Tin monsters and fighting them

How? Pure tin is capable of releasing tentacles – they are called "tin whiskers" or "whiskers". Thin to invisibility - 1-10 microns, and long - up to 10 mm, they rapidly grow from any area with pure tin, whether it is the coating of components or tinning on the board. And then, oriented by the electric field, they greedily attach themselves to neighboring conductors, causing short circuits that provoke either inexplicable malfunctions or fatal breakdowns. In a low-current circuit, they can torment you for years with glitches of unknown nature, either growing or melting. If the power is sufficient, the whisker turns into plasma upon short-circuiting, causing a cascade of failures. It is unpleasant to drop a cigarette and burn your pants, but it is more annoying to drop it on an oily rag in a warehouse with flammable materials.

Why does this happen? It has been reliably established that at 13.2°C tin suddenly decides to change its tetragonal β-phase to a cubic α-phase, destroying the macroscopic structure down to the state of fine powder. The speed of the process rapidly increases with decreasing temperature, and the cubic phase, even at a higher temperature, when in contact with normal tin, causes its transformation. The process is accompanied by the growth of drying ulcers on the surface of the metal, which is why the phenomenon is called tin plague.

Known since ancient times, the plague was described by Aristotle. According to historical anecdotes, it was one of the reasons for the defeat of Napoleon's army in Russia. Allegedly, in the cold, buttons, spoons, and mugs turned into gray powder. It is considered proven that the unreliability of tin soldering was one of the reasons for the failure of Robert Scott's expedition to the South Pole. In a letter written in sunny -70-degree March 1912, Scott addresses his widow: "...I think there is no chance. We decided not to kill ourselves, and to fight to the end to reach the camp. Death in the struggle is painless, so don't worry about me."

The infection has not been eradicated to this day, appearing everywhere with insidious unpredictability. Like a quarrelsome wife, the allotropy of tin can turn a calm life into a disaster in an instant for the most unimaginable reason. It's not just a drop in temperature, but also contact with catalysts, including mercury compounds, diffused copper ions, and even its own oxide film, as well as ionizing radiation, vibration, and humidity. It is impossible to predict when the problem will appear; cases of short circuits have been documented both 5 days and 5 years later. Thus, by doing "good" with other people's hands, Eurobureaucrats have bloodied their own up to the elbows.

It is hardly possible to count how many freezes, spontaneous reboots, and data losses were caused by short-term short circuits alone, but even on household appliances and personal computers, tin whiskers are the main source of malfunctions. By the grace of God, in Russia, despite the cold, the infection does not take root well, but in its homeland, they have fully tasted the cup of good intentions. For example, in 2006, SWATCH suffered undisclosed losses, being forced to recall batches of watches worth $1 billion from dealers! And the damage is not limited to money:

• from 1975 to 1989, the number of failures in PACEMAKERS caused by whiskers continuously increased until regulatory authorities finally took notice, but the problem has not been completely resolved TO THIS DAY

• failures at nuclear power plants: Dresden NPP, South Texas NPP, Millstone NPP

• communication satellite outages: PAS-4, DirecTV 3, SOLIDARIDAD, GALAXY IV

• Phoenix and Patriot missile guidance errors – indeed, a five-hundred-kilogram rocket on perchlorate fuel, it's so environmentally friendly, why spoil it with lead solder

And these are just the most egregious cases that led to investigations and leaked into the public domain.

The problem, as described, is serious. Not just serious – frightening. And if we sometimes choose what to solder with ourselves, it is impossible to buy some components in a lead-free version.

It's time to talk about tackling the problem.

Let's make a reservation right away – if there was a guaranteed method to once and for all stop the growth of tin whiskers, we would probably have already forgotten about procrastination. Alas, there is no magic wand yet capable of solving this problem with a single stroke. But, as the shop master said, "It's better to try than to watch it break on its own".

So, a brief list of measures against this microscopic but very unpleasant problem:

1. Application of protective coatings: an attempt to "bury" the problem. The most intuitive option is to try to seal the tin, depriving the whiskers of room for maneuver. Just as we try to contain hogweed by covering the ground with film, engineers use special coatings here. It would seem, what could be the problem.

Firstly, tin whiskers have ATOMIC tips and can penetrate any coating. Secondly, if improperly selected, it can itself cause damage during thermal cycling. Due to the difference in coefficients of thermal expansion, the coating may crack and exacerbate the problem. As they say, "measure seven times, glue once." Remember that choosing a coating is like choosing a life partner: a mistake can cost you not only nerves, but also money.

2. Elimination of contamination: soldering hygiene. This is not even a measure, but a necessary condition. Cleanliness is the key to health, and this applies not only to dishes. Residues of flux, dust, grease, fingerprints, and other contaminants on the surface can serve as a kind of "nutrient broth" for the growth of whiskers. Therefore, thorough cleaning of the surface before soldering is not a perfectionist's whim, but an urgent necessity. The use of special cleaning solutions, brushes, and ultrasonic baths helps to ensure proper cleanliness. But, as practice shows, even the most sterile "operating room" for boards does not guarantee the complete absence of whiskers. "I washed the board like a cat in childhood, but the whiskers still grow", one of the clients once complained.

3. Reducing mechanical stresses: "Relax, don't do it". Mechanical impact regularly acts as a trigger, stimulating the growth of whiskers. Both vibration and the difference in material expansion create internal stresses that can lead to deformation and breakage of soldered joints, and in our case, also to the growth of unwanted "whiskers".

To avoid this, engineers try to minimize deformations: both by selecting materials that have the most similar coefficients of thermal expansion, and by optimizing the layout. Ideally, achieve a state where the board and components feel like best friends in a hammock: they are comfortable and calm, there is no tension, but this, unfortunately, is not always possible, especially when soldering dissimilar materials.

4. The path of least resistance is the use of special solders. Finally, we have come to the simplest and, according to most experts, the most effective method. Obviously, not all solders are equally useful in the fight against tin whiskers. And here comes the one who was destined to become the hero of this article... Yes, the very SAC (tin-silver-copper) - solder with silver and copper content, with a silver content in common formulations from 0.3 to 4%, of which the optimal combination of properties and price is SAC305 with a silver content of 3%. It is this, at first glance, modest percentage, as engineers and materials scientists claim, that is the very silver sword that can effectively cut off the whiskers of the monster.

From the point of view of crystallography, the addition of silver to the tin-copper alloy stabilizes the structure of the crystal lattice. As we discussed in the previous article, pure tin has a tetragonal lattice that is prone to defect formation and atom migration, which, in turn, initiates the growth of filamentary crystals. In SAC solders, silver prevents allotropic transformations by locking vacancies and interstices, which are the main conductors for the migration of tin atoms.

From the point of view of quantum chemistry, the addition of silver changes the electronic structure of the alloy, namely, the density of electronic states on the surface, reducing the thermodynamic probability of whisker formation, as well as contributing to the formation of a stronger and less prone to deformation interphase boundary between the solder and the contact pad.

In addition, it can be assumed that the addition of silver homogenizes the surface energy of the solder, which can also level the nucleation and growth process of tin whiskers. Ultimately, the formation of a stable solder joint with low surface energy and a small number of defects in the crystal lattice reduces the tendency of the material to grow unwanted formations, not only whiskers, but also cracks and polycrystalline inclusions.

But it's not just about anti-whisker properties. Many manufacturers have long adopted the SAC family also because silver-containing solders provide a number of pleasant additions:

• Good spreadability: even coverage is the key to success. Silver added to the alloy significantly improves wetting properties, allowing the solder to spread evenly over the surface and form a strong and durable bond. As solderers say - "Solder should spread like rumors in a village".

• High strength and resistance to thermal cycling: reliability is everything. Joints made with SAC solder have higher mechanical strength and significantly greater resistance to temperature fluctuations. After all, it is the cyclic fluctuations that are the main cause of "fatigue" of soldered joints, leading to their destruction.

• Reduction of dross formation: less waste, cleaner work. The addition of silver to the solder helps minimize the formation of oxides, which in turn leads to a reduction in slag output in the case of wave soldering and an overall improvement in soldering quality.

• Non-toxicity (relative to other lead-free): safety first. Although lead-free solders are still controversial, especially in the context of their impact on health and the environment, SAC materials are relatively non-toxic compared to other lead-free compositions.

As a result, the use of silver-containing solders is, without exaggeration, a treatment not only for the symptom, but also for the disease itself, as well as an improvement in mechanical and electrical characteristics.

Bismuth solders: why are they not as good as they seem?

In the context of lead-free solders, alloys with the addition of bismuth are often mentioned. Bismuth can indeed reduce the need for lead, providing, in addition, a lower soldering temperature. However, these alloys have significant drawbacks:

• Brittleness: Bismuth solders are generally more brittle compared to others. This is especially critical for devices operating under high stress conditions.

• Wetting issues: Bismuth impairs the wetting of contacts, leading to the formation of unreliable solder joints.

• Interaction with lead (and other metals): The presence of bismuth in solder paste can cause undesirable reactions with lead (if present in component coatings), resulting in poor solder quality and even joint delamination under high temperatures. This creates serious problems when using mixed technology, where lead-free solders are used on components with leaded leads, which, unfortunately, cannot always be avoided. In addition, bismuth forms a thermocouple with many metals. And the idea of soldering thermocouples in random places on the circuit is rarely attractive to anyone.

The price of peace of mind: why SAC is not a luxury, but a necessity

Of course, silver is a noble metal, and solders containing it may cost more than regular ones. But let's look at this situation from the point of view of economic feasibility and potential risks... No, let's look at it from the point of view of the company "SWATCH", which, as you remember, lost a billion greenbacks, saving a thousand times less on solder.

Moreover, in a number of critical industries, such as military, marine, aviation, and space technology, the requirements for reliability, durability, and fault tolerance are so high that the use of silver solder is not a recommendation, but a mandatory condition. Here, tin whiskers are not an annoying curiosity, but a potential disaster that can lead to human casualties and multi-billion dollar losses. And if you are a contract manufacturer working with these industries, then the choice is in favor of SAC. "We are not soldering daisies here", one military inspector told me when I was working at the factory.

Here comes the end of the text dedicated to such an engaging issue as the ban on lead in soldering. I tried to cover everything related to this topic: from the theory of formation to practical ways of dealing with it. I hope that I managed to make the mysterious trivial, and the inexplicable obvious. Remember that the technology of electronic production is an immense universe at the intersection of the continuums of many sciences, despite being thoroughly studied, it still remains magic even for the initiated. And the whiskers? Well, they belong not on the board, but you know where.

Author: Georgiy Alexandrov