Board fabrication plants are something that has received considerable attention in recent years for both safety and environmental reasons, seeking to reduce the total amount of water used and to eliminate processes with adverse environmental and health hazards.
The replacement of electroless copper has been promoted for two reasons. Firstly many of the formulations involve formaldehyde, a carcinogen, and secondly the processes consume large amounts of water. As a result, many manufacturers have adopted alternative 'direct metallisation' techniques, although there is now an electroless copper process that does not use formaldehyde. Probably the most attention has been given to direct metallisation systems based on palladium, but carbon/graphite treatment and conductive polymers are also in use.
You will know from your study of HASL that there are many environmental implications of this process. Not only does it use heat, but the coating contains lead, and the fluxes used are aggressive. The EPA study on this is particularly interesting because it provides comparative information for six different surface finishes, and there is no single winner.
For example, all the processes use hazardous materials1, some of which are subject to regulatory requirements, and one or more of which are materials that present a risk to the aquatic environment. Perhaps more surprising is the very wide range in the consumption of water and energy, with a range of 7:1 in water consumption and 10:1 in energy consumption for different processes applied to the same test board. ENIG is seen to be not as environmentally benign as some other surface finishes, a fact also borne out by the Cookson Electronics life-cycle analysis.
Other areas where savings can be made, both in cost and to the environment, involve the regeneration of materials such as spent etchants and the recovery and reuse of other materials. A case has also been made for the use of plasma de-smear to replace the permanganate process.
PCB manufacturers also have waste streams that are associated with photoresist: the spent developer solution, containing uncured resist and potassium carbonate; the spent stripper solution, containing cured mask, known as 'skins', and a high percentage of amines; the rinses from these processes; and the skins filtered from the stripper solution during processing to extend stripper life.
The main opportunities for cost-saving relate to the expensive concentrate used for stripping, where recycling the majority of the stripper can save substantial costs. Removing the skins, and separating them from fluids that have been trapped, is one of the challenges, where centrifuge methods have proven of value.
The critical area is that of water usage. When water used to be freely available, 'total loss' systems were prevalent,. Here the total volume of discharge was unimportant, provided that heavy metals and other contaminants were present in sufficiently low concentration. With more recent shortages of water, and its high cost, there is much emphasis on saving water consumption.
The techniques for this are generally relatively straightforward, but need a lot of attention to detail. Many savings can be made simply by implementing drag-out reduction and rinse water reduction techniques; the next step is the more capital-intensive work of re-circulating cooling water and use ion exchange technology to purify wastes. As with the semiconductor industry, correct processing can also help metal reclaim.
One of the problems in any attack on water usage is a lack of awareness of how much water is being used and discharged, and of the consequent cost to the business. One useful idea is that of a 'water mass balance', based on the principle of conservation that the amount of water entering a site must equal the amount of water, in all its various forms, that leaves the site. Based on the survey of water used, a water mass balance is a key technique for identifying where water is used, and where it might be wasted. Even relatively small continuous usages of water can have a substantial impact on costs: bear in mind that you are charged both for the water you consume and for the effluent you discharge.
If you are faced with the problem of implementation, register at Envirowise (formerly the DTI's Environmental Technology Best Practice Programme) http://www.envirowise.gov.uk/ for several relevant publications:
Other problems relate to devising effective techniques of removing persistent organic species from spent liquor, operational baths and rinse water. In particular, there are concerns about chelating agents such as EDTA (ethylene diamine tetra-acetic acid), which take up heavy metals during the process. There are also concerns about materials used as brighteners, photochemical strippers, surfactants and surface-modifying compounds.
A promising procedure is to use ozone and hydrogen peroxide either together or individually in the presence of short wavelength (254 nm) ultraviolet radiation. This generates highly reactive hydroxyl radicals that destroy virtually all organic substances, and have a greater oxidising potential than hydrogen peroxide or ozone separately.