Feature Tin Whiskers: Capsulization by Dr. Jennie S. Hwang drites). And tin whisker and tin pest are separate H-TECHnoLogiES gRoUP metallurgical phenomena (SMT Magazine, May 2013). However, whiskers share commonality Since lead-free implementation, concerns with dendrites in two aspects: Both are the re- about tin whiskers have intensified. For the sult of a physical metallurgical process, thus fol- past 12 years, studies and research by various lowing the science of physical metallurgy; and laboratories and organizations have delivered both could cause a product failure. burgeoning reports and papers, and my column Uncertainty about tin whisker growth is has devoted an entire series to this subject. This most insidious. Stock markets do not like un- article aims to capsulize the important areas of certainty, nor does the electronics industry. Our the subject. (Note: For expression, “whisker” is effort is to alleviate the uncertainty. used as both noun and verb.) The tin whisker issue and its potential mis- Practical Criteria haps have been recognized for more than six As some metals can whisker when accom- decades in electronic, electrical and industrial modating conditions are met, the goal should applications. Some metals are prone to whisker- be set with the differentiation between whisker- ing, or protruding from the surface of the sub- resistant and whisker-proof. strate. In addition to tin, the metals that have Overall, for testing or evaluation of the exhibited whiskers include zinc, cadmium, sil- whisker propensity of a system, the key ques- ver, gold, aluminum, copper, lead, and others. tions to be addressed are, is the system whisker- The whiskering phenomenon is distinct prone or whisker-resistant (not whisker-proof), and unique. It is the result of a process differ- and how does the system’s whisker resistance ent from other known phenomena (e.g., den- stand in reference to the intended benchmark? 16 SMT Magazine • July 2014 Feature TIN WHISKERS: CAPSULIZatiON continues To comply with RoHS regulations, Pb-free materials including pure tin (Sn) have been used as surface coating for component leads and metal terminals, and Sn-based alloys as sol- der materials in making solder joints. Because of its economics, availability, manufacturability, compatibility and solderability, pure tin makes a practical replacement for Sn-Pb as a choice of surface coating. Today, most component manu- facturers offer pure tin-coated components. The evaluation of tin whisker propensity and growth rate needs to be put in the context of relative formation rate under a set of conditions. For the electronic and electrical applications, the figure 1: Tin whiskers appearance, from mound renewed concern about tin whiskers are largely to filament. the result of conversion from tin-lead coating to lead-free (or tin coating) for component leads or PCB surface finish. Thus, the relative perfor- may also be self-annihilating as the electric cur- mance in reference to a tin-lead benchmark that rent can fuse the whisker if the current is suffi- has demonstrated satisfactory whisker-resistance cient (e.g., typically more than 50 milliamps is is a logical criterion, not the absolute perfor- often required). The self-annihilation ability var- mance. An SAC (SnAgCu) alloy is lead-free, but ies with the whisker’s size in length and diameter. a lead-free alloy is not necessarily an SAC. This This self-annihilating occurrence further contrib- clarity is particularly important as more viable utes to the observed inconsistent or mythical na- lead-free alloys become commercially available. ture of the events. Furthermore, the highly dis- And tin whiskering is highly sensitive to an alloy parate whisker growth rates have been reported, composition including impurities. ranging from 0.03 to 9 mm/year. And whiskers can grow even in a vacuum environment. Phenomena and Observations Among various findings, one experiment Tin whisker reflects its coined name, which indicated that whiskers can be eliminated by has long been recognized to be associated with controlling the plating process in an equiva- electroplated tin coating and most likely occurs lent way to controlling stresses in materials. with pure tin. Its appearance resembles whiskers. The very sharp decrease in internal stress of tin However, they can also form in a wide range of electrodeposits was observed after plating as shapes and sizes, such as fibrous filament-like quickly as within minutes. It is interesting to spirals, nodules, columns and mounds (Figure note that this fast stress release occurs regardless 1). Tin whiskers are often single crystals and whether initial stress in the deposit is compres- electrically conductive. They are normally brit- sive or tensile. In the case of compressive or ten- tle in nature but can be rendered ductile when sile stress, the value of the stress drops to very whiskers are very long and thin. low numbers, but it remains being of the same Whisker formation and its resulting shapes type as the initial stress form (i.e., high initial and sizes depend on time, temperature, sub- tensile stress reduces to much lower stress value strate, surface condition of the substrate, sur- but remains tensile and high compressive stress face morphology, plating chemistry, and plat- remains compressive). ing process. The rate of whisker growth also It has been observed that the inclusion of depends on a list of factors including the above- organic elements in the tin structure promote mentioned. tin growth. Organic inclusion or the level of in- Whiskers sometimes grow up to a few mm clusion is in turn affected by the plating chem- long, but usually less than 50 µm, and a few mi- istry[1,2]. And bright tin has exhibited to be most crons in diameter. Whiskers may grow, but they susceptible to whisker formation. Bright tin 18 SMT Magazine • July 2014 Feature TIN WHISKERS: CAPSULIZatiON continues figure 2: Tin whiskers—organic inclusions and grain size (four months aged at 50°C). plating chemistry is prone to creating an envi- ize the metal gas, metal arc can occur. A NASA ronment that creates greater organic inclusion report attributed a satellite failure to tin metal and higher stress level in tin crystal structure. vapor arc as the suspected root cause. It is ex- The nature of substrate, external mechanical pected that tin arc is more likely to occur under force, and temperature have been found to af- reduced atmospheric pressures or vacuum envi- fect tin whiskering as well. ronments. Concerns and potential impact 3. Break-off debris If/when tin whisker occurs, concerns and The whiskers, being brittle and conductive, impact primarily fall in the following four cat- can break off from the base of its substrate sur- egories (SMT Magazine, November 2013). face, which may create functional issues. This is particularly a concern for sensitive electronic 1. Short circuits devices, such as optical and computer disk driv- When a whisker grows to a length that bridg- er applications. The break-off behavior varies es the adjacent lead or terminal, this conductive with the service conditions and the characteris- whisker can cause an electrical short. However, tics of the whisker. if a whisker is formed but does not bridge its neighbor, there will not be an electrical short. 4. Unwanted antenna To complicate the phenomena, there are occa- Tin whiskers can act like miniature anten- sions where whiskers may not cause failure, or nas, which affect the circuit impedance and a failure may not be detected even when the cause reflections. In this case, the most affected whisker physically touch the adjacent lead due areas are in high-frequency applications (higher to lack of electrical current flow. than 6 GHz) or in fast digital circuits. 2. Tin metal arcing Causes and Contributing Factors Under high levels of current and voltage Regarding causes and factors, physical met- that is able to vaporize the whisker and ion- allurgy is the place to go to. Fundamentally, tin July 2014 • SMT Magazine 19 Feature TIN WHISKERS: CAPSULIZatiON continues whisker follows the basic physical metallurgy in lar grain sizes generated the following results: its principles on nucleation and crystal growth 235 µm whisker was formed from the coating through the classic theories of dislocation dy- containing 0.2% carbon; 12 µm whisker was namics and of other lattice defects in tin crys- formed from the coating containing 0.05% car- tal structure. Thus, for whiskers to appear from bon content[1,2]. the tin-based (or coated) surface, the causes and contributing factors should be intimately Surface Physical Condition related to the nucleation site creation and the Surface conditions, such as notches or subsequent growth paths. However, the actual scratches on the surface, are the source of atom- processes of nucleation and grain growth of tin ic irregularity, which could contribute to the whisker are dauntingly complex. driving force of tin whisker formation. The nucleation and growth can be encour- aged by stresses introduced during and after the Substrate Surface Morphology plating process. The sources of these stresses Physically maneuvering the surface mor- come from multiple fronts. This includes re- phology of the substrate in the level of rough- sidual stresses caused by electroplating and/or ness was found to alter the tin whisker propen- additional stresses imposed after plating, and/or sity—a rougher surface being less prone to tin the induced stresses by foreign elements, and/ whiskers[3], as shown in Figure 3. It is believed or thermally-induced stresses. Specific causes that a relatively rougher surface facilitates the and contributing factors are excerpted from my formation of an even interface between the tin previous article (SMT Magazine, March 2014): coating and the substrate surface that contains a thinner and more uniform intermetallic layer. Organic Inclusions Organic inclusions affect the tin crystal Oxidation or Contamination Level structure by distorting or crowding the crystal It is postulated that as the oxygen atoms lattice, thus creating the internal stress.