Properties of Acrylic Conformal Coating

Exceptionally user-friendly, liquid acrylic resin (AR) conformal coatings protect printed circuit boards (PCBs) and similar electronics from corrosion, dirt/dust, fungus, moisture and thermal shock. AR’s ease of application contributes to its status as the most frequently used conformal material.

Liquid Application Methods for AR

The primary means of AR liquid application are brush, dip (immersion) and spray. Each has merits and drawbacks relative to the other techniques.

Brush

Simple, cost-effective manual brush-coating is recommended for smaller-batch acrylic assignments. For larger volumes, brushing can be time-consuming and costly. Brushing also covers only one side of an assembly at a time, further slowing production. Skilled operators are required for film-application to assure quality outcomes of appropriate thickness. Overly-thick films can crack during thermal cycling, leaving the component vulnerable to contamination and dysfunction; films that are too thin cause similar problems. Additionally, brushing is ineffective for coating complicated/irregular PCB surfaces, which obstruct film uniformity.

Brushing is quick but can also be accompanied by potential quality issues, regardless of the operator’s experience or skill-level. This limits reliable brush-technique opportunities.

Dip

Unlike brushing, AR-dipping is very repeatable and fast, allowing higher-level production runs. Electrical assemblies are submerged in a dip-tank containing the liquid AR solution, through either manual or automated processes. Completely immersed, coating coalesces around the unit with dependable under-component penetration.

Because the coating is a liquid, inconsistent edge/tip coverage and irregular coating thickness can occur. Dip coating requires a higher level of expertise when it comes to the masking often required to protect keep-out areas.

Spray

Cost-effective manual or automated spray techniques work well with AR films. They provide a coating-surface quality superior to either brush or dip, assuming the designated assembly has been properly cleaned and AR-coating type has no adhesion issues relative to the substrate material. Automated spray processes are recommended for high-volume spray assignments, generating faster turn-around and better coating accuracy. Low/medium volume coating assignments are effectively enacted with a dedicated spray-booth coating operation utilizing an aerosol/spray-gun. In these cases, processing with a skilled operator can generate finish superior to the brush and dip methods.

Despite the relative drawbacks of each method, brush, dip and spray processes are easily implemented. In each case, the choice of coating method used will be guided by the selected AR type’s properties (chemical, electrical, physical, etc.) in relation to its compatibility with PCB material.

AR Performance Properties

In accordance with IPC-610, AR is typically applied at thicknesses between 0.002 – 0.005 inches (2-5 millimeters). Fungus-resistant acrylic is easily applied and cures in as little as 30 minutes at room temperature, making it a good choice if the objective is high throughput. Simple liquid application methods typically result in the fastest turnaround-time of all conformal coatings. AR’s moisture protection is also very highly-rated, adding to its utility for a wide range of coating uses.

As a one‐part system, simplified AR application combines with ease of rework to give both manufacturer and client lower material and production costs. AR coatings respond to solvent removal for repair purposes, however, this same trait – inability to withstand weaker solvents like isopropyl alcohol or xylene – also makes acrylic incompatible for coating assemblies experiencing prolonged solvent-contact during operation.

With effective operating temperatures ranging from -65°C through +125°C, AR does not withstand temperatures above 130°C, eliminating its use for electronics intended for higher-heat operational environments. Despite lower abrasive/chemical and solvent resistance, AR coatings nevertheless provide good acid/base protection as well as fungus resistance.

With moderate surface elasticity and good protective qualities, acrylic conformal films electrical properties include a high dielectric strength (300V/mm), a dielectric constant of 2.5 and a dissipation factor of 0.01. Superior dielectric properties withstand most occurrences of static/voltage discharge, adding to PCB functionality and longevity.

Superior moisture protection is AR’s most identifiable performance characteristic, providing reliable, long-lasting defense from moisture-based threats to component performance. Most prominent are humidity and other forms of condensation during the assembly operation. Similar liquid performance hazards — rain/salt seawater and water spray – do not easily penetrate acrylic coatings, which achieve maximum physical properties within minutes of application. Clear drying AR conformal films also:

• Can be soldered through if necessary
• Display low glass-transition temperatures
• Fluoresce under UV light, further simplifying inspection
• Retain considerable surface flexibility, post application

Summary

As the most commonly used compound in the conformal coating industry, acrylic resins provide superior moisture resistance, fair surface elasticity, overall component protection, high dielectric strength and simplified removal/rework. These AR film benefits are suitable for a wide range of less complex conformal coating assignments.