Topic

Cleaning issues

Contents

Introduction of fluorocarbon solvents

Rosin flux residues require cleaning, especially when activated fluxes have been used, and cleaning was very commonly carried out, especially throughout Europe and the USA, driven by a perception held by military customers that all flux residues are potentially harmful. The original approach used a variety of solvents but these gave problems:

Some solvents present health hazards and/or have low TLV limits.

Key Information

An occupational exposure standard (OES) is the concentration of an airborne substance, averaged over a reference period, at which according to current knowledge there is no evidence that it is likely to be injurious to employees if they are exposed by inhalation day after day to that concentration. Chemical manufacturers are required to issue product safety information which will contain OES information, although it may be given in terms of the US threshold limit value (TLV) which for practical purposes is the same.

Fluorocarbons had been developed during the 1930s as refrigerants, and in the late 1960s were introduced for the cleaning of electronic assemblies. The particular compound most commonly used was 'CFC-113', 1,1,2-trichloro-1,2,2-trifluoroethane1. This material:

For these reasons, fluorocarbons rapidly became the preferred means of cleaning, despite their expense. The solvents were marketed under a variety of trade-names, of which Arklone and Freon were the best-known in the UK.

1 The numbering system used for CFCs and similar compounds is confusing, but there is an explanation at http://www.epa.gov/ozone/geninfo/numbers.html.

 

[back to top]


Ozone depletion

Following scientific evidence of the damaging effect on the stratospheric ozone layer, which protects us from the harmful effect of the sun's radiation, over 100 countries agreed, in the Montreal Protocol of September 1987 (amended in London, June 1990 and at Copenhagen, November 1992), to phase out man-made ozone-depleting chemicals as quickly as possible. Not only was CFC-113 affected, but another of the common chlorinated solvents (1,1,1-trichloroethane, or methyl chloroform, and sold as Genklene or Chlorothene) was also covered by the resulting European-wide regulations, where phase-out was scheduled by the mid-1990s. The Protocol and its associated EC regulations imposed control on the supply to the market of the substances rather than their use as such, but the effect is the same, to force electronic companies to seek different ways of approaching the problem, and these materials have now essentially become unavailable.

The Ozone Depletion Potential (ODP) is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11, and indicates the relative ability of substances to damage the Earth's ozone layer. There are two groups of chemicals, classified according to ODP:

Class I materials, those with significant ODP, have already been phased out. HCFCs and other halogenated materials with some ODP are subject to increasing restrictions and will also be phased out, those with the highest ODP values first. For example, the commonly used HCFC-141b ceased production on 1 January 2003.

If you are interested in researching this in more detail, a good starting point in the US Environmental Protection Agency website at http://www.epa.gov/docs/ozone/.

2 For a complete listing, see http://www.epa.gov/ozone/ods.html (Class I) and http://www.epa.gov/ozone/ods2.html (Class II).

 

[back to top]


Other issues

The list of environmental issues involved with cleaning materials is in fact much more extensive than mere ozone-depletion, although it is that aspect which has been nearest the headlines. A fuller list includes:

The Environmental Protection Act 1990 controls emission from large cleaning operations (the trigger levels for which are constantly reducing) and, through Water Authority consents, is also affecting the ability of a company to discharge even treated effluent. Legislation in the EPA and elsewhere, regulates how many materials, including spent solvents, may be carried, transferred, stored, treated and disposed of. These regulations are likely to get tighter and involve higher costs in the future.

Probably the most unfriendly material from the GWP perspective is sulphur hexafluoride. Used as a cover gas in magnesium production and casting, as a dielectric gas and insulator in electric power equipment, as a fire suppression discharge agent in military systems, and formerly as an aerosol propellant, this has a GWP of 22,200 and a life of 3,200 years!

[back to top]


Hazards in cleaning

Almost all cleaning options have some potential to harm operators if not properly controlled to minimise risk. Precautions taken have to be reasonably practical but also to take into account technical, cost and environmental issues. Three aspects of Health and Safety hazard are generally considered:

[back to top]


Alternatives to CFCs

As with the replacement for tin-lead solder, there is no immediate drop-in replacement for the CFCs used in cleaning. The nearest to such a material, n-propyl bromide, was developed too late in the day, and its use has been somewhat controversial. Although it has an ODP rating of zero, is relatively cheap, non-flammable and fits a range of applications, it is somewhat toxic, with an OEL of 50 ppm, and some concern that this will reduce. In consequence, there has been no change to the four basic approaches to the withdrawal of CFCs that were proposed at the outset:

  1. Use a volatile organic solvent in a process similar to CFC vapour cleaning using either non-flammable HCFC solvents or flammable solvents with a lower global warming potential.
  2. Use an organic solvent with a high boiling point, either evaporating the solvent at elevated temperature or choosing a water-miscible material which can be rinsed with water and dried once the solvent has completed its task of removing the 'soil' - this is referred to as a 'semi-aqueous' process.
  3. Use water as a solvent, either in combination with detergent to remove rosin flux residues or in conjunction with water soluble fluxes. Some components are not compatible with water or are difficult to clean because of fluid entrapment in small crevices. Water-soluble fluxes are more active than rosin fluxes, providing a larger process window, but even traces need to be removed because they are acid.
  4. Use a no-clean process.

A somewhat unexpected problem with a number of the processes, especially those based on water/saponifier combinations, is that the lead from solder will actually dissolve in the solvent, whereas this was not a problem with CFCs. This points out the need to consider every aspect of the process when evaluating a proposed change!

With no 'drop-in replacement' for CFCs available, much attention has inevitably been focussed on removing the need for cleaning. Figure 1 gives an example of a review carried out by a user of the cost and difficulty of alternative cleaning approaches, from which the no-clean option can clearly be seen to be desirable, if less easy than using CFCs.

Figure 1: Difficulty : cost ratio for different cleaning methods, Source: Nortel

Difficulty : cost ratio for different cleaning methods

[back to top]


No-clean processes

No-clean processes are not the same as not cleaning! In the latter case there will be flux residues, and they may or may not be deleterious to the long term reliability of the end product. A no-clean process involves selecting flux materials which will be effective in use but whose residues are:

Of these, the major issues in recent years have been associated with pin testability and cosmetic acceptability, and work continues on solder ball elimination and maximising the process window.

No-clean systems

No-clean systems have been formulated for both wave-soldering and reflow soldering. The basis for both is a flux which has a higher proportion of either solvent (for wave soldering) or rheology modifiers (for reflow soldering) and a lower concentration of both flux base and activator. Moreover both flux base and activators are chosen so that their decomposition products are not harmful.

The ultimate of no clean is 'no residue', where the flux is expended entirely during the process of producing the solder joint. This also presents the ultimate challenge for the user! The issues with all no-clean/no residue processes relate to process control, so that solderability is achieved and, the joint satisfactorily wetted, before the protective qualities of the flux are dissipated.

[back to top]