Soldering_PbFreeSolderAndTinWhiskers

Changes in the preferred solder formulation used by commercial product manufacturers have created a pronounced presence in the market of solders without elemental lead (Pb) in them. The alloy compositions which contain elemental tin (Sn) and Pb melt at a lower temperature than those with Sn but without Pb. This impacts the formulations of flux that can be used with Sn-based Pb-free solders, operation of reflow systems and selection of printed circuit board materials, and the composition of solder used to pretreat or plate component leads. When Pb-free Sn solder is used to plate part leads, particularly copper leads, the Pb-free Sn solder has a tendency to “grow” tiny “whiskers” which are conductive and very strong which can lead to electrical short circuits. Whiskering from Pb-free platings can be quenched by tinning the surfaces with Sn-Pb solder though any small area that is not touched by the tinning process will still be capable of growing whiskers, including mechanical parts which are not tinned.

 

Tin Whiskering

What is tin whiskering?

Metal whiskering or Tin whiskers is a phenomenon or fault which occurs in electrical and mechanical devices which are plated with Pb-free Sn plating. Tin whiskers are small metal hairs that extrude from the plating and can grow long enough to cause an electrical short short circuit (http://en.wikipedia.org/wiki/Whisker_(metallurgy)).
"Tin whiskers" were a problem with early electronic solders, and elemental lead (Pb) was initially added to the alloy in part to eliminate them (http://en.wikipedia.org/wiki/Solder). Tin is a chemical element with the symbol Sn (for Latin: stannum) and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin has long been used as a solder in the form of an alloy with lead, tin accounting for 5 to 70% by weight.

The percentage relationship of Sn and Pb in a Sn-Pb solder that achieves the lowest melting temperature is by definition the eutectic point and is 63% Sn and 37% Pb (e.g. Sn63). Eutectic solder has long been preferred for aerospace applications in particular because it exposes the system to the least temperature stress during soldering, avoids tin whiskering, and produces highly repeatable, high quality solder joints which are very reliable in most applications. (http://en.wikipedia.org/wiki/Tin#Solder).


Tin whiskers (http://www.guardian.co.uk/technology/2008/apr/03/research.engineering)

According to "Mitigation Strategies for Tin Whiskers" report (2002) by Center for Advanced Life Cycle Engineering (CALCE), whiskers are elongated single crystals of pure tin that have been reported to grow to more than 10 mm (250 mils) in length (though they are more typically 1 mm or less) and from 0.3 to 10ľm in diameter (typically 1 - 3 ľm). Whiskers grow spontaneously without an applied electric field or moisture (unlike dendrites) and independent of atmospheric pressure (they grow in vacuum). Whiskers may be straight, kinked, hooked or forked and some are reported to be hollow. Their outer surfaces are usually striated. Whisker growth may begin soon after plating. However, initiation of growth may also take years. The unpredictable nature of whisker incubation and subsequent growth is of particular concern to systems requiring long term, reliable operation (http://www.calce.umd.edu/tin-whiskers/TINWHISKERMITIGATION.pdf).
 

Why are tin whiskers bad?

Whiskers can cause short circuits and arcing in electrical equipment. Tin whiskers don't have to be airborne to damage equipment, as they are typically already growing in an environment where they can produce short circuits.  Tin whiskers may have caused the failure of the Galaxy IV satellite in 1998. At frequencies above 6 GHz or in fast digital circuits, tin whiskers can act like miniature antennas, affecting the circuit impedance and causing reflections. In computer disk drives they can break off and cause head crashes or bearing failures (http://en.wikipedia.org/wiki/Whisker_%28metallurgy%29).

On April 17 2005, the Millstone nuclear generating plant in Connecticut shut down when a circuit board monitoring a steam pressure line short-circuited. In 2006, a huge batch of Swatch watches, made by the eponymous Swiss company, were recalled at an estimated cost of $1bn (Ł500m). In both cases, "tin whiskers" - microscopic growths of the metal from soldering points on a circuit board - were blamed for causing the problems. It's not the first time these mysterious growths have been blamed for electronics failures. In 1998 the Galaxy IV communications satellite sputtered out after just five years; engineers diagnosed its failure as due to "whiskers". The US military blamed them for malfunctioning F-15 radar systems and misguided Phoenix and Patriot missiles. In 1986, the US Food and Drug Administration recalled a number of pacemakers because of these same whiskers. In fact, they've been known about since the 1940s, and happen with cadmium and zinc, too: during the second world war, similar whiskers would short the cadmium tuning capacitors in aircraft radios. A decade later, tin-based relays in AT&T telephone switching centers were found to cause shorts (http://www.guardian.co.uk/technology/2008/apr/03/research.engineering).

Tin whiskers pose a serious reliability risk to electronic assemblies that are specific to the product that incorporates pure tin plating. According to M. Osterman from CALCE, the general risks fall into four categories (from http://www.calce.umd.edu/tin-whiskers/TINWHISKERMITIGATION.pdf):

  1. Stable Short Circuits In Low Voltage, High Impedance Circuits. In such circuits there may be insufficient current available to fuse the whisker open and a stable short circuit results. Depending on a variety of factors including the diameter and length of the whisker, it can take more than 50 milliamps (mA) to fuse open a tin whisker.
  2. Transient Short Circuits At atmospheric pressure, if the available current exceeds the fusing current of the whisker, the circuit may only experience a transient glitch as the whisker fuses open.
  3. Metal Vapor Arcing (Plasma) in Vacuum In vacuum a much more destructive phenomenon can occur. If currents of above a few amps are available and the supply voltage is above approximately 12 V, the whisker will fuse open but the vaporized tin may initiate a plasma that can conduct over 200 amps! An adequate supply of tin from the surrounding plated surface can help to sustain the arc until the available tin is consumed or the supply current is interrupted such as occurs when a protective fuse element interrupts.
  4. Debris/Contamination Whiskers or parts of whiskers may break loose and bridge isolated conductors remote from the original site of whisker growth. In addition, whisker debris may interfere with optical surfaces or the smooth operation of microelectromechanical structures (MEMS).


Since the European Union Waste Electrical and Electronic Equipment Directive (WEEE Directive) and Restriction of Hazardous Substances Directive (RoHS) came into effect on 1 July 2006, the use of lead in such alloys has decreased. Replacing lead has many problems, including a higher melting point, and the formation of tin whiskers causing electrical problems (http://en.wikipedia.org/wiki/Tin#Solder).

"So NASA does not want the economic risk of having the Hubble [Space Telescope] go down. But if one personal computer in a thousand goes down because of the whiskers, no one is going to do anything much about it," says John Ketterson, a solid state physicist at Northwestern University in Illinois (http://www.guardian.co.uk/technology/2008/apr/03/research.engineering).

 

Lead Free Soldering

What is Lead Free Soldering?

"Tin whiskers" were a problem with early electronic solders, and lead was initially added to the alloy in part to eliminate them (http://en.wikipedia.org/wiki/Solder).
On July 1, 2006 the European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) came into effect prohibiting the intentional addition of lead to most consumer electronics produced in the EU. Manufacturers in the U.S. may receive tax benefits by reducing the use of lead-based solder. Lead free solders in commercial use may contain tin, copper, silver, bismuth, indium, zinc, antimony, and traces of other metals (http://en.wikipedia.org/wiki/Solder). There is no U.S. legislation presently on the horizon pertaining to lead containing solders. Most of the focus presently on eliminating lead in electronic solders is from European community countries and Japan  (www.indium.com/_dynamo/download.php?docid=21).
There are many lead free solders exists, but the group comprising tin-silver-copper alloys is considered the mainstream alloy system that will replace tin-lead (www.indium.com/_dynamo/download.php?docid=21).
 

Is lead free soldering important?

It is commonly believed that lead is a highly toxic metal. Some studies show that past uses of lead and lead chemicals in water supply plumbing, paint and gasoline resulted in a measurable increase of lead in the environment and as a consequence, increased blood levels of lead in humans. Paint chip ingestion, mainly by children, resulted in numerous cases of lead poisoning. Consequently the use of lead in these applications has been prohibited for several years. The primary concern now is from groundwater leaching of lead bearing solder in electronic products ultimately disposed in landfills (www.indium.com/_dynamo/download.php?docid=21).

However, this is a very controversial subject. During debate in the 1990’s about the possible dangers of lead soldering, analyses by organizations interested in keeping access to Sn-Pb solder reported that the amount of Pb that might be introduced into the water system from electronics platings and solders posed less of a health hazard then what we already have from arsenic and considered the comparison to children eating lead-based house paint to be non-relevant. There has always been a question around whether this is fueled by cost advantages for high volume production. The horses have left the barn now making the argument mute. This does not necessarily make the pollution argument accurate.