Preparing for a solvent solution
07 March 2006
A massive requalification process is currently underway to identify, test, certify and learn to use new materials, chemicals and processes in time for July this year, when the ban on lead in most electronics products sold in Europe will come into force. Phil Kinnear argues that it would save time and money to look at conformal coatings at the same time.
Requalification requires dedication and many man-hours, and it does not come cheap: it can cost £100,000 to put a set of chemistry through the necessary technical and administrative processes. This begs a question: if the investment needed is this significant, doesn't it make sense to optimise the requalification spend by looking a little further than lead-free?
While we concentrate on getting the lead out of our processes, solvents are still entering our environment at an alarming rate, potentially creating more damage than lead has ever done. Airborne solvents emissions can affect the environment at two levels: they contribute to smog formation and general reduction of air quality at ground level, while in the upper atmosphere they interact with ozone and can contribute to global warming.
Legislation relating to solvents lags far behind that relating to lead, effectively - for electronics manufacture - leaving emissions reductions by and large to the good citizenship of users. But this is very likely to change, and there are already signs that, by law, we will have to change our manufacturing habits regarding solvents. VOC-free legislation is already in place in the US and Europe, and although European electronics manufacture still has room for manoeuvre, there is nothing to suggest that this will continue to be the case.
Conformal coatings
For electronics manufacture, one of the areas that relies heavily on solvents is conformal coating. This thin polymeric film protects PCB assemblies against moisture and pollution from the operating environment, ensures longer product life and protects against board failure.
Conformal coatings can be split into three categories: the more traditional solvent-based systems, which are well understood, represent 95% of conformal coating by volume in Europe and 50% in the US. Solventless systems make up the remainder, bar a very small number of specialist manufacturers who use water-based systems.
Solvent-based
Legislation is already moving to reduce the impact of solvents: toluene, the vehicle in up to 40% of solvent-based conformal coatings, has recently been reclassified by the UK's health and safety commission and its use banned by a number of countries throughout the world. This, together with the likelihood of bans on additional solvents, should put a large question mark over the continued long-term use of solvent-based systems.
Water-based
This technology, too, is fairly well known and well regarded, but it involves a long cure time. Although the coating cures to tack-free within 15 minutes (compared with 10-12 minutes for solvent coatings), moisture typically remains within the coating for 24 hours, so the boards cannot be electrically tested immediately following coating. For larger volume manufacturers, this is unacceptable. As an alternative to solvent-based systems, however, water- based products are ideal for lower volume manufacturers where the cure cycle times are not a problem.
Solventless
The advantages of these 100% solids 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 US, the majority of the coatings used are silicone systems which either react with moisture and cure from liquid to solid form, or, alternatively, are heat-cured at 120(C for 5-10mins.
UV curable solventless systems use the same principle, but require exposure to UV, for a "snap-cure" that takes just 10-20 seconds, a speed that makes them far more attractive than any of the other alternatives. The reason that their use has been limited until now is that they have typically needed a secondary heat cure system to ensure polymerisation of any conformal coating in the " shadowed areas" under the components, effectively cancelling all the advantages of high-speed UV cure. The UV coatings are typically inflexible, yet have a coefficient of thermal expansion that far exceeds the board assemblies they protect. As a consequence, they tend to crack with use.
The conformal coating industry has worked hard at eliminating these problems by developing a UV alternative that makes the most of the exceptional cure speed. This has been achieved using a two-phase curing system that relies first on UV exposure and secondly on moisture. The coating is tack-dry within 20 seconds and is at full cure in 2-3 days using only ambient temperatures and moisture. It is also flexible enough not to crack, even after aggresive thermal cycling, amply exceeding the stringent demands of the automotive industry.
On a per litre basis, solventless UV curable coatings are at least twice the price of solvent-based or water-based alternatives. However, solvent-based and water-based systems cure to leave just 20-35% of that litre on the assembly, while solventless systems leave 100%, effectively making the prices comparable.
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 far larger processing line than UV-cured solventless alternatives. Given all the advantages it is not surprising that electronics manufacturers have been asking for high-performance UV-curing solventless conformal coatings for years. Finally, with the latest developments in the market, they now have access to such materials.
The issues of no-clean
Although the military and aerospace sectors routinely clean their boards, the automotive industry, which uses a significant share of coatings, does not. The net result is that over 95% of conformal coatings used in Europe are applied to no-clean boards, creating the industry's biggest barrier to quality conformal coating, and more importantly, to board performance. This is because the numerous process chemistries and materials used in PCB and assembly manufacture leave all sorts of residues on the board. Because they are so many, and because their proportions will vary, it is almost impossible to predict how these residues will react with each other immediately, during manufacture, and over time in the field.
Indeed, one of the reasons that solvent-based systems are popular is that the solvent vehicle will tend to act like a cleaning solution, dissolving the dirt from the surface, and locking it up into the coating. Solventless systems do not have any sort of degreasing action, they sit on top of the residues.
Taking this into account, the ideal solution is a UV curing system that has a low solvents content that is sufficient to degrease the board but low enough to disperse quickly from the coating during the snap UV curing. This "halfway house" alternative offers the benefits of low viscosity, low coating thickness, good flow and quality finishing and effectively represents a drop-in UV conformal coating for those electronics manufacturers who are attracted by UV conformal coatings but are wary of making the step from solvent-based to solventless in one go. The only investment necessary is in the UV curing equipment.
Contact Details and Archive...
Related Articles...
Most Viewed Articles...