Playing with marked cards [1]

Anyone who plays with marked cards is either performing a card trick - or cheating. A card magician amazes the audience. But hardly anyone believes in real miracles these days. Even children usually just puzzle over 'how the magician did it'. At the gambling table, on the other hand, the trick has a long tradition and is constantly being developed further, so that cards with radioactive prongs are already being used today.

Mammoth skull and tusk in the 'Museum of the Rockies' in Bozeman (Montana)Electronic manufacturing may not use playing cards, but sleight of hand doesn't seem to be unknown. If we look at the fluxes, the manufacturers usually insist on trade secrets [2], as they do not want to reveal to the dear customer what can be found in the mixture. Although this is old-fashioned, it is still presented as such to the less well-off customer. Old-fashioned because any reasonably trained laboratory assistant or chemist can quickly find out what's what using modern analytical methods, such as gas chromatography, and the associated programs. Just read how archaeologists can reconstruct a mammoth's entire life from an old tooth [3].

Two significant incisions have had a major impact on flux chemistry. The Montreal Protocol [4] came into force on January 1, 1989 and removed halogenated hydrocarbons (HFCs, brand names Freone, Frigene and Solkane) and a number of other chemicals used in the washing of fabricated assemblies from users. This was significant as most fluxes were formulated with the various rosins and rosin is not water soluble.

Not long after, the EU stepped in and issued the RoHS (Restriction of Hazardous Substances in Electrical and Electronic Equipment) Directive on July 1, 2006, which greatly increased soldering temperatures by largely banning lead in solders - not exactly ideal given the sharp increase in energy requirements.

Although the HKWs were not 'ideal' solvents for the flux residues, they also managed to attack the resins that were preferably used in the mixtures. Without them, there were no really good alternatives and so the (largely) resin-free or 'no-clean' fluxes were created.

Sleight of hand with flux acids

Ants do not use their acid for reflow soldering, but to fend off even overpowering enemiesTheRoHS had caused a deeper intervention by requiring significantly higher temperatures for lead-free soldering - from 184 °C to around 230 °C. It should be explained once again that the main task of acids in fluxes is to remove existing oxides produced by oxygen during the melting process. The traditional acids therefore failed because they reacted below the new melting point and this is where the first sleight of hand began.

It is also important to know that the activity of an activator usually increases with increasing temperature and then fails above a certain value. This has to do with thermal decomposition or excessive volatilization.

Probably to the regret of solderers and the relevant chemists, the situation on the circuit board is far more complicated. Not only is the flux not the only contributor - the possible reactions of the whole chemical mishmash and the temperature differences on the board make the story much more confusing.

In most cases, activators such as oleic acid or fatty acid, both from the fatty acid class, were used in the milder fluxes. Carboxylic acids and dicarboxylic acids as well as amino acids were also found in the mixtures. Halogenides or organohalides were also added to even milder fluxes.

After consulting their thick chemical tomes, there are actually only a few acids left for the fluxes, as many are too dangerous (there have been tragic accidents and even the current ones can cause respiratory or skin irritation) or have properties that make them unsuitable for today's soldering processes.

Let us more or less arbitrarily select some commonly used acids and look at their properties. The questions to be asked are their effectiveness, the temperature range in which they should be used, possible chemical reactions or decomposition products as well as evaporation, which would probably make them disappear from the assembly.

Resin acid

Although natural resins have their own problems, such as harvesting differences depending on the year, they were popular in the lead-containing past because, on the one hand, the temperatures were just right and, on the other, the residues were largely inert - ionizable salts etc. were nicely embedded in water-insoluble resin.

But what made them so suitable was rendered absurd by the lead-free solders, because they reacted too early. Although the vaunted 'plateau' in the reflow profile was suddenly no longer desirable, the resin acids had largely outlived their usefulness.

Formic acid

A few years ago, pure formic acid was praised by some adepts in the industry as a panacea for soldering. In the meantime, however, this praise has almost disappeared, as a glance at the melting point and decomposition temperature alone is enough to make this acid largely uninteresting for lead-free technology.

Then there are the health hazards, because direct contact with formic acid or concentrated vapors irritates the respiratory tract and eyes and can lead to severe burns, which would not be very pleasant for solderers.

Succinic acid

In 1546, the German mineralogist Georgius Agricola discovered that a colorless, crystalline, aliphatic dicarboxylic acid can be extracted from amber - once the death traps for prehistoric antsAlthoughthe melting point is slightly higher, the boiling point indicates that the acid evaporates and/or decomposes just as the maximum temperature is reached in the furnace.

As we all know, solid soldering is still being used - and probably more than ever before. So, in addition to the use of nitrogen furnaces and condensation systems, other 'card tricks' need to be used to deal with the oxides. Since the standards and norms are lagging far behind developments (after all, they are written by the flux manufacturers), it is safe to assume that several ingredients are used that are actually critical - usually below the percentage that should be shown in the descriptions.

New approaches are also being made, and quite a few are based on old, once discarded practices. So we come back to the stacked cards, because how else would one invent methods that can detect 'Total Halogen Content in Halogen-Free Fluxes'?

Literature

Yanrong Shi et al: The Role of Organic Amines in Soldering Materials, IPC APEX EXPO Conference Proceedings
Kamila Piotrowska et al: Corrosion reliability of electronics: the influence of solder temperature on decomposition of flux activators, https://www.researchgate.net/publication/331439191
Emmanuelle Guéné; Céline Puechagut: Development of a Suitable Flux Medium for Cleanable and No-Clean Solder Pastes Based on Tin-Bismuth-Silver Alloy, SMTA Proceedings
Christopher J. Pontius et al.: Determination of Total Halogen Content in Halogen-Free Fluxes by Inductively Coupled Plasma and some Limitations of Ion Chromatography, Proceedings of SMTA International, Sep. 27 - Oct. 1, 2015, Rosemont, IL

References

[1] Term from Rotwelsch, a sociolect of marginalized social groups, which is often defamatory referred to as 'crook language'
[2] D. Baluch; G. Minogue; Fundamentals of solder paste technology, Global SMT & Packaging - December 2007
[3] www.science.org/doi/10.1126/<br />science.abg1134
[4] www.unido.org/our-focus-safeguarding-environment-implementation-multilateral-environmental-agreements/montrealprotocol