be applied before or after the bond, it is particularly suited to post-application on cyanoacrylate adhesives, since it ensures rapid fixturing between close fitting parts. Typical applications for activators include securing wires or coils to PCB’s, tamper proofing adjustable components and mounting stand off’s, edge guides and board stiffeners.

Moving onto accelerators, while this product type is used to speed up cure time, users should be aware of the ‘trade off’ in the bond strength. For example, when bonding a very porous material with a cyanoacrylate, the application of an accelerator may increase adhesion by causing the cure before the adhesive is fully absorbed. Where possible it is therefore better to use an activator than an accelerator.

 

Flux Removers (or defluxers as they are sometimes known)

Flux removers are designed to remove flux residues and other contaminants that may be found on printed circuit boards due to rework, repair or manufacture.

Used before the application of an encapsulant or conformal coating, flux removers play a critical part in ensuring reliable circuitry.

Microcare MCC-PW2101 Power Clean is a flux remover that promises to deflux high temperature lead-free solder residues. Fast drying and non-flammable, this PowerClean product will even clean oils and inks.

Another popular Flux Remover is Microcare MCC-UFR107 Universal Flux Remover which is designed to safely remove contamination and soils that can impact the functionality and reliability of electronics. The fast drying and low aroma properties make it ideal for cleaning electronics during rework and repair, spot cleaning or in-field service repairs.

 

Conclusion

Surface preparation is undoubtedly an important process for effective adhesion, however the approach to this process, cannot be considered as ‘one size fits all’.

Important factors to be considered include the application in question, material type and the form of contamination. The key surface preparation products and techniques have been outlined above, along with an explanation of their suitability for certain materials and applications.

If you have any further questions on this topic please don’t hesitate to Ask The Glue Doctor – who will be happy to answer all of your technical questions.

At Ellsworth Adhesives Europe, we can proudly say that we supply adhesive solutions to almost all of the industrial markets including aerospace, military, construction, white goods and LED and lighting, however there is one market that we know more intimately than most and that is the electronics market.

As a distributor of adhesives and adhesive dispensing equipment, we represent a number of key manufacturers within the industry – and many of these manufacturers have product ranges specifically designed for the electronics sector such as Henkel Loctite, Dow Corning and Robnor Resins.

Henkel is the world’s leading and most progressive manufacturer of qualified materials for the electronics industry, including semiconductor packaging, printed circuit board (PCB) assembly and advanced soldering solutions. There are hundreds of different ways in which Henkel electronic adhesives can enhance the quality and efficiency of electronic components.d

The Technomelt range from Henkel, for example, provides protective encapsulation for electronic components, making it the perfect adhesive for low-pressure moulding applications. Henkel Technomelt, available from Ellsworth Adhesives Europe, is an innovative hot melt adhesive that both bonds and protects. In the low-pressure moulding process, the components are placed without a housing in a mould and encapsulated with the hotmelt. Once the moulding has cooled and solidified, the components – now provided with a protective outer shell – are ready for further processing. Hotmelt moulding uses one-part adhesives, resulting in a simple, very fast and clean production process. The low application pressure ensures that the process is gentle and therefore suitable for encapsulating even the most small and delicate components with a hotmelt adhesive.

Like Henkel, Dow Corning has a portfolio of adhesives tailored to suit the specific requirements of the electronics market. Dow Corning is a globally integrated provider of materials, application technology and services. A key benefit of Dow Corning adhesives is their potential to significantly increase reliability of the electronic module or component because of their stress-relieving properties. Dow Corning adhesives have a “rubbery”, elastomeric nature and are not traditional, highly rigid adhesives. This low stress environment can reduce damage to module components and interconnections.

Dow Corning manufactures a variety of noncorrosive, thermally conductive silicone adhesives that are ideally suited for use in bonding hybrid circuit substrates, power semiconductor components and devices to heat sinks as well as for use in other bonding applications where flexibility and thermal conductivity are major concerns. The flowable versions are also perfect for use as thermally conductive potting materials for transformers, power supplies, coils and other electronic devices that require improved thermal dissipation.

Launched last year, TC-2030 from Dow Corning offers outstanding thermal conductivity and excellent elastomeric properties after aging, providing long-term reliability. Good adhesion to various substrates, such as anodized or cast aluminium, tin-plated copper and printed circuit boards, make this material well suited for use across a wide variety of electronics markets. TC-2030‘s thermal conductivity of 2.7 W/mK is higher than traditional thermally conductive adhesives and with a viscosity of 190 Pa.s, it also offers good dispensing characteristics and can be easily applied by a dispenser once the two-part formulation is mixed. The electronics market has shown a need for improved heat dissipation in printed circuit board applications with high power components and Dow Corning’s high thermal conductivity TC-2030 provides manufacturers with a viable option to address this need.

Moving from silicone adhesives to polyurethanes, for our final application focus we turn to Europe’s leading formulator of epoxy and polyurethane resin systems, Robnor ResinLab.

Robnor’s electrical and electronic polyurethane range offers a comprehensive choice for users who have a variety of requirements for their application. Robnor polyurethanes are used in applications demanding economy, toughness, high insulation and thermal impact resistance. Furthermore, this range of polyurethanes can be used as an alternative to epoxy and silicone materials and offers lower unit costs and faster production while providing excellent general performance.

One of the most common applications for Robnor polyurethanes is potting for electronic devices. Robnor EL199HP is used by a smart metering solutions provider for potting its electronic RFID transmitting data devices in mechanical water metering systems. EL199HP provides a low cost integral solution with resin and dispensing equipment thanks to its low viscosity, low hardness and high flexibility. EL199HP offers good adhesion to ABS & FR4 and is also UL94-VO compliant.

Our Sales Engineers could fill a whole series of books with their knowledge of adhesives and industrial applications – this small snapshot of their experience in the electronics industry is just the tip of the iceberg. Please contact the Ellsworth team with any technical questions you may have on your own specific application and our Sales Engineers will be happy to assist.

Encapsulants are a staple material for designers of electronic devices, providing both insulation and protection for electrical and electronic components. Typically this material is applied in thick layers and completely embeds the electronic circuitry, keeping it safe from the harmful effects of moisture and other contaminants, through isolation. Encapsulants can also protect the circuit from thermal and mechanical stresses and provide electrical insulation for high voltages.

With a material this versatile, you might be fooled into thinking you can’t go wrong, however selecting the correct encapsulant is crucial to securing the results you desire. As a distributor of adhesive materials and adhesive dispensing equipment, we have an extensive portfolio of encapsulants from a range of industry leading brands. We are happy to share with you our expertise on this electronic material and can provide unbiased advice on the different encapsulant options available, helping you to decipher which one is right for you.

 

Encapsulants Types

There are a number of different encapsulants on the market, from potting and moulding compounds to glob top and chip on board encapsulants. These product categories can then be further broken down into material type, with the main chemistries including silicones, epoxies and urethanes. The choice of material type depends on the application for which it is required. Generally, most silicone, epoxy and urethane systems offer the electrical and environmental protection required for ambient conditions. The difference between encapsulant chemistries has more significance as requirements become more complex and extend beyond the basic ambient protection all encapsulants provide.

 

Material Choice

Silicones have several qualities which are desirable for encapsulating circuits including minimal shrinkage, deep section cure, repairability, excellent dielectric properties and the ability to cure to a flexible, stress-relieving elastomer.

Silicone encapsulants are considered to be safer than urethanes and epoxies, which carry certain risks including potentially harmful ingredients and the possibility of exotherm. Another great benefit of this material is its wide range of operating temperatures which stretches from -45 to 200 degrees.

Silicones are usually easy to mix two-component systems, consisting of the resin itself as well as a catalyst such as hardener or accelerator. Dow Corning has a range of silicone encapsulants which are supplied as solvent less, two part liquid component kits in convenient ratios. Some are designed for 1:1 mix ratios with parts A and B, while others are formulated for mixing at 10:1 ratios with a base and curing agent.

In contrast to two component silicone encapsulants, epoxies do not require primers. Epoxy encapsulants are suited to a range of applications thanks to their ability to adhere unaided, to a range of substrates. Epoxies offer superior resistance to environmental hazards such as high temperatures and chemical exposure. Furthermore they are processed easily and are friendly to the environment and the workplace.

Robnor ResinLab has a range of epoxy encapsulants used to encapsulate electronics. Robnor ResinLab PX700K-1 for example is commonly used by manufacturers requiring excellent physical and chemical resistance qualities, while Robnor PX439XS is the encapsulant of choice for users requiring excellent heat resistance and dimensional stability. In addition to this, Robnor PX439XS has low running costs and does not require any oil or oil spillage contaminant infrastructure.

Urethanes and polyurethanes are the final material to be considered, these encapsulants provide a good balance of durable protection and flexibility making them the material of choice for applications requiring elongation and abrasion resistance. Generally, urethane encapsulants are much softer than epoxies and can cure in a wide variety of gel times with lower stress than an epoxy. Additionally, urethane encapsulants are the preferred potting compound for adhesion to plastics. Robnor Resinlab has a range of urethane encapsulants that perform well in low temperature applications (up to -40C) as they maintain a high degree of flexibility.

 

Encapsulant Variations

Moving on from the material composition of encapsulants, let us explore two further variations on this complex yet fascinating product type; UV curing pottants and chip on board encapsulants.

UV curing pottants are the encapsulant of choice for users looking to increase production throughput since they cure in seconds upon exposure to UV light. UV potting compounds are ideal for shallow component potting and sealing of connectors, inductors, detectors, capacitors, RF circuits, relay screws, sensors, tamper proofing, and PCB encapsulation. Dymax, the leading manufacturer of UV curing materials and systems, has a full range of light-cure potting materials designed to cut the costs and processing times associated with microelectronic assembly. Dymax Series 9000 is a selection of single part encapsulants offering high ionic purity and excellent adhesion and resistance to humidity, while the Dymax 9100 series comprises a range of resilient chip-encapsulant materials designed with a light and secondary ambient moisture cure system to tackle shadow areas.

If strength is a key application requirement, Chip on Board encapsulants could provide the solution. This encapsulant type is specially designed to boost mechanical strength and provide environmental protection to wire bonds, leads and aluminium. Henkel Loctite has two ranges of chip on board encapsulants, Loctite Hysol and Loctite Eccobond. These Henkel ranges are favoured by manufacturers seeking outstanding performance and high throughput for low cost assembly processes. Henkel encapsulants provide reliable protection and value in the same package and are available as UV curing or thermal materials.

 

Conclusion

Now that the main encapsulant types and their properties have been established, it should be much easier to find the product suited to your requirements, however it is important that these requirements are carefully evaluated. Ask yourself: What is essential to my application? Consider elements such as flexibility, strength, cure time, budget and temperature sensitivity. By doing this, your search should be narrowed down considerably.
With so many encapsulant products in the market, even if your needs are demanding, there is bound to be at least one product that ticks all of your boxes. And remember, if in doubt, don’t hesitate to Ask the Glue Doctor who will be happy to help!

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