The pervasiveness of electronics in every manufacturing industry has provided unique challenges. Manufacturers are tasked with protecting these components in environments that make electronics vulnerable to even minute amounts of moisture, debris, and environmental contamination.
A useful solution to this problem is conformal coating. Conformal coating is a thin (usually 25-75µm thick) chemical or polymer film (Parylene and acrylic are popularly used, depending on the application) that covers an electronic component to act as a barrier against contamination and defense against moisture. While this capability enhances the protection and reliability of electronics, manufacturers have often overlooked a key element to dependability in this system: surface condition.
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4 Common Factors that Could Lead to Coating Failure
1. No Clean Flux Residues
The name of these very common flux residues is a bit of a misnomer in that it gives the impression that once this is applied, there is “no cleaning” necessary before conformal coating. All flux residues, no clean flux residues notwithstanding, need to be properly cleaned or volatilized so they don’t interfere with the coating placed over them. It is also very important to understand the chemical compatibility between the residue and the conformal coating before proceeding. Often there is a sense of safety when using a no-clean flux residue that can lead to over-application, causing de-wetting (when the conformal coating pulls back from the residue), ultimately resulting in cracking and sometimes corrosion if moisture is allowed in.
2. Solder Mask
Another common and necessary part of electronic component construction is the solder mask. This is the typical green, lacquer-like coating that covers the copper layer of a PCB in between the solder pads where components are attached. This covering protects the copper traces that connect the components that populate the board from coming into contact with other metals and conductive materials. This mask helps prevent solder bridges, solder jumpers, and other issues that may lead to shorting and unreliability. If the condition of the solder mask is not considered, however, then the conformal coating overtop may not be able to secure a consistent and reliable bond. This is usually the layer that has direct contact with the conformal coating and therefore is crucial to the adhesion process.
3. Low Surface Energy
Commonly, PCB and the pre-conformal layer coatings have a large polymer presence which is an intrinsically low-energy material. Surface energy directly relates to the level of attraction a surface has to other materials. A surface with high energy is usually very amenable to bonding, and a low-energy surface is not. The low-energy surface requires treatment or cleaning in order to reach a level of surface energy that is optimal for adhesion. In the case of the PCB, if it is not properly treated for adhesion (common polymer treatments include plasma, corona, or flame treatment), a conformal coating will not perform as expected. This leads to delamination, chipping, blistering, and other failures that leave the electronics vulnerable to their harsh environments.
4. Improper Treatment & Cleaning
Often, treatments and coatings prior to applying the conformal coating can be administered improperly due to a lack of surface chemistry understanding. Atmospheric plasma treatment, a very widely used method for preparing electronics for adhesion, bombards the surface with excited molecules, free electrons, and ions that change the chemical makeup of the surface, raising its surface energy. Often, there is over-treatment of plasma or flame treatments, over-application of a coating, or just insufficient cleaning of the surface prior to the coating.
Cleaning techniques range from wiping with isopropyl alcohol to cleaning with compressed air to remove particulate and debris. Insufficient cleaning is often due to not considering the chemical surface state and, therefore, not being able to clean or activate the surface to a degree that is optimal for adhesion. Improper surface treatment results in poor adhesion, which can lead to fish eyes, dimpling, moisture corrosion, and damage due to debris that a strong and complete conformal coating can resist. Another factor that is often overlooked is the effectiveness of treatment over time. If the immediate result of a coating passes all currently utilized tests, then typically, the inspection stops. However, it's very common for that bond to become unstable over time, while in storage or even within 48 hours of the application of the coating. If treatment is not being done with this in mind, the problem will never be solved.
In order to ensure that none of these factors slips under your radar, you need a way to monitor and measure the surface state and the effect of each protective layer. You need to know when, where, and how to evaluate the readiness of a surface for adhesion. It comes down to rethinking the entire approach to adhesion.
To learn more about how to manage your adhesion process and ensure your coatings are free from failure, download the eBook "How to Increase Electronics Manufacturing Quality Through Cleanliness."