The steps involved in manual and automated contamination removal.
Printed circuit boards are subjected to many harsh environmental conditions, including extreme temperatures, strong chemicals, corrosive salts, dust and moisture. Encapsulating them with a protective conformal coating makes sense. Conformal coatings keep harmful elements from touching delicate components and degrading performance of the boards. However, for optimum PCB longevity, functionality and reliability, it is imperative boards are perfectly clean and dry before conformal coating.
PCB contamination comes from many sources: transport, handling, storage and manufacturing. The most common examples of PCB contamination are fingerprint oils and salts, flux residue, tape or other adhesive residue, solder balls, and even some inks or chip bonder.
Any contaminants or soils on PCBs may interfere with the proper bonding of the conformal coating to the PCB substrates. Salts or oils from fingerprints left on the boards can cause defects in the conformal coating (FIGURE 1). These include uneven coverage, pinholes, craters, blisters and fisheyes.
In addition, flux residue absorbs and holds moisture. If residue is sealed under the conformal coating, it will likely cause cracking and peeling when the conformal coating is cured and the trapped moisture releases. If that compromised board then makes it into the field, dust, water and salts can penetrate it, resulting in a variety of damage, including delamination, parasitic leakage, dendrite growth, electrochemical migration and shorting. In extreme conditions, it may also result in complete component failure. To prevent performance problems, cleaning the contaminants and drying the PCBs prior to conformal coating is essential.
Clean and dry before coating. Typically, the cost prevents many manufacturers from cleaning and drying before conformal coating. Those steps may seem like an unnecessary expense due to the extra time and cleaning fluid costs involved. However, cleaning and drying prior to coating improve PCB life-expectancy and reliability. They also help prevent unpredictable board performance, costly PCB failures, disruptive product recalls and expensive product returns, not to mention the added time and expense of removing a conformal coating to rework a malfunctioning PCB if it fails due to environmental exposure. So, cleaning and drying are typically worth the extra investment in time and materials.
Two of the more popular methods of cleaning and drying PCBs are either manual cleaning at the benchtop or automated cleaning using a vapor degreaser.
Benchtop cleaning and drying. There are four steps to successfully cleaning PCBs at the benchtop: wet, scrub, rinse and dry (FIGURE 2). These four steps ensure a good conformal coating adheres to the PCBs, cures without damage, and resists delaminating when exposed to extreme conditions. First, wet the board with a pure cleaning fluid. Second, scrub it using a good quality scrubbing brush. Third, rinse away any byproducts with more clean fluid. The final and fourth step is to dry the PCB completely to prevent moisture from being trapped under the conformal coating.
Manual drying is accomplished in a few different ways during benchtop cleaning. One of the common methods to dry PCBs is with a simple lint-free wipe. Gently push the wipe under low-mounted SMT components to absorb any excess spray of cleaning fluid. Or clean and dry at the same time by scrubbing components through a wipe placed on top of the board. Another way to dry PCBs before coating is with a dust remover. This can be a quick and effective way to blow out trapped moisture from underneath larger components that a wipe alone may not reach. The dust remover can be targeted to a very precise area on the PCB and works well to eliminate moisture in hard-to-reach areas of the circuit board. In addition, combining a duster with a wipe will double the drying power and prevent the fluid from being pushed back onto the board, spreading contaminant around.
Vapor degreaser cleaning and drying. As PCBs continue to shrink in size due to the demand for smaller electronics, manufacturers are squeezing multiple micro-components like flip chips, µBGAs and CSPs into tighter spaces on the boards. Low-standoff components like Mosfets and bottom-termination packages like QFN (quad-flat no-leads) are now commonplace. As a result, these tiny, stacked, high-density and complex PCBs are more susceptible to functionality problems, especially if exposed to harsh conditions. So, good cleaning and conformal coating is an absolute necessity.
But the dense construction of these PCBs makes them hard to clean and even more challenging to dry. Cleaning fluid can get trapped under these components, leaving the PCBs dirty, at risk of cross-contamination and unprepared for conformal coating.
A solution for effectively cleaning small, complex PCBs is to use a vapor degreaser and a modern, sustainable cleaning fluid. Vapor degreasing uses cleaning fluid immersion, combined with vapor rinsing and vapor drying, to remove all types of contaminants, including fluxes, pastes, particulates and residue (FIGURE 3). The fluid gets into and under all components, but more important, it gets out of the tiny channels between stacked components.
Vapor degreasing machines range in size from small benchtop models to huge floor systems. No matter what size vapor degreaser is used, the cleaning and drying processes are the same. The vapor degreaser boils a cleaning fluid at a low temperature, usually between 40˚C (105˚F) and 65°C (165°F), to produce a pure, clear and dense vapor blanket. PCBs are lowered into the boiling cleaning fluid in the boil sump to heat, loosen and remove most contamination. They then move to the rinse sump, where any vestige of contamination is rinsed away. Finally, the PCBs are raised and held inside the vapor blanket, where the parts dry and cool. The entire process takes approximately six to 20 minutes per batch.
The cleaning fluid has a low surface tension, permitting it to permeate the entire board, including under and between tightly spaced components to wash away fingerprint oils, fluxes and residue. A low-temperature fluid minimizes risk of damage to components, and since the cleaning fluid is ultra-pure, it leaves no residue behind.
The vapor blanket dries PCBs quickly. The vapor passes under the low-mounted components, so the PCBs come out of the vapor degreaser dry and cool enough to handle. This is important since PCBs need to be cool to help the conformal coating adhere to the board.
Removing conformal coatings. If necessary, conformal coatings can be removed for board rework and repair. Removing conformal coatings is challenging, however. They are designed to be durable and, by nature, hard to remove. If rework is required, the best way to dissolve the conformal coating is to use a cleaning fluid with a chemical composition similar to the contamination. For example, silicone conformal coatings offer great protection, while being light, durable and easy to rework. For silicone coatings, choose a remover that is siloxane-based, so it is chemically very similar to the silicone conformal coating.
Acrylic, epoxy and urethane coatings are more difficult to remove. They often need stronger, more aggressive cleaners and may require soaking to soften the coating before removal. Other conformal coatings, especially those used in military applications, are rock-hard and simply cannot be removed chemically. They often require physical abrasion like sandblasting, which can damage the PCBs.
For reliability and functionality, ensure PCBs are clean and dry prior to conformal coating. By properly preparing the PCBs prior to coating, manufacturers ensure optimized production yields, throughput and quality, and also avoid the major costs of coating removal and PCB rework.