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Better Consumer Electronics Reliability: Coatings and Adhesives

Written by Pierce Geary

In a recent study conducted by Instrumental, the top ten most common manufacturing defects were examined. The number one defect that manufacturers fight against is a deficiency in glue.

It’s a simple fact that electronic products are relying on adhesive bonding far more than mechanical fasteners in their assembly. The rise in glue applications, the unfamiliarity of adhesion processes, and the difficulty of detecting and fixing adhesion issues makes it clear why this is the most pervasive defect area.

In the Instrumental study, 4 out of the 10 most common defects can be linked to some kind of adhesion, bonding or cleaning issue. These problems are all interrelated, and, if solved or prevented, can have a dramatic effect on scrap rate, time spent troubleshooting and root causing defects, and make a measurable impact on warranty claims and recalls.

Consumer electronics are built using several adhesive bonding processes including wire bonding of components on circuit boards within the products, encapsulation of those boards with conformal coatings, and exterior coatings with oleophobic and hydrophobic characteristics.

For electronic devices to perform at the levels demanded by consumers, each of these adhesion applications needs to be 100% reliable throughout the life of the product. Late-process performance testing is not enough to ensure zero defect manufacturing.

In-process monitoring, testing, inspection and data collection is the only way to guarantee perfect adhesion within every product at every Critical Control Point (CCP) where varying conditions can affect adhesion outcomes.

Consumer electronics process control must include a robust understanding of what elements must be controlled for perfect gluing and coating adhesion.

Small and Complex Electronics Makes Cleaning and Detection Difficult

Electronic component manufacturing has followed a decades old prediction about the exponential miniaturization of the chips that populate circuit boards and power everything around us. Moore’s law, named after its creator, Gordon E. Moore, one of the cofounders of Intel, said that the number of transistors that could cost-effectively be placed on a chip would double every couple years. This trend has held true since it was first uttered back in 1965.

The profoundly small components we can build and assemble has allowed for extraordinary processing power in packages inconceivably small to even someone like Moore back in the mid-60s.

 

To learn more about how surfaces at the molecular level affect production outcomes, download our eBook: Checklist: Adhesion Failure Root-Cause Analysis for Manufacturers

 

The crowded nature of modern PCBs has made soldering and wire bonding components, and cleaning residues a much more precise and delicate matter. To prevent solder bridges, inconsistent conformal coatings, or dendritic growth caused by residual flux and other contaminants on the boards, surface cleanliness is essential.

Detecting contamination on surfaces that are as small and variable as a populated PCB is extremely difficult and is often left up to subjective, visual inspections because of the lack of equipment with the proper sensitivities to micron-level contamination.

Common contaminants that interfere with adhesion reliability, like flux residues, oils and silicones are not detectable through a visual test and are also missed by most cleanliness validation techniques that only evaluate ionic contamination levels.

In order to neutralize the threat of these contaminants, even on the tiniest surfaces, manufacturers need a well-designed and well-maintained aqueous cleaning solution as well as a plasma treatment process. When these systems are fully optimized and precisely monitored they can take care of the totality of residues and contaminants that plague electronics assemblies.

“Fully optimized”, “precisely monitored” and “well-maintained” are the operative words in that statement. The only way to ensure that these cleaning processes are actually creating an optimal surface for adhesion is to measure the cleanliness of the surfaces before and after each cleaning and treatment step. Using smart sensors to gather data that certifies the surface quality at each CCP means manufacturers can get longer use out of their cleaning equipment, only change equipment parameters when it is absolutely necessary, and rely on a truly consistent cleaning process.

Plasma Treatments

Plasma treatment systems provide high precision, molecular level surface engineering of materials like circuit boards. These systems are often added to a production process as a magic wand to make all adhesion and gluing problems disappear; manufacturers are disappointed when things don’t quite turn out that way.

If implemented with process controls that are sensitive to the molecular level chemical changes taking place during a plasma treatment, these processes can be a game-changer for manufacturers.

Common plasma treatment processes being employed by electronics manufacturers include batch argon or argon/oxygen plasmas in low pressure vacuum chambers or atmospheric pressure plasma in air or He/O2 immediately before an automated pick and place die attach step, or for activating populated board surfaces before conformal coating.

Because plasma treatments use a reactive gas to perform the cleaning action, the plasma is able to access and clean places on boards that are difficult to chemically clean with traditional methods. A well-implemented plasma cleaning process will also lightly oxidize the underlying board surface to provide chemical “teeth” for adhesion of conformal coatings and adhesives.

Process control and process validation is accomplished most reliably by fast, repeatable, and accurate tests done on real parts, near the action in a manufacturing process.

How to Guarantee Uniform and Reliable Coatings

From touch screens to sensors, consumer electronics are chock full of coated surfaces intended to extend the life of the devices and make sure they can operate as they were intended to.

The way to ensure these coatings work properly is to control the three elements necessary for strong, reliable adhesion for any material:

  1. The composition of the adhesive, coating, ink or paint
  2. The application of the adhesive, coating, ink or paint
  3. The quality of the bond surface

For electronic devices (and nearly all manufacturing applications) the composition of the coating has been heavily researched and precisely formulated to be able to have the desired characteristics.

The application of the coatings definitely needs to be monitored, but it is still a widely understood process and the controls for calibrating dispensing parameters are often discussed throughout the industry.

Controlling the quality of the surface the coating is applied to, however, is very often overlooked and, in our experience, is the most common reason for coating failures in every industry.

For coatings to properly adhere to a surface it must be free of chemical contaminants by way of a cleaning and/or activation process. A wash process can remove large particulates and then a plasma treatment process can activate the surface, creating a chemically clean surface ready to be coated.

It’s extremely important to have a cleaning and treatment process that delivers a high surface quality and a validation system to quantitatively verify that cleaning.

An in-line, automated surface analysis system is the best way to keep up rapid production rates while maintaining consistency and accuracy. Data from these systems can be shared instantaneously throughout an organization so Quality Engineers can evaluate the analytics and make decisions quickly, from anywhere in the world.

Hydrophobic and Hydrophilic Coatings

Self-cleaning coatings come in two varieties and serve very distinct yet equally important functions. There are hydrophobic coatings and hydrophilic coatings, and which one you choose is based on how you want the water and moisture to interact with the surface of a lens to  remove dirt and dust.

Hydrophobicity and hydrophilicity refer to the way liquids behave when they come into contact with a surface. Water will spread out and “wet” a surface that is hydrophilic. If glass isn't perfectly clean, water “breaks” instead of wetting out uniformly on the surface; as the water dries, dirt and minerals that were dissolved in the water appear as water spots.  

However, if the glass is clean, the water runs off in sheets. As water dries on clean glass, it remains spread out, and any dirt or minerals are left as a thin, uniform (and invisible) film that doesn't interfere with the ability to see through the glass. Hydrophilic sensor coatings function in the same way: water tends to run off in sheets, taking a majority of the dirt and dust with it. The thin, uniform film of water that remains and evaporates is spread out and doesn't leave behind water spots to obstruct the sensor’s ability to image its surroundings.

Oleophobic Coatings

Oleophobic coatings work in much the same way hydrophobic coatings do, although they primarily repel oils. Grease and oils from hands are deposited on absolutely everything we touch, especially highly chemically reactive surfaces that have been cleaned or treated during manufacturing processes.

Oleophobic coatings are often used on electronics screens in the form of nanoparticle coatings, which are extremely thin films deposited through plasma chemical vapor deposition (CVD). This process takes place in a chamber where the coatings are introduced in a gaseous state and bonded to the surface of the glass, metal or plastic. These coatings are invisibly shallow and transparent so they’re perfect for optics and photonic applications.

The Unique Challenges of Adhesively Bonding Electronic Devices

In trying to design the quotidian products our technology-rich society relies on, consumer electronics manufacturers have figured out how to make sure they resemble the space-age gadgetry we were all promised decades ago.

Using glues and adhesives to assemble consumer electronics like cell phones, smart watches, fitness trackers, tablets, and smart home appliances, manufacturers can build sleek and stylish devices that are perfect fodder for artistic commercials and fit into a modern lifestyle. But the process of actually building these products is an amalgamation of art and science.

The smooth contours and seamlessness of the designs is thanks to the adhesives that hold it all together. If there were tiny screws all over the back of cellphones, people would have a lot to say about it.

It’s these slick, organic designs that make these devices excellent candidates for adhesive bonding and also make them difficult to clean and inspect.

Curved edges and narrow channels in metals and plastic are extremely hard to reach when cleaning surfaces before applying adhesion. It’s at these interfaces that manufacturers have the most adhesive failures.

Atmospheric pressure plasma is uniquely able to reach those difficult surfaces but none of the traditional methods of surface quality evaluation are useful for rapidly inspecting curved or narrow surfaces reliably.

To know that you are fully cleaning every nook and cranny of your electronics assembly surface, contact angle measurements are the only accurate method. This technique uses a tiny drop of liquid that is deposited on the surface and then, using an intelligent algorithm, measures the extent to which the drop wets out on the surface. These measurements can be done very quickly, offers real-time analytical data and is extremely predictive of adhesion outcomes.

Employing Predictive Analytics to Ensure Reliability

The most reliable way to test each step in an adhesion process (i.e. surface quality testing of materials from suppliers, validating surface activation steps, verifying coating uniformity, etc.) is to use production-level contact angle measurements. This simple test gives Quality Engineers more quantitative data regarding the state of their material surfaces in a production setting than they have ever been able to gather through other means.

Understanding the necessity of controlled, clean surfaces for assembling and coating reliable consumer electronic devices is the bedrock for creating a surface quality specification. You can draw a straight line from the chemistry of the surface to the reliability of the products. Having a measurement of this relationship puts manufacturers at a strong advantage for lowering production costs and increasing value for their customers.

To learn more about how surfaces at the molecular level affect production outcomes, download our free eBook, “Checklist: Adhesion Failure Root-Cause Analysis for Manufacturers.” This guide walks you through discovering the places in your adhesion process you might have overlooked and what you can do to make sure every variable is accounted for.

Adhesion Failure Root-Cause Analysis for Manufacturers

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