The pervasiveness of electronics in every manufacturing industry has provided unique challenges. Manufacturers are tasked with protecting these componentsin 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 a defense against moisture. While, this capability massively enhances the protection of, and therefore the reliability of, electronics, manufacturers have been overlooking a key element to dependability in this system: surface condition.
Here are four major factors that could lead to poor adhesion which manufacturers are often unaware of the impact they have:
- 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 volatized 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. This confidence leads to over-application, causing dewetting (when the conformal coating pulls back from the residue), ultimately resulting in cracking and sometimes corrosion, if moisture is allowed in.
- Solder Mask Another common and necessary part of electronic component construction is solder mask. This is the typically 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 material. This mask helps prevent solder bridges, solder jumpers and other issues that may lead to shorting and unreliability. If the curing 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.
- 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 find purchase. This leads to delamination, chipping, blistering and other failures that leave the electronics vulnerable to their harsh environs.
- Improper Treatment Often, treatments and coatings prior to applying the conformal coating can be administered improperly due to a lack of understanding the chemistry of the surface. Atmospheric plasma treatment, a very widely used method for preparing electronics for adhesion, bombards the surface with excited molecules, free electrons and ions that changes the chemical make-up of the surface, raising its surface energy.
At BTG Labs we’ve found there is 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 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 fisheye, dimpling, moisture corrosion and damage due to debris that a strong and complete conformal coating can resist.Another factor not considered is the effectiveness of a treatment over time. If the immediate result of a coating passes all currently utilized tests then typically the inspection stops. It is very common, however, 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.
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Image to right: A printed circuit board close up showing the green solder mask and components attached to solder pads.