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Maximize Your Investment: Re-qualify for Lead-free
December 31, 1969 |Estimated reading time: 7 minutes
A massive re-qualification process is underway to identify, test, certify, and learn to use new materials and processes for RoHS. Re-qualification requires dedication and many man-hours. if the investment needed is this significant, Why not optimize re-qualification costs by looking a little further than lead-free?
By Phil Kinner
massive re-qualification process is underway in the global electronics manufacturing industry to identify, test, certify, and learn to use new materials, chemicals, and processes in time for the RoHS deadline. Re-qualification requires dedication and several man-hours, and can cost $175,000 (U.S.) to put a set of chemistry through necessary technical and administrative processes. But if required investments are this significant, wouldn’t it make sense to optimize re-qualification dollars by looking beyond lead-free?
While we concentrate on getting the lead out of processes, solvents are entering the environment at an alarming rate, potentially creating more damage than lead. Airborne-solvent emissions can affect the environment at two levels. They contribute to smog formation and general air-quality reduction at ground level. In the upper atmosphere, they interact with ozone and can contribute to global warming. Legislation relating to solvents lags behind that of lead, leaving emissions reductions up to users, requiring them to reduce their environmental impact voluntarily. But this likely will change; and there are already signs that, by law, manufacturing habits regarding solvents must also change. VOC-free legislation is in place in the U.S. and Europe, and although European electronics manufacturers still have room to maneuver, there is nothing to suggest that this will continue. Stricter local legislation can override general Europe-centric rulings, and widely used solvents - increasingly recognized as environmentally damaging - are being reclassified and banned.
Conformal Coatings
Conformal coating relies heavily on solvents. The thin polymeric film protects PCB assemblies from moisture and operating environment pollution, extends product life, and protects against board failure (Figure 1). Widely used in defense, aerospace, automobile electronics, and industrial controls where failure could be catastrophic, conformal coatings are finding their way into applications such as domestic appliances (Figure 2).
Figure 1. Conformally coated board.
Conformal coatings can be divided into three categories. Traditional solvent-based systems represent about 50% of conformal coatings by volume in the U.S., and 95% in Europe. Solventless systems make up the rest, barring a small number of specialist manufacturers using water-based systems.
Solvent-based. We have touched on the problems of solvents, and legislation is moving to reduce their impact. The U.K.’s health and safety commission recently reclassified toluene, the vehicle found in up to 40% of solvent-based conformal coatings, and its use was banned by a number of countries and manufacturers worldwide. Combined with the likelihood of bans on additional solvents, this should place a large question mark over the continued long-term use of solvent-based systems. Add to this the waste factor - 60-80% of every liter bought is lost to the environment - as well as the hazards and consequent costs of transport, storage, and handling, and solvent-less based coatings quickly become less attractive.
Figure 2. Conformal coating room.
Water-based. Water-based alternatives are well known and regarded, but involve long cure times. Although the coating cures to tack-free within 15 minutes (compared to 10-12 min. for solvent coatings), moisture typically remains within the coating for 24 hours. Therefore, boards cannot be tested electrically immediately following coating. For larger-volume manufacturers, this is unacceptable. As an alternative to solvent-based systems, water-based products are ideal for lower-volume manufacturers, such as specialist military manufacturers.
Solventless. To date, solventless systems have seen more success in the U.S. than in Europe. The advantages of these fully solid systems include the fact that 100% of what you put onto the board turns into coating; there are no volatile chemicals; and they are easy to transport, store, and handle. In the U.S., the majority of solventless coatings used are silicone systems that either react with moisture and cure from liquid-to-solid form or are heat-cured at 120°C for 5-10 mins.
UV-curable solventless systems use the same principle, but require exposure to UV for a snap-cure that takes about 10-20 seconds. Their use has been limited until now because they typically required a secondary heat-cure system to ensure polymerization of any conformal coating in shadowed areas under components, effectively negating the advantages of high-speed UV curing. Due to their high viscosity, these coatings also tend to be at least 100-μm thick - double that of solvent- and water-based alternatives. This can be seen as a disadvantage, although some believe that the thicker coatings offer better protection. UV coatings typically are inflexible, but have a coefficient of thermal expansion (CTE) that exceeds the board assemblies they protect. Consequentially, they tend to crack with use.
The conformal coating industry has worked to eliminate these problems by developing a UV alternative that makes the most of the exceptional cure speed. This was achieved using a two-phase curing system that relies on UV exposure and moisture. The coating tack dries within 20 seconds, and reaches full-cure in 2-3 days using only ambient temperatures and moisture. It does not suffer from problems of shaded areas, as its viscosity prevents it from flowing under components. However, it can be applied using robotic spray equipment that is commonly used in the industry. It also is flexible enough so it does not crack, even after aggressive thermal cycling - exceeding the demands of the automobile industry.
On a per-liter basis, solventless UV-curable coatings are at least twice the price of solvent- or water-based alternatives. However, solvent- and water-based systems cure to leave 20-35% of that liter on the assembly, while solventless systems leave 100%. Therefore, the cost per square meter of solventless coatings, at the same dry thickness, is comparable to solvent- and water-based coatings.
But this is not the entire story. First, solvent materials cost more to transport, store, and handle; and with a 12-13-minute thermal cure, they also cost more to process. They also require a larger processing line than UV-cured solventless alternatives. By replacing 10- or 13-foot-long thermal-curing ovens and replacing them with 3-foot-long UV-curing units, manufacturers can increase throughput by 120% while saving 50% of their processing footprint. Given all the advantages, it is not surprising that electronics manufacturers have been asking for high-performance, UV-curing solventless conformal coatings for years.
No-clean Issues
Although the military and aerospace sectors routinely clean their boards, the automobile industry, which uses a significant amount of conformal coating, does not. The net result is that more than 95% of conformal coatings used in Europe, for example, are applied to no-clean boards - creating the industry’s largest barrier to quality conformal coating and board performance. This is because many process chemistries and materials used in PCB and assembly manufacturing leave several different residues on the board. Because they are several, and because their proportions vary, it is almost impossible to predict how these residues will react with each other immediately, during the manufacturing process, and over time in the field. Conformally coating a board that has been through the electronics manufacturing process and has not been cleaned could be compared to putting wallpaper up without first cleaning and fixing the walls, or driving a new car through a field and getting it covered in dirt, dust, and other contaminants before taking it to be painted.
With narrowing process windows and tightening performance specifications, no-clean seems anachronistic. We know an automobile should be cleaned for a perfect paint finish, so why not clean an assembly that may run a crucial control system?
Hopefully, automotive electronics manufacturers will revert to cleaning boards. Until then, conformal coating manufacturers must live with the reality of no-clean and offer the best performance possible. One of the reasons that solvent-based systems are popular is that the solvent vehicle tends to act like a cleaning solution, dissolving the dirt from the surface and locking it into the coating. Solventless systems do not have any sort of degreasing action. They sit on top of residues, so board adherence depends on adhesion of residues to the board. Taking this into account, the ideal solution is a UV-curing system that has a low solvent content sufficient enough to degrease the board, but low enough to disperse quickly from the coating during snap-UV curing. The fact that the solvent used is considered VOC-free in the U.S. further increases the appeal of this methodology. This alternative offers low viscosity, low coating thickness, good flow, and quality finishing. It also effectively represents a drop-in UV conformal coating for those desiring UV conformal coatings, but wary of moving to a solventless solution.
Conclusion
Although conformal coatings are compatible with lead-based and lead-free manufacturing practices, and the move to lead-free does not have a direct impact on the choice of conformal coatings, current re-qualification efforts could be made more efficient by taking a long-term view of legislation. Today, the focus is on lead; tomorrow it likely will be on solvents. Re-qualification is a massive investment in terms of resources and money. It makes sense for manufacturers to include solventless and low-solvent conformal coatings in re-qualification processes.
Phil Kinner, chief chemist, HumiSeal Europe - Chase Specialty Coatings, may be contacted at 44 1276 691 10 0; e-mail: sales@humiseal.eu.