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SMT talked to suppliers and users about problems, trends, regulations, training, cost control and ongoing research regarding cleaning materials and equipment.
Gail Flower, Editor-at-large
Over the years, materials and equipment for PCB cleaning have changed dramatically. In 1995, solvents containing chlorofluorocarbons – most of the products in use at the time – were banned as ozone-layer-depleting substances. Cost considerations made up the next area of concern when considering aqueous, semi-aqueous, and solvent-based chemistries as electronics cleaning formulae. Another basic decision centers on usage: how cleaning savvy are in-house engineers; how compatible are materials with the available equipment and components; how well does a product perform; what is the surface tension; is the odor affecting worker health and comfort?
SMT editors talked to a representative group of suppliers and users about the latest materials, equipment, and current practices, asking a set of basic questions to gauge the current market’s response.
Q: What is the most difficult board cleaning problem?
“The proliferation of lead-free alloys with no-clean flux has given rise to residue-related issues, including electromigration (dendritic growth) and electrical leakage failures,” said Michael Konrad, CEO at Aqueous Technologies. Many assemblers have discovered that their older defluxing equipment will not remove flux residue from lead-free post-reflowed assemblies, he added. Removing flux and other residues in a lead-free environment requires higher impact pressures than with leaded materials.
Most board cleaning problems revolve around getting the flux out from under closely spaced components (<3 mils), cleaning mis-printed circuit assemblies, and dealing with cleaning waste streams, according to Austin American Technology’s CEO, Steve Stach.
PCB fabricators require thorough, uniform cleaning of deep, small-diameter, high-aspect-ratio blind vias and thru-holes prior to plating, said James D. Getty, director of applications and business development, March Plasma Systems, adding that this cleaning step is called “desmear” (for less removal) or “etch back (for more removal),” and can be complex with hundreds of vias and tens of layers to the PCB.
“Full material compatibility with the chemistry in use remains one of the big show-stoppers as they are related to sensitive metals, markings, labels, etc.” said Harald Wack, Ph.D., president of Zestron. “Users in the North American market are beginning to feel the limits of DI water as the sole cleaning agent for removing LF-OA flux residues. This problem is directly related to improving the cleaning performance under low-standoff components where residues are trapped and not removed by DI water.”
Doug Winther, president of Technical Devices Company, agreed that low-standoff components are the most difficult for users, particularly with lead-free.
Figure 1. Cleaning can be batch or in-line. (Photo: Austin American Technologies)
“Designing PCBs for cleanability has become a challenging task due to miniaturization and complexity of leading-edge products,” said Mike Bixenman, CTO of Kyzen. Smaller components mean smaller spaces between traces; the smaller the area, the more difficult the reach for cleaning.
“The biggest problem for the electronics industry is removing white residues from lead-free soldering materials,” said Mike Jones, VP of marketing at MicroCare Corp. No-clean products require more cleaning and rework than older products because they are so poorly activated, he added. For those using no-clean materials, “their processes and production would be enormously enhanced if they simply decided to clean every board and then made cleaning an integral part of their manufacturing process, rather than an afterthought.”
Aside from cleaning stencils, users often use ultrasonic technology to clean single- and double-sided misprinted boards, according to Bill Schreiber, president of Smart Sonic Stencil Cleaning Systems. Manual misprint cleaning tends to deposit fugitive solder balls into vias and other difficult-to-clean crevices. Spraying would simply broadcast raw solder paste throughout the wash chamber, contaminating the reflowed side of the board. Ultrasonic uses gravity, making solder balls simply fall away from the PCB.
Getting microBGAs and fine-pitch stencil apertures clean while staying lint-free, especially when using type 3 and 4 pastes, is the most frequent discussion point heard by Chris Merow, VP of sales and marketing at Blue Thunder Technologies.
Ron Edgar, a hardware development engineer with several years of experience in designing high-rel boards for telecom, identified rework as the most difficult area of concern for cleaning. “I worked in final assembly and test for many years, and when a faulty part was reworked, a technician would replace the component and clean it off using brushes and wipes. The dissolved flux would sometimes lurk under large components, such as BGAs, in the shadows, creating possible long-term effects.” In the area of high reliability, an investment in proper cleaning and testing is not an area to trade off against costs.
Q: What are the most stringent requirements and regulations for cleaning?
“When qualifying a cleaning process, assemblers are required to qualify changes to solder, fluxing, and cleaning,” Kyzen’s Bixenman said. Some common standards are ANSI/JSTD-001, “Requirements for Soldered Electrical and Electronic Assemblies;” IPC-A-610, “Acceptability of Electronic Assemblies;” and IPC-TM-650, “IPC Test Methods.” Under 650 falls visual inspection for residues, contamination, corrosion, mask and substrate condition; mechanical test for tensile and shear strength where applicable; electrical SIR and electrochemical migration; chemical testing for ionic cleanliness; surface organic contamination; conformal adhesion; component compatibility; and residue analysis as required.
“New environmental regulations put more restrictions on what chemicals can be used in cleaning processes,” said Winther of Technical Devices. Many companies have to restructure their cleaning requirements balance cleaning effectiveness and environmental friendliness. “REACH regulations in Europe and the ever-tightening CARB regulations in California mean engineers are no longer certain what cleaning agents will be permitted in the years ahead,” added MicroCare’s Jones. Schreiber, Smart Sonic, agreed that the one sure thing about environmental regulation is change. “What seems to be the most stringent requirement today will most likely be surpassed soon by future regulations. I think VOCs are the biggest concern on user’s minds as the REACH and RoHS programs have targeted most of the harmful raw materials used in today’s cleaning chemistries.” The Southern California Air Quality Department has the most stringent VOC requirements domestically (25g/L); other states are now investigating evaporative losses, said Zestron’s Wack. Both domestically and internationally chemical oxygen demand (COD) and biological oxygen demand (BOD) levels are under investigation. REACH is currently being implemented with success, to the dismay of most chemical vendors.
“Lead-free alloys and board and component miniaturization have made the removal of flux and other residues more difficult,” said Konrad of Aqueous. While it is widely accepted that a cleanliness specification of <10 µg NaCL/in2 is sufficient, many applications now require substantially cleaner results, such as <2 µg NaCL/in2. As electronic assemblies are expected to last longer, run faster, cost less and be smaller than ever before, the threshold for acceptable residues is at the lowest level ever. Wack of Zestron added that lead-free, no-clean processes are the most demanding, especially if not cleaned within 24 hours of soldering. The more geometrically complex the assembly, the more difficult the cleaning. “The most stringent cleaning requirements I find are generally self-imposed,” noted Austin American’s Stach. “These unique requirements can result from product requirements like electrical performance or reliability or can be based on corporate culture in companies desiring a ‘green’ business model. Self-imposed cleaning standards usually exceed IPC 610 and are usually justified to protect the bottom line.”
Q: How can users control costs yet do a better job of cleaning?
Users should work hand-in-hand with suppliers, contended Wack. “A typical in-line process can be optimized by addressing cost drivers: evaporative and drag-out losses as well as overall energy consumption,” said Wack. He suggests vapor recovery systems to eliminate evaporative loss and minimize VOC emissions, process optimization to reduce drag-out losses, chemistries that use lower temperatures, and bath monitoring controls.
“PCB manufacturers should switch from a wet chemical desmear and etch-back process to a dry plasma-based process,” asserted Getty, March Plasma. “Plasma cleaning eliminates almost all liquid waste issues, saves on the high cost of wet cleaning chemicals, and improves the PCB manufacturing environment by reducing exposure to toxic wet chemicals. It is inherently a ‘green’ process.”
For cleaning PCBs, Jones suggested a total cost analysis of the cleaning process including direct costs such as solvent and electricity as well as indirect costs such as floor space, training costs, labor, environmental compliance, and capital expenditure. Modern defluxing equipment requires only a fraction of the consumable materials (water, chemical, filters) than machines built just a few years ago,” said Konrad. Modern defluxing equipment takes up less space, less electricity, less water, and less drainage than in the past. Many models require operators to merely load and unload the equipment while wash, rinse, cleanliness test, dry, SPC, chemical management, and effluent treatment happen at a button’s push.
To do a better job of cleaning with stencils, Merow of Blue Thunder Technologies recommends using a longer roll of aggressive, high-porosity, ESD-safe, rayon/polyester-blend wipe to reduce the amount of changeovers in the course of a year by 30%.
Schreiber recommended looking for hidden costs associated with cleaning. “Always evaluate and select the chemistry first. The chemistry determines the reaction with the contaminant. Consider environmental impact, user health and safety, operational cost, presence of odors and vapors, cycle time, maintenance requirements, storage, transportation, and waste management costs.”
Fluid management is critical in maintaining an economical cleaning process. Individual module containment specifically with wash chemistry remains essential. Air management reduces chemical odors in the workspace while minimizing fume exhaust from the machine. Fluid control is also critical in maintaining proper wash bath concentration.
Winther recommended regular maintenance on cleaning equipment to reduce down time as well as improve efficiency.
Stach suggested asking questions of cleaning equipment and materials suppliers. How long will the cleaning chemistry last before changing? What concentration is required to do the job? What is the typical cycle time? What is the cost of waste management? Evaluate costs based on the accumulated answers.
Q: How can users control costs yet do a better job of cleaning?
Ning-Cheng Lee, Ph.D., handles basic research at Indium Corporation. “Flux technology for lead-free alloys differs considerably from that for eutectic Sn/Pb solder systems, mainly in soldering and cleaning,” he said. The flux used in lead-free requires higher oxygen barrier capability and higher thermal stability.
“RMA flux, no-clean flux with high solid content, and water-washable systems exhibit greater prospects for upgrading to lead-free applications,” Lee explained. For Sn/Zn/Bi solders, the demand on flux capacity and oxygen barrier capability increases. Lead-free pastes require higher-temperature reflow, need higher flux capacity, and form more tin salts. Lee has found some success with semi-aqueous and aqueous solvent spray-able cleaners. Besides spraying for critical mechanical agitation, ultrasonic seems to work well. Improvements in flux thermal stability assist with soldering and cleaning.
At Stokes Institute’s Department of Mechanical and Aeronautical Engineering, University of Limerick, Ireland, Maurice Collins, Ph.D., is also investigating the effect of flux residue on creep corrosion over solder masks. He is working with lead-free processing and using aggressive flux chemistry to achieve wetting and overcome oxide film formation. The results of his study will be presented at SMTAI 2009 in San Diego.
Jun S. Choi, senior product manager at SII NanoTechnology USA Inc., has been working with benchtop XRF systems to do micro-particle contaminant analysis of boards using XRF with mapping.
Yeqing Su works on packaging development at Freescale in China. He uses plasma cleaning for robust die attach processes to improve chip-substrate adhesion. He also uses plasma cleaning pre and post wire bonding to enforce adhesion between mold compound and substrate. Stach, Austin American Technology, indicated that this type of cleaning will soon filter down to board level.
Cleaning remains an essential step in PCB processing. Lead-free materials and processing only add to the challenge. Fortunately, the work done in universities, at suppliers’ labs, at EMS sites, and in associations will continue to provide solutions for PCB cleaning. SMT
Gail Flower, editor-at-large, SMT, may be contacted at email@example.com.