Plating of tin and tin alloys from methanesulfonic acid baths

Transcription

1 Plating of tin and tin alloys from methanesulfonic acid baths Prerna Goradia, Vijaykumar Ijeri, Komal Shah, Trupti Dere, Sudhir Gurav Grauer & Weil (India) Ltd., Kandivali East, Mumbai ABSTRACT The electrical and electronics industry are heavily dependent on tin and tin-alloy coatings for solderability; most of which is done by electroplating. More than 95% of tin or tin-lead plating is done from acidic electrolytes, while other alloys with copper, zinc, etc are done from alkaline and cyanide electrolytes. For pure tin deposition, sulphuric acid based baths are the most common ones due to its low cost. Fluoborate baths have been used to deposit tin-lead alloys because of solubility of lead. However, due to environmental concerns both lead and fluoborates have to be phased out. Methanesulfonate electrolytes are a good replacement to fluoborates with some additional advantages of higher metal solubility, conductivity, less corrosive and slower oxidation of stannous to stannic. This paper will discuss the advantages and applications of methanesulfonate electrolytes for the deposition of tin and tin alloys. INTRODUCTION Tin is a silvery, malleable metal that doesn t get easily oxidized in air and is used to coat other metals to prevent corrosion. It resists corrosion from water but can be attacked by acids and alkalis. The first alloy, used in large scale since 3000 BC, was bronze, an alloy of tin and copper. Because of its low toxicity, tin-plated metal is also used for food packaging, giving the name to tin cans, which are made mostly of steel. Tin melts at a low temperature of about 232 C. Sim ple eutectic systems occur with bismuth, gallium, lead, thallium and zinc. In 2006, about half of tin produced was used in solder. The rest was divided between tin plating, tin chemicals, brass and bronze, and niche uses. Tin plating is used extensively in the electrical engineering industry to provide protection and to confer solderability. Tin electroplating is also widely used in manufacturing printed circuit boards (PCBs) and electronic components. Most electric circuit connections are made by soldering, therefore the surfaces of the conductors being connected are coated by tin or a tin alloy having excellent solderability. Additionally tin coating protects the components and connections from corrosion in aggressive atmosphere. Thickness of tin coatings used in electronics is usually up to 15 µm. Tin has long been used as a solder in the form of an alloy with lead, tin accounting for 5 to 70% w/w. Tin forms a eutectic mixture with lead containing 63% tin and 37% lead. Such compositions are primarily used in solders for joining pipes or electric circuits. Since the European Union Waste Electrical and Electronic Equipment Directive (WEEE Directive) and Restriction of Hazardous Substances Directive 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. Tin pest can occur in lead-free solders, leading to loss of the soldered joint. Replacement alloys are rapidly being found, although problems of joint integrity remain. Tin-copper and lead-tin-copper alloys are used in tri-metal sliding bearings as anti-friction coating of µm thick. In addition to this very thin (1µm) pure tin coating over the bearing surface is used for better cosmetic appearance and corrosion protection. TIN PLATING Tin is one of the easiest metals to electrodeposit, but commercial electroplating started quite late in the 1930s because the metal could be coated readily by hot dipping too. Hot dipped tin provided a

2 brighter finish, which was not possible with the tin electrolytes available then. One of the advantages of electroplating is that no limitation is imposed on the thickness of tin that can be applied. The required thickness can be attained by adjusting the bath parameters and time. A growing realization of the advantages of electrodeposited tin as a protective coating led to an increased interest in research on tin plating. Tin is usually plated with a bright or matte finish. Bright tin is obtained from electroplating solutions containing brighteners, i.e. organic additives causing formation of very fine grains of deposit. It has excellent cosmetic appearance; however these coatings are characterized by high internal stresses and contain large amounts of organics. Matte tin coatings are made in electrolytes with few grain refiners, but without brighteners. It has dull appearance but the level of internal stresses in matte tin deposits are much less than in that of bright tin. Acidic stannous sulphate baths have been the main stay of bright tin plating for many years now. It is a convenient electrolyte with easily available stannous sulphate, sulphuric acid and proprietary brighteners. The disadvantage with sulphate system is that it cannot be used at high current densities (high speed plating) and low solubility of lead salts that is required for Sn-Pb alloy deposition to avoid whisker formation. Another problem associated with sulphuric acid electrolyte is its tendency to oxidize organic materials and metal ions with multiple valences, with tin being one of them [1]. The conversion of stannous to stannic form in the plating bath is a waste of available metal to deposit. The fluoborate baths have enjoyed some popularity due to their near 100% cathodic efficiency and fast plating rate. In addition, the ratio of tin and lead can be adjusted to produce any solder composition. However, fluoborate plating solutions are more hazardous, with fluoboric acid being a strong acid; it can cause severe burns of the skin and is corrosive to eyes, gastrointestinal and respiratory tract. METHANESULFONIC ACID ELECTROLYTES: Wayne Proell was one of the first to recognize the utility of alkane sulfonic acids for electroplating applications [2]. He found that alkane sulfonic acids, having between one and five carbon atoms in the alkyl group, formed water soluble salts of various metals. The alkane sulfonates did not undergo any appreciable hydrolysis, regardless of the temperatures used. Proell indicated that it was possible to plate many metals from alkane sulfonate baths, including cadmium, lead, nickel, silver and zinc. Although the usefulness of the alkane sulfonic acid system was known for several decades, the system gained some commercial acceptability only since the 1980s. The simplest of the alkanesulfonic acids, Methanesulfonic acid (MSA) is a colorless liquid with the chemical formula CH 3 SO 3 H. It became a potential electrolyte for usage in tin, lead and tin-lead plating. The high solubilities of metals in MSA, especially tin, lead, silver, copper, zinc make it an ideal electroplating electrolyte. With the need to remove lead from electronic components, much effort is being expended to find alternative solder coatings and the ability of MSA to solubilize metals that are insoluble in other organic or mineral acids makes it an ideal choice for next generation electronic components. Another attractive feature of the MSA based plating systems is the ease of treating MSA effluents. MSA is a strong acid (pka = 21.9) which is almost completely ionized at 0.1 M in aqueous solution, and hence a strong electrolyte with good conductivity. MSA has a low tendency to oxidize organic compounds. MSA solutions have a unique resistance to the oxidation of metal ions to their higher valence states. This oxidative stability of metal ions in MSA solutions is perhaps best known for the stannous / stannic system [3]. MSA is less toxic than fluoroboric acid. The LD 50 (oral, rat) values were found to be 495 mg / kg and 1158 mg/kg for fluoboric acid and MSA respectively [4]. In addition, fluoroboric acid and fluorosilicic acids have lachrymatory properties, and both acids can evolve HF. MSA is considered readily biodegradable ultimately forming sulfate and carbon dioxide and is also considered to be a natural product, as MSA is part of the natural sulfur cycle [5]. By contrast, fluoroboric acid in effluent undergoes hydrolytic dissociation into boric acid and fluoride, both of which are environmentally troublesome.

3 In general, the low toxicity of MSA, especially when compared to fluoroboric acid makes it a safe electrolyte to handle. Pure Tin MSA baths High tin baths typically contain g/l of tin metal and enable high speed plating at high current densities. Figure-1 shows the rates of deposition within a range of current densities that give uniform bright deposits in a mildly stirred solution. To operate at even higher current densities, it becomes necessary to have more vigorous solution movement TINBRITE HS 5 µm / min A / sq. dm Figure 1: Rate of deposition at different current densities from a room temperature TINBRITE HS bath containing 50 g/l of Sn metal. At an average current density of about 8 A/sq. dm, the thickness obtained from MSA bath and fluoborate baths are ~ 4 um and ~3 um per minute respectively. A regular sulphate bath cannot be operated at such high current densities. Such high rates of plating are beneficial in continuous plating of wires or connector strips and to build high thickness on electrical components. The resulting bright tin deposits are suitable for connectors, contacts, wire and other items requiring a bright tin finish. Figure 2: Continuous wire plating (left) and thick tin deposits on electrical parts (right)

4 A close examination of the bright deposits reveals that the grains are too fine to be distinguished even at 50000X by SEM. This fine grained structure gives a smooth surface topography with consequently low roughness. Figure 3: SEM images of a panel plated with TINBRITE HS. The left image is at 1000X, the right image is at 50000X (the large particles are of occluded dust) When brightness is not essential as in case of plating on wires that will further be subjected to drawing, the TINBRITE HS bath may be used with simple grain refiners (i.e. without brightening agents). Such deposits will have less stress. There are predominantly two sizes of grains (6 10 µm & 1 3 µm) that allow for relaxation and stress relief within the deposits. (Figure-4) Figure 4: SEM images of a panel plated with TINBRITE HS (without brightening agents). The left image is at 1000X, the right image is at 5000X showing two kinds of grain formation. Ductility is important for tin coatings, especially for wire-drawing, and even in many other applications tin coatings are subjected to some level of mechanical deformation. Adhesion and ductility over copper / brass were evaluated qualitatively by bending plated panels back and forth at 180 degrees twice and examining the deformed panel areas for signs of cracking, blistering and/or peeling. These bright as well as dull tin deposits also pass the quench test (i.e. when a 10 um coated panel heated to 150 o C for 1 hour is dropped in water at room temperature there are no blisters or peel off)

5 Figure 5: Optical microscope images of a panel plated with MSA tin before (left) and after bend tests (right). No peeling off of the coating is visible around the length of the groove where the panel was bent. A uniform matte finish can also be obtained from such baths with different set of grain refiners. The surface morphology and grain structure obtained with Growel s MATTE TIN MSA is shown in figure 6. The difference is obvious by comparing the SEM images of figures 3, 4 and 6. Figure 6: SEM images of a panel plated with MATTE TIN MSA. The left image is at 1000X, the right image is at 5000X showing grain sizes. This bath can be operated at room temperature upto a current density of ~ 7 A / sq. dm. For higher current (higher rate of deposition), the bath needs to be heated to 50 o C to avoid burning and dendrite formation. Whisker resistance for the tin depsoits has been verified by inemi / JEDEC tests.

6 10 8 µm / min A / sq. dm Figure 7: Rate of deposition at different current densities from MATTE TIN MSA bath containing 50 g/l of Sn metal. When high speeds are not necessary, it is possible to plate at lower current densities with tin metal content ~ 20 g/l. This also saves on drag out losses, and has better coverage on recessed areas of components µm / min A / sq. dm Figure 8: Rate of deposition at different current densities from a bright low TIN MSA bath containing 20 g/l of Sn metal. WHISKERS: No discussion of tin plating is complete without mentioning about whiskers. In the early 1950s, bright tin was the finish of choice. It was popular not because it was effective or safe or economical but because it was pretty. Unfortunately, the same organics that made the plating bright and shiny also caused serious internal stress and problems during the soldering process.

7 It is well documented that internal compressive stress is a driving force for the growth of tin whiskers. This stress can come from the naturally occurring compressive stress that occurs when tin is electrodeposited from a plating bath. More the organic brightening additives in the plating bath, the higher the internal stress in the tin deposit will be. The highest stressed deposits are obtained from baths that yield bright or specular (mirror-like) deposits, often used for decorative electroplates. Somewhat less prone to whiskers is the matte or dull finished tin, which is usually less stressed, but still will grow (usually fewer and/or shorter) whiskers. Other sources of internal stress in tin electroplate are intermetallic compounds formed from the base metal onto which the tin is electroplated. For example, the tin is often plated onto a copper component terminal lead or printed circuit trace. Intermetallic compounds such as Sn 6 Cu 5 or Sn 3 Cu that form from such base can cause internal compressive stress in the tin layer. This internal stress can accelerate whisker growth. Whiskering is a serious concern in this era of electronically connected society. Whiskers pose a danger when they become long enough to bridge across leads or circuit tracks on printed circuit boards (Figure-9). Different approaches have been tried out to eliminate such occurrences, and debates on which of them is the best are still ongoing. Figure -9: Bridging of leads by tin whiskers (left) and SEM image of whisker growth (right) Hot dipped or reflowed tin tends to grow fewer and shorter whiskers as well, if precautions are taken to avoid stress-causing intermetallics. The high temperatures during board assembly can alleviate stress in the finish and, therefore, eliminate the potential for whiskering. This effect can also be overcome by introducing a nickel diffusion barrier between the tin and copper layers. Unfortunately, pre-solder-dipped components could not survive high-temperature burn-in because the eutectic melting point was too low; many of these devices became permanently soldered into the test sockets. Nonetheless, solder dipping components with tin/lead and plating with pure matte tin became the two prevalent and competing lead-frame finishes [6]. The author states that there is a widespread misconception that lead or bismuth can reduce the incidence of tin whiskers, but all forms of tin and tin alloys have the potential to whisker, including tin/lead, tin/bismuth and tin/copper alloys. The key to reducing or eliminating whiskers is not the addition of lead but a balanced combination of proper plating additives, good material selection, and stringent process controls and operating parameters.

8 Elimination of compressive stress in the electroplated Sn finish layer will suppress whisker growth to a great extent. Although this elimination of compressive stress in the Sn plate has been successful in minimizing whisker growth, verification of the stress level in a plated deposit requires X-Ray diffraction on the deposit, an analysis that needs to be executed in a laboratory. It would be much preferable to discover an alloying addition which (in the same manner as Pb) can be detected in the deposit by XRF (X-Ray Fluorescence) and can be implemented in a routine inspection system [7]. Alloying of tin with some other metals is another way towards mitigation of whiskers. The May 2005 inemi Tin Whisker User group [8] recommendation suggests a number of practices to be followed to mitigate tin whiskers. Alloying with bismuth or silver is one of them, in addition to matte tin plating. With the Pb-free / RoHS juggernaut gaining momentum around the world; in response, new processing parameters are being developed by the large players on the worldwide electronics stage. What s bubbling back up can at best be described as confusing [9]. JEDEC in the U.S., EIC in Europe, and JEITA in Japan are not currently in consensus. The majority at JEDEC says no to Bismuth, and has put their support behind the use of Matte Sn plating. At JEITA, they recommend the Sn-Bi alloy. While both base their recommendations on a great deal of research, their arguments against the other s solution seem a bit shaky. The good news is the two are not basically incompatible; Matte Sn-plated and SnBi-plated leads can coexist in the same production environment. There are several reasons why half the world prefers Bismuth. First of all, it s been shown to be effective in the suppression of solder whisker formation under NEMI (National Electronics Manufacturing Initiative) standards for storage and thermal cycling environments. Bismuth also lowers the melting point of the plating alloy to just over 200 C making it compatible with standard SnPb solder profiles. Plus the addition of Bismuth to the plating improves the wetting performance, especially over time. Finally, testing has proven that the long-term reliability of the solder joints of SnBi-plated leads is comparable to that of standard SnPb-plated leads. Lead traditionally has been used in lead frame finishes but is particularly harmful because it's extremely mobile when used in plating. All individuals who work in a plating facility are exposed to harmful levels of lead through both direct contact and the inhalation of fumes. Because of its effects on the development and function of many organs, and its devastation of the central nervous system, lead has been classified as a probable human carcinogen by the EPA. The medical dangers posed by lead plating processes, by themselves, warrant an effort to reduce or remove lead from this step in the manufacture of electronics. For demanding automotive applications, where the use of bright tin-lead has been permitted for a limited period of time, continuing efforts are being made by both the electronics industry and suppliers of plating chemistry to replace tin-lead with pure tin or other alloys. TIN ALLOY PLATING: Processes for the electrolytical deposition of tin lead alloys are known since 1921 and are the most convenient solderable coatings. The standard potentials for tin and lead are very close (Pb: V and Sn: V). Alloy deposition is therefore possible in all alloy compositions. Both metals have a high hydrogen overvoltage, so deposition of tin lead alloys is possible from strong acid solutions without complexing agents with high current efficiencies [10]. The eutectic alloy with 37% Pb has been used extensively by the PCB industry, while those with 5 15% Pb are used in electronics and electrical assemblies. While fluoboric acid based Sn-Pb baths have been common in use, the MSA based baths are gaining popularity. The development of lead-free solder plating chemistries has not been easy because of the inherent difficulties in plating these alloys i.e. melting temperature requirements, good solderability and ductility. The plating industry narrowed upon binary alloy plating processes such as tin bismuth, tin copper, or tin silver based on their desirable materials properties to replace eutectic tin lead (Table- 1).

9 Alloy Composition Melting Properties (by wt.) temperature ( o C) Sn-Bi 98 : Good strength and thermal fatigue, best whisker resistance Sn-Cu 99.3 : Good creep and thermal fatigue, susceptible to whiskers Sn-Ag 96.5 : Good wetting and shear strength, whisker resistance Table-1: Properties of plated tin alloys [Modern Electroplating, 5 th Edn., M. Schlesinger and M. Paunovic, 2010] Considering the cost and properties, Sn-Bi alloy plating appears more attractive. Bismuth has very good solubility in MSA, and is required in small quantities in the alloy. LCD 98.5 : 1.5 MCD 98.3 : 1.7 HCD 98.1 : thickness (µm) Figure 10: Thickness and Sn Bi alloy distribution across a 5 A, 3 min Hull panel from a Tin-Bismuth MSA bath containing 25 g/l Sn, 0.5 g/l Bi and Growel s proprietary brighteners. Matte Sn-Bi plating with ~ 2 % of Bi is also possible with different kinds of organic additives (Figure- 11). It is interesting to note that variants in matte finish can be achieved by different organic additives. Light reflected from surfaces with flat grain arrangement, gives a satin like appearance. Figure 11: SEM images of matte (left) and satin (right) Sn Bi alloy deposits from a methanesulfonic acid bath containing ~50 g/l Sn, ~2 g/l Bi and Growel s proprietary additives.

10 Copper-tin alloy (bronze) plating from cyanide bath has been in practice for a long time. They are a good substitute for nickel free decorative and some technical applications. The two widely used bronze plating processes are white (~55% Cu, ~45% Sn) and yellow (~ 85% Cu, ~15% Sn) bronzes. The non magnetic nature of bronze deposits is advantageous in high frequency connectors. They also have good resistance to outdoor exposure and wear. Yellow bronze can provide a good underlayer for gold plating of nickel free jewelry. Cyanide free plating of Cu-Sn alloys has been a long standing wish of the plating community. Manz et al [11] have reviewed on the use of phosphonate, pyrophosphate and MSA based baths. The MSA baths are the most recent ones. White metallic finish with ~ 60 % Sn is obtained with such baths (Figure - 12). It is possible to vary the compositions with suitable complexing agents and additives. LCD 62.8 : 37.2 MCD 61.3 : 38.7 HCD 60.4 : thickness (um) Figure 12: Thickness and Sn Cu alloy distribution across a 0.5 A, 2 min Hull panel from a Tin-Copper MSA bath containing 1 g/l Sn, 2 g/l Cu and Growel s propriety brighteners. Sn : Cu ratio is mentioned within the bars. The development of bronze MSA baths is still in progress. Because of the excellent solubility of many metals in MSA, it is possible to get alloys of different compositions and also extend the range of alloy plating chemistry. The benefits of MSA include: 1) Excellent metal solubility 2) Excellent solution conductivity 3) Ease of effluent treatment 4) Stability 5) Low toxicity and biodegradability As environment friendly tin and tin alloy plating gains acceptance by the industry, the future processes will be free from toxic lead and hazardous fluoborates. NOTE: Coating thickness measurements and alloy compositions were determined by X-ray fluorescence methods (CMI-OXFORD instruments, USA). Imaging of surface morphology and alloy compositions were done by Scanning Electron Microscopy (FEI Quanta 200 ESEM, Netherlands). REFERENCES: 1) N. M. Martyak, R. Seefeldt, Electrochimica Acta 49 (2004)

11 2) W.A. Proell, US Patent (1950) 3) M. D. Gernon, M. Wu, T. Buszta and P. Janney, Green Chemistry, 1 (1999) ) Springborn Laboratories, Inc., An Acute Oral Toxicity Study in Rats with 70% Methane Sulfonic Acid, SLI Study No , 07/11/97 5) S. C. Baker, D. P. Kelly and J. C. Murrell, Nature, 350 (1991) ) last accessed on 08/10/2013 7) Electroplated Tin and Tin Whiskers in Lead Free Electronics by F. W. Verdi; last accessed on 08/10/2013 8) 9) Mark Cantrell, California Eastern Laboratories in JAPAN S BEST KEPT Pb-FREE SECRET, 10) Modern Electroplating, 5 th Edn., M. Schlesinger and M. Paunovic, (2010) 11) U. Manz, S. Berger, K. Bronder, K. Leyendecker, B. Weyhmuller, G. Wirth, NASF SUR/FIN 2012