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Conducting failure test analysis and quality control evaluations of circuits that have been protected with conformal coatings can be difficult. The use of air-abrasive equipment may offer a solution. This article examines the pros and cons of using air abrasion to handle the rework, evaluation and repair of components used in harsh environments.
By J. Patrick Byrne and Tim Stampe
Companies producing high-reliability SMT boards that have been protected with conformal coatings face the challenge of effectively conducting failure test analysis and quality control evaluations of the circuits. Rugged, high-tech circuit boards used in the control systems of environmental test chambers, heavy equipment and military vehicle control panels must function properly in harsh environments. Because of this, these boards are sprayed with various formulas of conformal coatings to protect the surface mount devices (SMDs) and traces, or they are heavily potted on individual components (Figure 1).
Figure 1. Coatings or pottings can protect components from harsh environments.
To conduct rework, evaluations or repairs, these conformal coatings or pottings must be removed without damaging solder joints, components or traces underneath. Damaging the product during evaluation defeats the goal of testing to find out why the product failed, or what may need to be replaced or redesigned. Because of the delicacy involved in the process, it typically has been a labor-intensive, manual operation requiring skilled operators.
Understanding the Challenge
Depending on the product, protecting parts from elements such as temperature, chemicals and humidity can be accomplished through the use of a sprayed layer of silicone coating, or a coating similar to shellac or varnish applied over the circuit board. In extreme cases, the board may need to be potted. Potting puts the board or component into a small pot and covers it with a black epoxy that hardens. Problems arise when operators have to perform rework or evaluation engineering, such as failure analysis. The coatings and potting materials are difficult to remove without damaging the component underneath.
Because scraping can damage the circuits, even very thin coatings may never be removed safely or completely. While enough coating can be removed to do a spot test, this leaves some coating on the product. Leaving any coating after removal creates difficulties. Without complete removal of the coating, repairs cannot be re-soldered effectively. Solder will not bond properly if even a small trace of coating material is left on the assembly.
With potting, this process is more difficult because of the thickness and hardness of the potting material. Operators can chip away at it with a sharp chisel or tool to remove the top half-inch of potting, then hope to be able to scrape down to the components safely without compromising the investigation, or the integrity of the piece. It is a precise and time-consuming task.
Many companies faced with this challenge are testing the use of air abrasive or microabrasive blasting equipment to handle the removal of coating or potting materials. It takes a period of adjustment and experimentation with abrasive powders and air pressures. However, if done correctly, it can solve 95% of these rework and test problems.
Microabrasive blasting offers a wide range of abrasive media in micron sizes. Everything from bicarbonate of soda to organic materials, plastic and heavier cutting abrasives, such as aluminum oxide, are available. The media is mixed with clean, dry air and projected through a small stylus to pinpoint miniscule areas on any surface to be abraded. Using the correct media, air pressure and timing, this blast can pinpoint even the smallest area for effective material removal. If used properly, it is completely safe to underlying solder and traces; and the small orifice of the blasting nozzle restricts the spray to the specific area being cleaned.
Because this process creates dust, work is conducted in a sealed chamber that evacuates the particles through a vacuum system. Various commercial dryers are also needed to maintain absolutely dry air and media (Figure 2).
Figure 2. Microabrasive blasting is conducted in a sealed work chamber.
At one time, the biggest drawback to this process was the creation of electrostatic discharge (ESD). This made the process impossible to use in many electronic applications. Today, there are ESD-safe work chambers that flood the blasting area with ionized air and supply ESD-protective accessories for operators using the system. ESD is a problem with boards because it can damage them without the operator’s knowledge, and the damage won’t show up until the board fails later in the field. These latent failures are an important issue for any product’s quality-control group. A compromised board may test well and appear to be functional, but it will fail when the end user puts normal stress on it. For this reason, it is imperative that any type of air abrading system be used within an ESD-controlled environment (Figure 3).
Figure 3. The blasting chamber has an ESD-controlled environment and additional grounding for the operator (note band on arm).
Figure 4. Microabrasive blasting is precise, allowing the operator to pinpoint small areas.
Because only a select area is worked on, precision is important (Figure 4). It is best to remove the precise amount of material needed and no more. Operators should never begin a removal until they know exactly where to look for the failure or at what point they want to conduct a test. Walnut-shell and bicarbonate-of-soda abrasives work well in this type of application. If used properly, these materials remove most coatings, but will not damage the board, traces or components (Figure 5). Plastic media and crushed glass also are used. However, it is recommended that the application and media be tested properly before using it on any assembly.
Figure 5. Walnut-shell and bicarbonate-of-soda abrasives remove most coatings and do not damage the board, traces or components.
Testing also applies to air pressure (psi), which depends on the type of coating to be removed. Suitable pressures for a thin coating at 70 psi may have no effect on a thicker or more resistant coating. Pressures up to 95 psi are safe for many thicker materials. It should be kept in mind, however, that testing is recommended before use on an actual product.
The entire process requires a high-volume air supply, for the continual ionization of the work area to prevent static buildup and for the process itself.
Working with technical support from the microabrasive-equipment supplier is valuable. Sending parts and materials out for testing saves time and experimentation in-house, even after equipment has been setup and established. Once the formula is developed, it becomes the responsibility of the operator to choose the right nozzle size for each job, distance from the board and time length of the blast. Because coating and potting materials are different, this takes some experimentation.
Figure 6. After using another method to remove the bulk of potted materials, blasting can finish the cleaning process down to the component.
Microabrasive blasting can remove most spray or thin coatings easily; however, it is not as effective on thick or rubbery types of materials. Some potting materials are difficult to remove, so blasting is used as a finishing process. Many thick, hard potting materials break easily at about 120°. Therefore, they should be heated to de-pot in an oven and held at that temperature for mechanical removal of a safe amount - down to a thin layer. Blasting can finish the cleaning process down to the component (Figure 6). Blasting is useful for removing a final layer on the circuit board, particularly when it is over a solder joint. Mechanical methods run the risk of damage, but the coating has to be 100% removed. A blaster can do this without damaging the solder joint or traces. Lead-free solder is relatively soft as well, and cannot tolerate removal without causing distortion to the look and functionality of the component.
For companies dealing in high-mix, high-volume, as well as those expanding operations to multiple divisions, it is imperative to implement as much automation as possible, and adhere to a lean manufacturing objective. This can be done by removing as much of the human-error factor as possible. Using semi-automated or operator-regulated machines dedicated to the task of conformal coating removal does not necessarily reduce the number of employees working on the line. It does, however, free them up to be more productive and attend to a wider variety of duties. It eliminates the redundant work of hand-removal of coatings for test, which is done by scraping points of coating away with hand picks and files. It also reduces the probability of damaging the boards during product rework or evaluation.
Test and rework areas must be cleaned completely of all coating materials. Microabrasive blasting ensures that this is done all the time. As the abrasive particles hit the coating, they appear to almost absorb the material and carry it away. Manufacturers moving toward world-class operations are seeking higher quality. For coated, high-reliability SMT boards requiring rework, test and evaluation, some older methods are not always effective. Microabrasive blasting can be an effective and safe method, ensuring a safer way to handle the application with higher quality control.
J. Patrick Byrne, technical manager, Comco Inc., may be contacted at (800) 796-6626, ext. 107; e-mail: firstname.lastname@example.org. Tim Stampe, senior service technologist, Vansco Electronics, may be contacted at (204) 453-3339, ext. 404; e-mail: email@example.com.