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The Need for a Stencil Cleaning Handbook
December 31, 1969 |Estimated reading time: 6 minutes
By Bill Schreiber, Smart Sonic Corporation
During the infancy of surface mount technology, cleaning 50-mil-pitch brass stencils and emulsion wire screens was a fairly straightforward task for the third-shift maintenance crew using a CFC solvent vapor degreaser. In the past 30 years, product changes have been driven by environmental regulations and the "faster, smaller, cheaper" technology syndrome: brass to stainless steel to nickel foils; chem-etch to laser-cut to electroformed stencils; solvent to semi-aqueous to aqueous chemistries; vapor degreaser to spray to ultrasonic cleaning technology; and RMA to water-washable to no-clean to lead-free solder paste. What will be the standard tomorrow? The only thing for certain is change. The IPC is providing help with its new Stencil and Misprinted Board Cleaning Handbook, IPC-7526.
As few as 10 years ago, stencil cleaning was considered just a maintenance job. Stencils were cleaned before returning them to inventory. However, knowing that over 70% of assembly defects originate from the printing process, concern over stencil cleanliness has been heightened, and stencil-cleaning technologies are scrutinized.1 But committing to a stencil-cleaning process was similar to committing to a video system VHS or Beta, VCR or Laser Disc, CD or DVD? In addition, video systems did not have environmental repercussions associated with them, as do stencil cleaners. Many assemblers found themselves going through three or more different stencil cleaning methods to keep up with technology and environmental regulatory changes.
In 1989, everyone was happy with their stencil cleaning process, before researchers at UCI discovered that gaping ozone hole over Antarctica, and got most of the industrial world to sign the Montreal Protocol, eliminating CFCs by 1995. It's one thing to have to research a new cleaning process because you're not happy with the current one. But, it is nothing short of aggravating to have to change a perfectly good process just because the government says it is necessary to save the Earth as we know it. After all, assemblers are paid to make PCBs faster, smaller, and cheaper not to replace a perfectly good cleaning process.
But where there's change, there's opportunity and the 1990s provided opportunities for the stencil cleaner manufacturers. CFCs had to be eliminated. Almost everyone was using an RMA flux. Saponifiers proved capable of converting the RMA flux to a water-soluble soap, so deionized (DI) water could be used to remove the soap residue. It seemed simple just switch to aqueous saponifiers and spend about $100,000 on new board and stencil cleaning equipment, and get on with the business of making faster, smaller, cheaper PCBs. Saponifiers could have been the end of the story, if it wasn't for the fact that saponification requires temperatures in excess of 140°F (60°C).
Since the beginning, solder paste screens and stencils were cleaned using CFC solvents at low temperatures. By switching to saponifiers, it was soon discovered that stencils were heat-sensitive. The metal-etched foil was bonded to an aluminum frame and polyester screen using a heat-cured epoxy adhesive. Plus, the expansion coefficients (CTEs) of aluminum, stainless steel, and polyester are significantly different, causing additional stress on the adhesive bond when exposed to hot wash solutions or hot drying air.2 Debonding of the epoxy quickly became an issue when stencil cleaner manufacturers adopted the use of hot saponifier chemistries.
After selling hundreds of stencil cleaners to use hot saponifiers, the industry was forced to adopt another chemistry. Unfortunately, the deep pockets needed to perform the research rested with the PCB cleaning companies. The small companies that made stencil cleaners were accustomed to spraying a chemistry used to clean PCBs. This concept was sound when used with CFC solvents because the solvent could be used at ambient temperature. But when saponifiers proved to be incompatible with SMT screens and stencils, the only other chemistry commonly used to clean flux was isopropyl alcohol (IPA).
Stencil cleaner manufacturers were quick to begin making IPA stencil cleaners, and IPA cleaned RMA solder paste fairly well. IPA stencil cleaners were manufactured with the electrical components isolated from the alcohol to prevent an ignition spark. Unfortunately, it was later discovered that an ignition could originate from a mechanical spark of metal spray nozzles hitting a metal stencil, an unsuspected static discharge, or from other electrical equipment in the area.2, 3
On another front, the solder paste manufacturers were busy developing "no-clean" and "water-washable" solder paste. Stencil cleaner manufacturers focused efforts on cleaning water-washable solder pastes because the chemistry water was readily available. However, process engineers had some basic questions to answer. Should they adopt a water-washable process and spend about $100,000 on cleaning and wastewater equipment, or adopt a no-clean process and not clean at all? Those that went water-washable usually had equipment that could be converted or retrofitted from IPA to H2O. Those that went no-clean thought that they would save the $100,000 and proceeded to remove their in-line cleaning machines.
Fortunately for stencil cleaner manufacturers, no-clean did not mean that the stencils did not need to be cleaned. Enter more change, yet more opportunity. Stencil cleaner machines now need to clean no-clean solder paste and, in some cases, water-washable and RMA as well. Many users moved away from alcohol and saponifiers in favor of a chemistry that could clean this broad spectrum of fluxes. Other solvents had inherent VOC and health/safety hazards associated with them. Semi-aqueous technologies were briefly introduced for cleaning stencils, but it was reasoned that if water was to be introduced into the process, why not just use water and eliminate the solvents completely? Special water additives (surfactants) were needed to clean solder paste at low temperatures. Surfactants are wetting agents that can be formulated into detergents to clean specific contaminants using designed process conditions and various cleaning machines. This offered an environmentally safe, aqueous answer to the problem. But what wasn't understood was why the pallets were becoming so dirty. It was because the in-line PCB cleaner went away; therefore, pallets weren't cleaned either.
Cleaning chemistry manufacturers were given a task formulate a detergent that will clean solder paste and post-solder flux residue. Also, it should clean SMD adhesives. It now appeared detergents were the way to go, and the last stencil cleaner ever needed could be purchased finally if it wasn't for fine-pitch.
Fine-pitch apertures of an SMT stencil created a new opportunity. To the naked eye, the stencils appeared to be clean, but misprints mysteriously increased. It seemed that the surface of the stencil was getting cleaned, but many of the smaller apertures were still contaminated (Figure 1). A small amount of solder paste left in the aperture would dry like cement, and result in a snowball effect. The dry paste would attract more paste, and soon the aperture became blocked, resulting in insufficient paste deposition. If the existing machine and chemistry could not clean the fresh paste, dry solder paste would prove impossible to clean. The need for an alternative stencil cleaner was once again evident. The microscopic cleaning action (cavitation) of ultrasonic technology could deliver the cleaning solution into ultra-fine-pitch apertures. But, were ultrasonic and PCB technologies compatible? In the 1950s, the U.S. military tried to clean boards using ultrasonic systems, but circuit boards at that time were very delicate, and ultrasonic technology was very aggressive and uncontrollable. Current ultrasonic technology is an extremely controllable precision-cleaning technology, and SMT boards are very durable making the two technologies compatible, given the proper ultrasonic frequency (40 kHz) and low-power-density parameters.4
ConclusionWhat's an assembler to do? If you are a survivor of this debacle, you probably learned about stencil cleaning technology the hard and expensive way. For all others, the IPC has published the Stencil and Misprinted Board Cleaning Handbook, IPC-7526. This wealth of information is a concerted effort by industry experts to guide readers through the pros and cons of stencil and misprint cleaning chemistry, machines, and waste management technologies currently available to meet today's cleaning needs and environmental restrictions. The handbook is available as a free download from the Smart Sonic Website: www.SmartSonic.com/article.html or the IPC Website: www.ipc.org/onlinestore, Search 7526. SMT
Bill Schreiber, president, Smart Sonic Corporation, may be contacted at (818) 610-7900; e-mail: bill@smartsonic.com.
REFERENCES1. A standard developed by IPC, Stencil and Misprinted Board Cleaning Handbook, IPC-7526, February 2007. 2. Clouthier, Richard R., "SMT Stencil Printing: A Decision That Could Impact Production," Electronic Packaging & Production, July 1996. 3. Kanegsberg, Barbara, "Cleaning Systems for Low-flashpoint Solvents," Precision Cleaning, March 1995. 4. Kenyon, William, "Why Not Ultrasonic Cleaning?" SMT, October 2004.