EFFECT OF ANNEALING AND NICKEL UNDERPLATE ON SUPPRESSION OF TIN WHISKERS

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1 The 5 th PSU-UNS International Conference on Engineering and 679 Technology (ICET-2011), Phuket, May 2-3, 2011 Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand EFFECT OF ANNEALING AND NICKEL UNDERPLATE ON SUPPRESSION OF TIN WHISKERS A. Wahi 1 *, N. Wahi 2*, A.Ourdjini 3* 1 Universiti Teknologi Malaysia, Faculty of Mechanical Engineering, Malaysia 2 Universiti Teknologi Mara, Faculty of Architecture,Planning & Surveying, Malaysia 3 Universiti Teknologi Malaysia, Faculty of Mechanical Engineering, Malaysia * a_azh84@yahoo.com Abstract: Immersion tin coating is gaining interest as an alternative surface finish. However, induced tin whiskers are a major problem in tin plating substrates which affect reliability and overall performance of electronic circuit. Tin whisker forms as a spontaneous growth when subjected to compressive stress after plating. The present study is aimed at shedding more light on the formation of tin whiskers and mitigation methods of controlling and inhibiting whiskers in immersion tin coatings. Tin plated samples were stored under ambient conditions for up to three months to observe the growth of tin whiskers. Two mitigation methods, annealing treatment and nickel under plating, were used to suppress the growth of tin whiskers. It was observed that tin whiskers were present in all samples and that their density increased with increasing storage time. From the two mitigation methods used, the result showed that the nickel underplate appears to be promising in only reducing the density of tin whiskers, whereas annealing showed no effect. Key Words: Whiskers, Tin coatings, annealing, stress 1.0 INTRODUCTION In the electronics industry immersion tin is a reliable surface finish for both PWB and IC substrates application. It is used in automotive, communications and electronics industries. However the growth of tin whiskers in tin immersion coatings is regarded as a critical phenomenon and potential risk in electronic circuits. It can cause short circuit or electrical breakdown in electrical devices. One of the techniques to avoid whiskers is addition of lead in the tin material. However, the directive on the Restriction of the use of certain Hazardous Substances (ROHS) in electrical and electronic equipment restricts the used of lead in electronic equipment due to the concern of human health. Lead is found to be the cause of blood and brain disorders and damage to the nervous system. whiskers. Two mitigation methods were used: annealing treatment and nickel underplate, to investigate their effect in reducing or eliminating the growth of tin whiskers. 2.0 RESEARCH METHOLOGY The substrates used in this experiment are made from sandwich copper. The dimensions are 45mm x 50mm x 1mm (width x length x thickness). Some of the substrates are plated with electroless nickel with immersion tin surface finish and some of them are plated by immersion tin only. In this study, a medium phosphorus nickel solution was used to plate a layer of nickel onto the copper substrate. For electrolysis nickel, it involved deposition of a thin nickel layer of less than 5micronmeter in thickness. Before nickel plating process, the sandwich copper needs to undergo pre-treatment process to remove oxide and activate the surface. The combination of chemical composition and typical process parameters for plating solution is shown in Table 1. Table 1: Electroless nickel plating solution chemical composition and parameters Chemical compositions Nickel Sulphate 28g/L Sodium Acetate 17g/L Sodium hypophosphite 24g/L Thiorea g/L Parameter Quantity ph Temperature o C After nickel plating process, the samples were plated with tin. The chemical compositions of tin plating were shown in Table 2. The plating temperature is 71 o C with the variation of the temperature is + 3 o C. In the present study, the accelerated test was done in order to gather more information of the stress on the tin

2 680 Table 2: Chemical composition of immersion tin plating Stanneous Chloride Hydrochloric Acid (37% ml) Sulfuric Acid (50% ml) Sodium Hypophosphite (gm) Thiourea Phenolsulfonic Acid 3.0 RESULT AND DISCUSSIONS 3.1 Tin whisker accelerated test 20g/L 25g/L 50 ml/l 16 g/l 200 g/l 5ml/L For tin (Sn) whisker accelerated test, the Sn plated substrates were stored in ambient condition for up to 3 months. The growth of whiskers in Sn plated samples was monitored monthly. The morphology of the substrates were analyzed by Scanning Electron Microscope (SEM) as soon as plated by tin which is shown in Figure 1 and. bent shapes. In PCB's the compressive stress at Sn layer is due to the CTE mismatch between the substrate and Sn layer. This compressive stress generated in the plating is regarded as a main driving forces for the whisker formation. [1,2] Research work by Lee [3] concluded that the Sn whiskers grow from the major orientation of the tin film. In this condition, the tin surface oxide film can be sheared along the boundaries of the grains. The shear force results from different strains generated in different grains in the direction normal to the substrate plane due to the biaxial compressive stress developed in the tin film. The different strains originate from the elastic anisotropy of tin. To release the compressive stress in the tin film, tin whiskers grow from the grain whose surface oxide is sheared. Some correlation between the thickness of Sn plating and whisker density was studied by Panashchenko and Osterman [4]. They found that the thinner Sn plating produced fewer whiskers. Average whisker length was also higher for thinner Sn plating, although maximum whisker lengths could not be correlated to plating thickness. The results from the present study are in good agreement with these observations as the Sn plating layer is considered thin. 500x Magnification 1000x Magnification Figure 1 500x magnifications & 1000x magnifications After 1 month storage in open atmospere at room temperature, whisker growth observed as shown in both Figure 2.0 and. After the second month, the whiskers continue to grow in different shapes and sizes, as shown in Figure 3.0, and. After 3 months, whiskers of various shapes, length and diameters were observed on the Sn surface as shown in Figure 4.0, and. It was observed that the whiskers cluster at certain areas on the surface. The figure shows that the density of whiskers increases with increasing storage time. The Sn whiskers appeared in the nodules, kink and Figure 2.0: Tin whiskers observation on Sn substrate surface under ambient condition after 1 month.

3 681 Figure 4.0: Tin whiskers observation on Sn substrate surface under ambient condition after 3 months 3.2 Effect of annealing of specimen with tin whiskers Figure 3.0: Tin Whiskers observation on Sn substrate surface under ambient condition after 2 months In annealing, the specimen is heated to a predetermined temperature for a time sufficient to allow the necessary changes to occur, followed by relatively slow cooling. In general, the annealing process can soften a cold-worked structure by recrystallising or inducing grain growth. It can soften certain agehardenable alloys by dissolving and cooling rapidly the second phase to obtain a supersaturated solution, thus relieving internal stresses. From this study, it was found that annealing resulted in a reduced whisker density. This effect could be seen from Figure 5.0, and. After 1 month storage, no whiskers were observed to grow on the Sn surface. However, whiskers started to appear after the second month and after 3 month storage, the whiskers density not only increased but their sizes and shapes also increased. Analysis of the Sn plating also showed that the tin layer had coarsened and re-crystallized during heat treatment, thereby relieving the stress, induced by the plating process.

4 682 applied immediately after components are plated [8]. IBM indicated that matte pure tin over copper, which was annealed at 150 o C for one hour within two weeks (24 h preferred) of plating could be adopted as one of their acceptable pure tin parts for its server and storage system [9] 3.3 Effect of Nickel underplate ISn on tin whiskers Another suppression method to reduce Sn whisker growth is Nickel underplating. This is achieved by depositing a Ni barrier layer between the copper substrate and Sn layer. The results of the present investigation, however, have shown that the presence of 1 µm Ni underplate layer had some effect in suppressing whisker growth during the first 2 months storage as no whiskers were observed, as shown in Figure 6.0 (a, b), but after 3 months storage, tin whisker started to grow (Figure 6.0 ) but with a much reduced density. No needle-like whisker growth was found on any Ni underplate ImSn finish. Figure 5.0: Tin whiskers on Sn plated substrate after 1 month, 2 months and 3 months Unfortunately, like many other aspects of tin whiskers, all of the factors related to effectiveness of annealing on whisker formation are not known. The exact conditions such as temperature, hold time, and heating and cooling rates that are required to sufficiently remove the residual stress in tin plated finishes are also not known. The present results have shown that annealing may be effective in some situations, but it does not necessarily eliminate tin whisker growth. 1962, Glazunova [5] noted that annealing at 150 o C significantly increased an incubation time and decreased tin whisker growth of tin plating on steel substrate. Other studies of annealing showed similar effects [6]. Lee [7] showed that the application of annealing for one hour at 150 o C changed the structure of the tin deposit. Philips, Infineon, and STMicroelectronics also have suggested that annealing is only effective if it is

5 683 some effect on reducing whiskers formation, especially the Ni underplate ISn. 5.0 References [1] K. S. Kim, C. H. Yu, J. M. Yang(2006), Tin whisker formation of lead-free plated leadframes, Microelectronics Reliability 46, Figure 6.0: Tin whiskers formed on Ni underplate ImSn finish after 1 month, 2 months and 3 months storage There is evidence that a Ni underplate could be an effective way of reducing or stopping Sn whisker growth as the present results show. The fact that a Ni layer of only 1 µm could prevent Sn whisker growth for the first 2 months is clear indication that a probably a much thicker Ni underplate layer would be more effective. Indeed, previous research led by Whitlaw et. al. [10] reported that no Sn whisker growth was found even after 22 week of ambient storage for Nickel of 3µm plating. Dittes et. al. [14] also conducted an experiment to evaluate the use of nickel (Ni) and silver (Ag) underlayers for tin whiskersupression. Typical lead-frame materials were plated with Sn. Some of the samples had a Ni under-layer with thickness ranging from 0.2 to 1.5µm and a few of the samples had Ag under-layer of 2-6µm. The plated samples were kept under varying temperature humidity storage conditions and some samples were subjected to temperature cycling conditions. Analysis of the intermetallics between the tin plating and the nickel under-layer showed the formation of Ni 3 Sn 4. This intermetallic compound (IMC) formation reduces the stress in the Sn plating. Whitlaw observed that even after 14 months of room temperature storage no whiskers were found on Ni- or Ag-plated copper whereas whiskers grew on specimens without under-layer. 4.0 Conclusions From the study on tin whisker growth the following conclusions are made: 1. In tin plating on copper substrate, under ambient conditions, Sn whiskers were found to start growing within the first month of storage with various shapes and sizes. 2. When the specimen is subjected to bending, large numbers of Sn whiskers grew within the first month of storage under ambient conditions. The residual stress in the vicinity of the bent seems to have resulted in the formation of higher density tin whiskers compared to the unbent flat surface area. 3. Nickel underplating between the tin layer and the copper substrate and annealing process appears to have [2] P. Oberndorff et al (2005), Whisker formation on matte Sn influencing of high humidity, Electron Components and Technology Conference, [3] B.-Z. Lee and D. N. Lee (1997), Spontaneous Growth Mechanism of Whiskers Growth [4] Lyudmyla Panashchenko and Michael Osterman (2009), Examination of Nickel Underlayer as a Tin Whisker Mitigator. [5] V. K. Glazunova(1962), A study of the influence of certain factors on the growth of filamentary tin crystals, Kristallografiya, vol. 7, no. 5, pp [6] M. Toben, A. Egli, J. Heber, and A. Winkier (2001), Tin whiskers: An overview of the mechanisms that drive their growth, in Proc. 34thAnnu. IICIT Connector and Interconnection Symp. Trade Show,Waretown, NJ, pp [7] B.-Z. Lee and D. N. Lee (1998), Spontaneous growth mechanism of tin whiskers, Acta Mater., vol. 46, no. 10, pp [8] P. Oberndorff, J. Klerk, M. Dittes, and P. Crema (2004), Whisker formation on Sn-plated components, in Proc. Conf. Electron. Berlin, Germany, Sep. 6 8, 2004, pp [9] IBM, IBM Eng. Spec(2004), IBM server and storage system s environmental requirements for purchased electronic components, (including restriction on hazardous substances RoHS),., Doc.: 77P0594, EC:J [11] Whitlaw K, Egli A and Toben,M(2004), Preventing Whiskers in electrodeposited Tin For SEiconductor Lead Frame Applications, Circuit World, Vol.30, No.2, pp [12] Dittes, M, Oberndorff, P and Petit, L(2003), Tin Whisker formation results, test methods and countermeasures, Proc. IEEE Electronic Components and Technology Conf., pp