What are tin whiskers?
Tin whiskers are very fine growths (of diameter of 1-50 microns) that grow in a straight, convoluted or spiral manner and are of varying diameter, between the electrical connections/ adjacent pads or all kinds of components, whose attachment together with the growth between them can cause electrical shorts and significant damage to the functioning of electronic systems.
Already in 1951, problems with tin whiskers were discovered in electronic circuits, but although they could not be explained, the problem was successfully coped with using lead solders.
After the entry into use in 2006 of the green standard (ROHS) and the beginning of the prohibition of the use of solders containing lead, and with the transition to working with pure tin solders, the problem began to seriously increase.
In fact, until today there is no absolute explanation of the phenomenon of the growth of the tin whiskers, but experiments made recently have succeeded in discovering a one to one connection between the variables that support and encourage the growth of tin whiskers.
The most likely explanation for this phenomenon (apart from the absence of lead) is mechanical pressure, such as residual stresses caused by electro-plating, stresses resulting from bubbling of various metals, heat that causes volumetric changes that introduce stresses. Furthermore, the presence of oxygen and carbon can produce these growths.
It is known that the tin whiskers grow from the base and around it, and there they are thicker. The cause of their growth is very small stresses, from currents that pass through the pad (in other words, energy), and even mechanical stresses involved in assembly and packaging, and even coatings from materials that introduce shrinkage or tensile stresses in the circuits that result from changes to the volume of the coating during polymerization or vulcanization.
It has been proved that the phenomenon of tin whiskers has tremendously increased in recent years during transition to assembly and soldering aids that are lead free. However, the reason for the increase has not yet been sufficiently researched, but it is clear that the rate of increase is frequently related to the quantity of lead in the solder products.
It is emphasized that it has been proved until now that the sole reasonable means of prevention, for the problem of tin whiskers, known until now throughout the world is use of the parylene coating (that “wraps” the pads and performs, on one hand, encapsulation and sealing in the best way, and does not introduce any stresses into the circuits/components, on the other way).
Parylene C, in contrast to regular conformal coatings, as set forth above, does not introduce stresses into the components of the electrical circuits. Furthermore, parylene is known to protect the electrical circuits against gases, liquids, and acids.
The penetration of parylene and its perfect following of the shape of the component considerably reduces the quantity of free oxygen found between the component and the atmosphere (we in fact “choke” the component by means of parylene), that is believed to be of great help in preventing the phenomenon.
A table showing examples of failures from various industries that resulting from tin whiskers phenomenon.