Tin Pest and Purple Plague

April 24, 2012

The development of integrated circuitry was not without its surprises, and it offered new research opportunities for my fellow materials scientists. One problem involved the connection of the electrical signals from the silicon chips to the outside world.

Nowadays, copper is used as the conductor in high density integrated circuits to route voltage and current between transistors and other circuit elements. This copper Damascene process is replacing the earlier technology in which aluminum was used for these on-chip connections. The connection to the package terminals is done with thin, highly conductive and very flexible gold wires.

Bonding of the gold wires to the aluminum can be done by pressing a heated gold wire on the aluminum bonding pad with a little rubbing provided by an ultrasonic excitation of the bonding ferrule. Pull testing will prove that the connection is mechanically strong, as will proper electrical conductance of the joint.

The problem in the gold-aluminum bond happens when too much heat is used to make the connection. In those cases, you end up with a low conductance joint characterized by either a white or purple coloration. These problems were poetically named the white and purple plagues. An illustration of a purple plague connection of gold to aluminum is shown in the figure.

A purple plague join (2) between gold (1) and aluminum (5) on a silicon chip (3). Growth of the AuAl2 intermetallic can cause a void in the aluminum (4). (Via Wikimedia Commons).

These "plagues" are actually intermetallic compounds that form between the gold and aluminum. Purple plague is the AuAl₂ intermetallic, and the white plague is the Au₅Al₂ intermetallic. As their name implies, intermetallic compounds have properties somewhat like compounds and less like alloys. Their properties can be quite unlike that of compositions of slightly different proportions of the same elements. The gold-aluminum phase diagram is shown below.

The gold-aluminum phase diagram. The intermetallic compounds, AuAl2 (purple plague) and Au5Al2 (white plague) are shown. (Modified Wikimedia Commons image).

These plagues are prevented by keeping the bonding temperature low, so the intermetallic phase is not thermodynamically favored. This is accomplished by using less heat and more ultrasonic rubbing.

These plagues happen at elevated temperature, but there's an interesting process for tin that happens at ordinary temperatures. This isn't a plague, it's a pest, tin pest. Tin pest is a deterioration of tin caused by a phase transformation of the pure element at 13.2 °C (about 56 °F). Above this temperature, tin is the silvery, ductile metal we usually see, but below this temperature, it's brittle, grey, and nonmetallic.

Alpha (left) and beta (right) forms of tin. (Via Wikimedia Commons).

Phases are labeled by Greek letters in alphabetic order from low temperature to high, so the low temperature form is called α-tin, and the higher temperature form is called β-tin. Fortunately, this transformation has a high activation energy, so it proceeds very slowly at the start. The initial phase, however, "seeds" subsequent transformation, so it's a propagating transformation. The volume increase caused by the phase change causes a tin body to crumble into an α-tin powder. At very low temperatures, this disintegration is rapid and complete.

Tin pest was noted in early church organ pipes. These were sometimes made of silver, but mostly zinc, probably as a mitigation against tin pest. They say that an army marches on its stomach, but it travels best when fully clothed. There's a theory that Napoleon's army was defeated during its Russian campaign because tin pest caused it to lose its tin buttons. Fortunately, what we call "tin cans" are actually made from coated steel.

Pipe organ at St. Stephan's Cathedral, Passau, Germany. This organ has 17,774 pipes. Some early organ pipes were made from tin, an inexpensive and readily available metal. The climate in northern Europe was such that they were attacked by tin pest. (Via Wikimedia Commons).

Tin pest would have remained just an interesting tale in the history of metallurgy, were it not for recent efforts to remove lead from solder. The European Restriction of Hazardous Substances Directive (RoHS), and directives in other countries, have caused a problem to electronic circuit manufacturers, since the low melting point lead-tin eutectic solder can no longer be used. In some cases, pure tin has been used, leading to tin pest problems.[1] Lead-free solders use alloying elements, such as antimony or bismuth, silver and indium, to both lower the melting point and prevent tin pest.[2]

One interesting tin pest problem was published on arXiv by scientists at the Laboratoire National des Champs Magnétiques Intenses, Grenoble, France.[3] Superconducting magnets need to cycle to very low temperatures, and it was found that joints made with the Sn₉₆Ag₄ lead-free solder had a 37% lower shear rupture strength at 77K than those made from the standard Sn₆₀Pb₄₀ solder. They attribute the problem to tin pest.

For the tin man's sake, we can only hope that Oz maintains an idyllic temperature at all seasons of the year.[4]

References:

1. Michael Barthelmy, "Problems With Pure Tin Coatings," NASA, January 17, 2001.
2. Kenneth A. LaBel and Michael J. Sampson, "Developing a NASA Lead-free Policy for Electronics - Lessons Learned," TRISMAC 2008 Conference, April 15, 2008.
3. R. Pfister and P. Pugnat, "Tin Pest: A Forgotten Issue in the Field of Applied Superconductivity?" arXiv Preprint Server, April 6, 2012.
4. The Wizard of Oz, 1939, Victor Fleming, George Cukor, Mervyn LeRoy, Norman Taurog and King Vidor, Directors, Internet Movie Database.