Battling the Growth of Tin Whiskers
February 1, 2007
Tin whiskers are small and hard to detect and inspect. Typically no bigger than 1 to 5 microns in diameter and as much as several millimeters in length, they are small, but powerful; capable of turning a $250 million communication satellite into an orbiting paperweight and short circuiting pacemakers, radar systems, fuses, relays, GPS receivers and even disabling a nuclear power plant.
“One of the problems with whiskers is that they are not easily seen,” says Ron Gedney, a retired engineer from IBM, who works on this issue with the International Electronics Manufacturing Initiative (iNEMI). “One wonders how many ‘no defect found’ faults have actually been caused by whiskers, but never detected.”
Tin whiskers are single crystals of tin that spontaneously grow from the surface of tin and tin alloy platings, says Robert D. Hilty, Ph.D., Technology-Director Materials Research, Tyco Electronics (Middletown, PA). They can grow through conformal finishes thinner than 100 microns. They can grow in ambient conditions, hot and humid environments, and in vacuum chambers that replicate space conditions.
They can take many shapes, can bend and kink during growth, and grow to lengths that could be disastrous. Their length becomes a problem as they grow and come in contact with leads and short out electronics. This is believed to have been the cause of a range of product failures including the disabling of the Galaxy 4 communications satellite in 1998.
On Earth, tin whiskers caused havoc at a nuclear facility. In April 2005, a tin whisker, sprouting from the lead of the diode on a universal logic board that had too high of a tin content, caused a short. The short gave a false low-pressure reading that tripped automatic safety protocols and forced an unplanned, emergency shutdown of the Millstone Power Station, a nuclear reactor facility in Southeast Connecticut.
In a subsequent technical briefing on the event by Westinghouse Electrical Corp., “Potential Tin Whiskers on Printed Circuit Board Components,” the company says that tin whiskers were observed on printed circuit boards (PCB) of “various vintages” and on PCBs that were manufactured by myriad contractors.
According to the bulletin, the diodes observed with tin whiskers on the SSPS universal logic board were manufactured to Mil-Spec MIL-PRF-19500/117, which requires that the component’s leads be coated with tin containing a minimum of 3% lead.
Different needs, different toolsWhile some manufacturers that incorporate electronic assemblies into their products must diligently inspect for the presence of these other-worldly looking whiskers, other manufacturers must nondestructively conduct material tests to make sure that the correct alloy recipe has been used so that the whiskers do not grow.
As can be seen in the Millstone Power Station case, lead is a key ingredient. Gedney says that while the tin whisker problem has been around for 50 years, it was essentially solved decades ago by going to a tin-lead composition. By infusing the material composition with as little as 3% lead, the problem can be solved. According to the Westinghouse brief, the diodes with whiskers had a coating on the leads that had a too high tin content. But, what once was a problem that had been successfully mitigated by the use of lead in solder and electroplating is now once again a potential problem as lead is no longer an option for many manufacturers. Restrictions on lead use is mandated by the European Restriction of Hazardous Substances Directive (RoHS) that went into effect on July 1, 2006.
“Take the lead out, and back come the tin whiskers,” Gedney says.
Manufacturers, with the exception of some aerospace and military suppliers who are exempt from RoHS, are already working in a lead-free environment and are using the best mitigation practices available. But, the problem is not going away. It is believed that whiskers will eventually grow in most products-over a period of a day, a week or even years-and researchers are looking for ways to retard their growth or design products so that they will not be affected by their growth.
But, a solution may be a ways off. Manufacturers need ways to inspect for tin whiskers today.
“Inspection for whiskers is challenging,” says Hilty. “We (Tyco) use both optical and scanning electron microscopes (SEM) to inspect for whiskers. There is a learning curve involved in doing this inspection-the more of these whiskers you look at, the better you are at finding them.”
In May 2005, the iNEMI Tin Whisker User Group released a standard that approves the use of microscopes. The JEDEC standard, “Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes,” was developed by the group that consists of eight manufacturers of high-reliability electronic assemblies including Agilent, Alcatel-Lucent, Celestica, Cisco Systems, Delphi Electronics & Safety, Hewlett-Packard, IBM, Sun Microsystems and Tyco Electronics.
The standard lays out a method of measuring and comparing whisker propensity and developed a consistent inspection protocol for tin whisker examination, including the use of microscopes. According to the standard, microscopes must meet these criteria:
- Optical stereomicroscope with adequate lighting capable of 50X to 150X magnification and capable of detecting whiskers with a minimum axial length of 10 microns. If tin whiskers are measured with an optical system, then the system must have a stage that is able to move in three dimensions and rotate, such that whiskers can be positioned perpendicular to the viewing direction for measurement.
- Optical microscope with adequate lighting capable of 100X to 300X magnification and capable of measuring whiskers with a minimum axial length of 10 microns. For tin whisker measurements, the optical system must have a stage that is able to move in three dimensions and rotate, such that whiskers can be positioned perpendicular to the viewing direction for measurement.
- Scanning electron microscope capable of at least 250X magnification. An SEM fitted with an X-ray detector is recommended for elemental identification.
Traditionally SEMs have been the de facto inspection technique because it offers tremendous depth of field and high magnification capability, says Hilty. They can be used to measure the whiskers, bend by bend, kink by kink, and can be used to differentiate tin whiskers from other whisker-like debris on the surface such as glass fibers. Scientists studying why and how tin whiskers grow use a variety of technologies including SEMs, focused ion beams (FIB) and X-ray diffraction. However, when it comes to inspection, SEMs and optical microscopy are primarily the two techniques used.
However, tin whisker expert Hilty, as well as microscopy expert Mike Metzger, general manager Industrial Microscopy & Metrology for Nikon Instruments (Melville, NY), say that SEMs have two major drawbacks: slow inspection speed and expensive equipment.
Metzger says SEMs work well down in the nano world, but whiskers, which are micron sized, can be seen with microscopes. In fact, he says, they are relatively big in size in the microscopy world.
Using a microscope, Metzger says, means that the whiskers can not only be seen, but with the help of a digital camera or a digital video system, the whiskers can be documented with both images and macro data such as when the image was taken and the circumstances in which it was taken.
Determining the correct microscope is important, Metzger says, and manufacturers often have a choice between a stereo microscope and a compound microscope. He says that the considerations for choosing between the two are field-of-view and magnification power. Stereo microscopes have a low magnification of 5X to 100X but a bigger field-of-view, while a compound microscope starts at about 50X to 1,000X (a 100X objective lens with 10X eyepiece), but has a dramatically smaller field-of-view.
Magnification for whiskers can fall within either of JEDEC’s microscope requirements. Gedney says that at least a 50X microscope is needed to see the whiskers. In order to find smaller whiskers, says Hilty, those less than 50 microns, magnification of at least 70X is required.
“From an inspection standpoint, how do you characterize this?” says Metzger. “You could look for tin whiskers with an SEM, but that is kind of overkill. A compound microscope can be used, but the magnification is often too high of a magnification. So, for documenting and inspecting tin whiskers, the best option falls between stereo and compound microscopes.”
Because the whiskers can be assorted sizes, Metzger points to the need for a flexible system such as the Nikon AZ100, which is a three-turret lens system that has been designed to fit inbetween a stereo and compound microscope. It features mono-zoom imaging, meaning that it has one optical path for zooming. Zooming is normally found in a stereo microscope, but a single optical path is more likely found on a compound microscope and gives the inspector a more accurate representation of the object. In contrast to the single optical path, a stereo microscope has two converging optical paths and the operator’s brain combines the two images into a single image.
Another product that might be an option are video measuring microscopes that capture high resolution video images and have a motorized stage. This might be important depending on the product to be tested. A PCB board might have 300 locations to inspect, and an automated system might speed the process and reduce errors.
Lighting is essentialAnother pivotal aspect to inspecting for whiskers is lighting. Jay Brusse, a researcher with the QSS Group Inc. (Lanham, MD) working out of the NASA Goddard Institute for Space Studies (New York), has developed a series of descriptive movies of tin whisker inspection that shows the importance of lighting.
In the videos, which test current PCBs and transistors dating from the 1960s, a stereomicroscope is used to inspect for whiskers. The microscope has a ring light attached to it and Brusse uses a dual-flex lamp with three fiber optic illuminators that can direct the light at a sample under the microscope.
Brusse’s videos show that a ring light situated above the sample can mask some whiskers and, in general, make it difficult to see them. Using the flex lamps from two directions and at glancing angles, the whiskers can clearly be seen by a reflective shine. According to text shown in the video, bright glint is characteristic of illuminating a highly reflective metal object (whisker) at an angle that reflects light up the microscope objective.
According to Metzger, this is an example of using bifurcated light pipes to do an inspection, which he says is a classic inspection technique. The light pipes cause what he calls an oblique illumination that helps to expose any surface irregularity. Many microscopes have oblique illumination built into the objective lens, a process called Dark Field EPI illumination, in which light is automatically directed at an angle at the part surface.
Because of the nature of tin whiskers-growing at unpredictable rates and at unpredictable times-creating a set of protocols might be a good way to inspect for whiskers. That may mean setting up regimented inspection protocols for the inspector to follow, or doing lot sampling and re-inspecting parts at regular intervals.
For instance, Tyco performs ongoing inspection of its products. It pulls one part per plating line per week and does an initial inspection. This part is stored at ambient conditions in the plating shop and re-inspected at three- and six-month intervals.
Also, Metzger suggests having a protocol for inspecting the parts at various angles and fixture positions because each sample to be inspected might be completely different than the sample that came before it. A PCB with no components standing on top of it might mean the inspector might only need a shallow angle of illumination. If it has resistors and other components, the angle may need to be increased, which reduces the affectivity of the oblique angle, but allows the technician to still see the defect.
Material compositionThe tin whiskering problem is one of many unknowns. Why exactly do they form? Why will one plate form many whiskers while another, seemingly identical plate, form few? How does lead work to retard their growth?
The causes of the tin whiskers are multi-factored. Compressive stress caused by mechanical, material and environmental factors is generally considered the leading candidate for tin whiskering. Whiskers can be formed by material impurities, intermetallic and coefficient of thermal expansion (CTE) reactions, surface and grain boundary diffusion, recrystallization and grain orientation. Of course, sometimes they are not formed which is why developing acceptable testing methods has proved challenging. “You can have two tin grains right next to each other,” says Hilty, “and one will turn into a tin whisker and the other one won’t, and it is unclear why.”
Typically, copper as a base material and tin as a solder material are used in soldering and electroplating applications. When lead was present this formulation was acceptable, but without lead these materials interact, a problem called intermetallic diffusion. This interaction can increase stress and may increase whiskering. A possible cause of this is that copper migrates up to 10 times faster than tin, says Gedney.
Thermal cycling conditions can lead to compressive stress in the tin because of CTE mismatch between tin and base materials such as the nickel alloy 42, says Hilty. Tin whisker growth can be rapid for these situations. The JEDEC standard lays out thermal cycling conditions that can be used to try and generate whiskers on samples.
An essential question to solving the problem may be in understanding why lead, in such small amounts, works in the first place. Gedney says that a likely reason for lead’s effectiveness is that it changes the structure of the film. “Looking at a cross section of the tin-copper coating on a lead, you find that the grains of tin grow perpendicular to the surface and so it is a columnar structure,” he says. “With lead, the columnar structure tends to break up.”
But lead is a no-no for manufacturers selling into the European Union because of RoHS. RoHS does not just target lead, but other materials including cadmium, mercury and chromium. Any company with products entering the European Union must be able to document that their products do not exceed maximum permissible levels of the restricted substances.
For most manufacturers, to meet RoHS compliance means ensuring that there is not too much of the substances in the material. Other manufacturers, most notably those in aerospace and defense, are exempt from most lead-free regulations, and their needs are a little different. These companies want to make sure that there is enough lead in the solder to eliminate whiskering.
Therefore, being able to determine the presence of lead can be vital. Technology that can determine the metals in a part, as well as its amounts, can be a key tool to determining the propensity for tin whisker growth.
Reliable, accurate and fast screening for the RoHS-restricted elements in all different types of materials has been a daunting task. In practice, XRF analysis has proven itself to be the perfect analytical tool for fulfilling the main requirements of industry and service laboratories. The International Electrotechnical Commission (IEC), which is responsible for the industrial standardization of RoHS procedures, ranks X-ray fluorescence (XRF) as the best analytical tool for quantitative screening in RoHS (IEC 62321), according to Bruker AXS (Billerica, MA).
Manufacturers of XRF systems have designed products specifically to identify the banned substances. In October, Bruker AXS introduced a turnkey system called RoHS-QUANT that does quantitative screening of chromium, lead, bromine, mercury and cadmium elements in polymers and plastics of electrical and electronic equipment, in accordance with the new regulations.
Manufacturers of portable X-ray fluorescent technology such as Thermo Fisher Scientific Electron (Billerica, MA) and Innov-X Systems (Woburn, MA) also have worked aggressively to develop products for just such a purpose. Analyzers such as Thermo Electron’s Niton XLt797 for RoHS compliance screening and Innov-X Systems’ Hi-Rel Solder Analyzer can provide instantaneous readings of the substances in the metal and their levels. Thermo Electron’s Niton XLt797 for RoHS compliance can screen for lead in as little as 60 seconds. In addition to verification of lead-free solder, the Niton analyzers can screen for the toxic elements banned by RoHS.
“XRF Systems can do very fast analysis,” says Ken Stehr, RoHS account manager for Thermo Fisher Scientific. “In a matter of seconds we can determine the lead contamination of a particular part. The portability aspect of it allows manufacturers to take the instrument out to where the samples are located or ship it out to vendor sites to do on-site audits even before the material gets into the manufacturing stream.”
For more information on the companies mentioned in this article, visit their Web sites:Bruker AXS, www.bruker-axs.com
Innov-X Systems, www.innov-xsys.com
Nikon Instruments, www.nikonusa.com
QSS Group Inc., www.qssgroupinc.com
Thermo Fisher Scientific Electron, www.thermo.com
Tyco Electronics, www.tycoelectronics.com