Stencil Cleaning: Choices and Proven Strategies
08/01/2003
Process, environmental and safety considerations
By Michael Konrad
Much discussion and debate have taken place in engineering circles regarding the 21st century question: "To clean or not to clean?" No-clean flux or cleanable flux? Batch or inline technologies? Open- or closed-loop? Chemical or water-only processes?
Consultants have made money selling the attributes of no-clean and cleaning technologies alike. What is frequently overlooked, however, is the process of cleaning stencils and screens.
Regardless of your belief (clean or no-clean), one thing is certain: stencils and misprinted assemblies need to be cleaned. Unreflowed solder paste does not go away on its own and the longer you wait, the more difficult it is to remove.
Fortunately, removing solder paste from stencils is relatively easy. Unlike post-reflowed boards, unreflowed solder paste is not "cured" onto a surface. Solder paste removal is relatively easy. Regardless of the "hype" various equipment manufacturers put out, no single technology maintains a monopoly on solder paste removal. Just about every available technology is capable of adequate solder paste removal.
So what sets the different technologies and methods apart? Ironically, it is not cleanliness. Instead, it is the type of contamination being removed, process and environmental considerations and safety considerations.
Removing Contamination
Solder pastes can be placed into one of two categories: Water-soluble and non-water-soluble. Water-soluble organic acids (OA), rosin (R), rosin-activated (RA) and rosin mildly activated (RMA) solder pastes may be cleaned using water only. Non-water-soluble (R, RA, RMA and no-clean) all require either water with a chemical additive or a solvent. A heated solution is not generally required for solder paste removal.
All adhesive removal applications require either a water/ chemical mix or a solvent. A heated solution is normally required for adhesive removal applications.
Process Considerations
There are basically two technologies used in stencil cleaning equipment: spray-in-air and ultrasonic.
Spray-in-air technology uses a spray pump and multiple spray nozzles. Wash solution (water or a chemical/water mix) is sprayed onto the stencil. The flux component of the solder paste is placed into solution while the solder balls are blasted off the stencil. Spray-in-air stencil cleaners rely on spray pressure to penetrate the stencil's apertures and remove the solder balls. Too much spray pressure and the possibility of stencil damage is increased. Too little spray pressure and solder balls may be left behind.
Complete wash-and-rinse solution real-time filtration is vital to ensure that the stencil will not become damaged by solder balls being propelled from the pump through the spray nozzles. Because spray-in-air systems re-use the wash solution, the paste removed from the stencil must not be pumped back through the spray system and onto the stencil. Additionally, excess paste in the spray pump may lead to premature pump seal failures.
When using a spray-in-air system, it is critically important to ensure that the chemistry selected is equipped with a suitable de-foaming package. An additional de-foaming additive may have to be added as the wash solution begins to load. Excessive foaming causes leaks and also contributes to pump cavitation, a situation that leads to inadequate cleaning and shortened pump life.
Ultrasonic technology remains a popular choice for solder paste and adhesive removal. Ultrasonic stencil cleaning systems utilize highly controlled sound waves to remove solder paste and adhesives from a stencil. Most ultrasonic stencil cleaning systems utilize a sound frequency of 40 kHz, which is exceptionally safe for the stencil and effective in the cleaning process.
Unlike spray-in-air systems, the ultrasonic process does subject the stencil to any degree of force. Solder paste is removed from the stencil without the pump-generated pressure found in a spray-in-air system. Because the ultrasonic cleaning cycle uses sound waves rather than a spray pump, the solution is not agitated, allowing for a greater selection of compatible cleaning chemistries.
Environmental Considerations
The primary purpose of a stencil cleaning system is to remove lead and excess solder. Lead is a highly toxic substance and its disposal is regulated. Much consideration should be given to the environmental suitability of any stencil cleaning process.
Stencil cleaning equipment is available in either open- or closed-loop configurations. In open-loop configurations, stencil cleaning systems must be equipped with a filtration system capable of capturing the smallest piece of solder paste. In no circumstance should a stencil cleaning system be connected to a drain without suitable filtration.
Most stencil cleaning systems subject stencils to two wet processes: wash and rinse. If using a spray-in-air system, the wash solution recirculation loop should have adequate solder paste filtration. If the particulate lead is removed from the wash solution, its disposal is both less costly and more environmentally compliant. In ultrasonic stencil cleaning systems, also ensure that the wash tank is equipped with a filtration system that removes solder paste from the wash solution. This ensures the removal of particulate lead and lengthens the useful life of the chemistry, both reducing operating costs.
The rinse section of any stencil cleaner must also be equipped with a filtration system. Although the majority of solder paste is removed in the wash section, residual quantities of solder paste are present in the rinse water.
Zero-discharge configurations are considered to be environmentally superior to open-loop configurations. There are two methods of obtaining a zero-discharge configuration. The traditional zero-discharge method uses the stencil cleaning system and a standalone evaporation system. In this scenario, the open-loop stencil cleaner is connected to an evaporator. Spent wash solution and rinse water are directed to the evaporator. The evaporator heats the stencil cleaner's effluent to a temperature of about 220°F. This action causes the solution to boil, evaporating the water and leaving behind the solids. As the solids content grows higher, the solution begins to require a higher temperature to boil. When the solution no longer boils at 220°F, the evaporator must be drained. The balance of the solution left in the evaporator is called sludge, which must be removed from the evaporator and hauled away as hazardous waste. Although configurations using an evaporator prevent the cleaner's effluent from being directed to a drain, they still produce a waste product that must be managed.
Closed-loop configurations actually capture the effluent, remove the contamination and then re-use the solution on subsequent cycles. A closed-loop stencil cleaner normally does not require a drain or a connection to an evaporator. It does, however, produce a waste in the form of spent filter cartridges. The spent particle filters contain lead and must, therefore, be disposed of accordingly, normally in a similar manner as empty solder paste containers.
A closed-loop stencil cleaner works by removing specific contamination from the wash and rinse solution. If the application is dedicated to water-soluble solder paste removal, closed-loop becomes easy and less costly. Because a strictly water-soluble solder paste removal application does not require the use of cleaning chemistries, the wash water may be directed through a particle filter and a set of carbon and resin beds. The particle filter removes the particulate contamination (solder balls). The carbon removes organic contamination from the solution while resin removes ionic contamination. By removing solder balls, organic matter and ionic properties from the wash water, it may be used and re-used indefinitely. The identical process is used for the rinse water.
Closed-loop systems require the periodic removal and exchange of the particulate filter as well as the carbon and resin media tanks. Because the particulate filter prevents lead solids from entering the carbon and resin tanks, these tanks may be easily regenerated, normally by the equipment vendor.
 Figure 1. Processes needed for various cleaning applications. |
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Safety Considerations
The final consideration when choosing a stencil cleaning technology is safety. What type of chemical is required to fulfill the equipment and process requirements? Safety considerations include flammability, toxicity and disposal methods.
All stencil cleaning methods can clean stencils completely. Some technologies present a greater risk to humans, while others present a greater risk to the environment. The selection process must consider human safety and environmental compliance while maintaining the integrity of the stencil.
MICHAEL KONRAD, president, may be contacted at Aqueous Technologies, 9785 Crescent Center Drive #302, Rancho Cucamonga, CA 91730, e-mail: [email protected].