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Step 8: Cleaning
December 31, 1969 |Estimated reading time: 8 minutes
By Dirk Ellis
Printed circuit board (PCB) cleaning — the value-added process of removing surface contaminants that have accumulated during various manufacturing processes and handling — is critical on several fronts. If not properly cleaned, surface contaminants can cause significant defects in the manufacturing process, ranging from poor adhesion of underfill and conformal coatings to lowered surface resistance to dendritic growth and corrosion. Cleaning also has an impact on the final product, from visual appearance to ensuring field reliability and optimum performance.
As electronic technology grows more complex — along with its manufacturing processes — the cleaning process also becomes more challenging. If the basic function of the cleaner is to wipe away the "sins" of the previous seven steps in the manufacturing process, ranging from residual fingerprints to residual paste and fluxes, cleaning is indeed both more difficult and critical than ever.
A brief trend seen in recent years to move away from cleaning, culminating in the rise of no-cleans, is in retreat. Interest in cleaning and the technology that goes with it is again on the rise. This increase might be correlated with a strong continuing increase in military applications and the Mil Spec requirements that govern their cleaning process. Due to a sort of "trickle-down effect," the cleaning processes employed by large companies also are becoming mainstream throughout the commercial industry. This is a positive trend.
Step by Step: The Cleaning Process
- Cleaning Systems. Removal of flux and process residues from a circuit board involves thermal, chemical and mechanical energy. A primary determinant of a cleaning system configuration is the type of chemical used to solubilize the flux residues and other contaminants. A water-only cleaning system affords the smallest footprint and lower cost of operation over a cleaner that accommodates aqueous chemistries. However, many common fluxes require an aqueous cleaning chemistry to cause the required chemical actions for complete removal of their post-solder flux residues.
The most common cleaning method used for SMT applications is either in-line or batch spray-in-air. Ultrasonics and vapor degreasing are two other less common batch cleaning methods. Batch processes are best for low-volume, high-mix applications, such as in cellular manufacturing. In-line spray-in-air solutions are for high-volume, dedicated or high-mix applications. Semi-aqueous solutions including spray-under-immersion and centrifugal technology also excel in high-reliability applications. This article will focus on spray-in-air systems.
Figure 1. Prewash drain valves allow switching from OA to saponification.
- In-line Prewash. The prewash system can significantly extend the life of the wash bath and enable faster cleaning conveyor speeds. Flooding nozzles are the method of choice for raising board temperature and primary washing action. These nozzles generally are powered by the recirculating wash pump.
For organic acid, water-soluble flux, detergents are built into the formula to help remove residue. The prewash must remove most of the flux contaminants. By routing the prewash effluent directly to drain (Figure 1), the detergents bypass the wash reservoir, meaning only small quantities of foam-generating detergents are exposed and removed during the subsequent recirculating wash stage. It also is beneficial to the process to remove and immediately route to drain any water-soluble mask during the product's prewash stage.
For RMA and many no-clean flux residues, the heated prewash spray systems properly prepare the residues for the more powerful recirculating wash stage. Flux residues exposed to the prewash are softened and any cleaning chemistry used has a longer time to react with the residues and other contaminants. The prewash in these applications is cascaded back to the recirculated wash. (Figure 1) In a batch process, the prewash often is omitted, and the process begins directly with the recirculating wash.
Figure 2. A coherent nozzle provides a sheet of water across the conveyor width for penetration under low standoff.
- Recirculating Wash. A recirculating wash stage should employ low pressure and high volume for proper chemical exposure and rates of solubilization. Products that are difficult to penetrate underneath and around or stubborn residues require more mechanical energy. This requirement can also come from the need to overcome the "puddling effect" caused from water sitting on the circuit board. In these situations, high volume with high-pressure cleaning may be needed.
Many types of high-pressure, high-volume nozzles have been used for electronic circuit cleaning over the years, with the most successful being coherent nozzle systems, specifically with a coherent nozzle that provides a coherent sheet of high-volume, high-pressure cleaning chemistry targeted across the entire conveyor width. (Figure 2) These systems retain kinetic energy (0.5 mv2) by maximizing their impact force, accomplished by keeping droplet size large (mass) and velocity high via pressure.
Figure 3. Chemical isolation with barrier between air knives and spray manifold.
- Chemical Isolation. For cleaning systems accommodating a cleaning chemistry additive to water, a chemical isolation stage minimizes chemistry dragout from the wash to the rinse stage. This phenomenon can cause too short of a life for closed-loop deionized filters/beds and/or foaming in the recirculating rinse reservoir. Chemistry is expensive, so it is crucial to retain as much as possible. In the meantime, if a circuit board to be processed is 20 × 20" and the chemical isolation section is 18" long, many problems can be associated with the board being in three modules at once. Typically, a mechanical blower and air knives remove the bulk of the chemistry from the circuit board and return it back to the wash tank. Next, a water spray dilutes any residual chemistry. Finally, the mechanical blower and air knives remove the remaining diluted chemistry from the circuit board before it enters the recirculating rinse. (Figures 3 and 4)
These steps become even more important in a batch system, since all process steps typically are carried out in the same process chamber, not separate modules.
Figure 4. Chemical isolation blower is on slide-out.
- Recirculating Rinse Stage. Remaining aqueous cleaning chemistries are completely rinsed using the same spraying technology used in the wash stage, assuring complete penetration underneath difficult-to-access components for complete flushing away of loosened contaminates. In this stage, the gravity cascade from the final rinse supplies the make-up water. The rinse tank then overflows and may be routed to drain, or can be recycled through closed-loop deionized filter/beds.
- Final Rinse. This is where the product is presented with the highest purity, typically heated 1- to 18-megohm-quality deionized water. Deionized water absorbs ions aggressively, effectively pulling contaminates off the circuit board. The spray nozzles in this section typically match the prewash nozzles to balance the cascading flow rate through the machine when running a water-soluble process. This stage assures any remaining ionic surface contamination is removed.
- Drying. This last stage of the cleaning process may be one of the most important if the board proceeds to in-circuit test (ICT) or conformal coat. The board must be totally dry for these processes. It is least important if the boards are put in queue for hours or days before the next process.
The high-velocity turbine blower is the industry standard for this process. With addition of auto belt tensioning, these blowers are almost to the maintenance level of a direct drive. Along with an efficient air knife design, the blowers create a mechanical squeegee effect. The circuit board, wet with pure deionized water is conveyed underneath air knives that blow off the water. For circuit boards mounted with complex components with blind holes or low standoffs, evaporation is the only way to completely dry these components, requiring the circuit board to achieve a temperature above the boiling point of water to ensure total evaporation. Many processes include a post-bake in a reflow oven; however, some in-line cleaners can perform convection drying. (Figure 5)
Figure 5. Convection dryer cross-section.
- Closed-loop Water Recycling. In a water-only application, the entire effluent stream can be recycled. In a chemical cleaning application, the rinse stream becomes a water-only process and is recycled conventionally. The water exits the cleaner and is either fed via gravity or pumped via a transfer sump pump to the main recycling unit. The unit provides the flow and pressure to push the water through a series of filters to re-deionize the cleaner effluent. Deionization occurs as the water is processed through a heavy metals removal, granular-activated carbon, cation and anion tanks. The basic filtration package will achieve water quality in the 1- to 3-megohm range. For water quality of 10 to 18 megohm, an additional mixed bed is added.
Benefits of closed-loop water recycling include lower operating costs, avoidance of environmental compliance issues, and consistent process controls. The process recovers heat energy, lowers water consumption, and minimizes deionized water costs. Since zero discharge exists in water-only applications, costs associated with permits and health and safety engineering support are eliminated. A properly designed recycle unit will include water quality, pressure and flow controls.
Current Trends, Future Challenges
Ironically, one of the cleaning industry's most pressing challenges was brought about by no-clean fluxes. It is only in the last five to 10 years that no-cleans have come into their own. While water-solubles and RMAs are designed to be cleaned, the practice of cleaning no-cleans poses a significant problem. Today, both equipment and material manufacturers are developing a process to clean no-cleans.
The lead-free process also is posing a number of cleaning issues. For instance, the higher elevated reflow, or spike temperatures, can cause flux residues to char, making them more difficult to remove. Some water-soluble pastes reflowed in air will need a chemical additive in the recirculating wash to completely clean. New-mix fluxes and pastes also present challenges. As lead-free practices mature in the U.S., so too will new cleaning approaches and technologies.
Cost of ownership continues to be a challenge for manufacturers worldwide. Energy costs continue to rise, and managing consumption continue to be a high priority throughout the manufacturing process — including cleaning. Equipment and chemical companies are working to reduce energy consumption, curtail system evaporative losses, use ambient temperature chemistries, and incorporate environmental concerns throughout the cleaning process, while striving to improve throughput and product quality.
Dirk Ellis, applications engineer, may be contacted at Speedline Technologies Inc., Camdenton, MO; E-mail: dellis@speedlinetech.com; Web site: http://www.speedlinetech.com.