Sn-whisker growth mechanism, suppression method

Sn-whisker growth mechanism, suppression method and its application Xi Dai Department of Materials Science, Fudan University, YangPu Region, ShangHai, China

Abstract: The tin whiskers like the Sword of Damocles always hanging in the sky of lead-free welding and plating which warning at all times against it. It’s particularly important to study Snwhisker growth mechanism, suppression method and choose the appropriate way because of which always cause immensely damage. The thesis analyzes and summaries eight whiskers growth mechanism and repress method, although the inducing factor of whiskers is various but all of them for releasing compressive stress. For this purpose to study Sn-whiskers and also based on actual product’s whisker issue to further prove the compressive stress, anneal and pure Tin plating have significant influence to Sn-whisker. Keywords: Sn-whisker, Compressive stress, Anneal, Inducing factor 1. Introduction The present era is Information Times, the electronic information industry is developing swiftly and violently based on the semiconductor integrated circuit technique. The electronic packaging technology is involved to it and widely used, the initial method via welded all those various components together by Sn-Pb materials on PCB or respectively, and those lead-leg plated with Sn-Pb alloy. But the plumbum is extremely hazard to human body and environment which principally damages the nervous system. Hence, the lead-free becomes the main stream trend, but it’s a severe challenge, because of the lead-free easily generates whiskers. The whiskers easily cause short cut, signal distortion etc. Sn-Whisker exists various morphology, such as columnar, kinked, hillock, rock candy shape etc., as shown in Fig.1. Actually, the Sn-whisker had been discovered in the 1950’s, hereafter, domestic and overseas researchers do lots of studies on it and came up with Sn-whisker generates mechanism, such as dislocation, recrystallization, mechanical stress, Sn finish thickness, finish oxidized surface broken etc., and also proposed effective method to suppress Sn-whiskers, like as adds additives of other elements, reflow heat treatment, underplating of barrier layer, matte tin-plating, avoid mechanical loaded on plating etc., also verified Sn-whiskers formation mechanism and suppression methods is effective by massive experiments. Thereinto, Eshelby J D, Frank F C, Osenhach J W and others did colossal seminal study and contribution to it, and then futurity researcher proposed other affection to whiskers, such as current density, intermetallic compound, electromigration, tin atom migrates along the gradient direction of chemical potential energy etc., and add rare-earth elements to Sn-plating, coating with OSP (Organic Solderability Preservative) to retard Sn-whiskers. After analyzed and summarized on whiskers formation and actual issue of product, it proved that it is able to reduce or even eliminate Sn whiskers risk if only to take corresponding measures according to relevant Sn-whiskers mechanism.

*Corresponding author Address: Suite 3A01, XingDi Mansion, 1698 YISHAN Road, Minhang District, Shanghai, China E-mail: [email protected] or [email protected] Tel.: 1 Fax:

Columnar

Kinked

Hillock

rock candy shape

Fig.1 Sn whisker morphology

2. Sn-whiskers formation mechanism 2.1. Compressive stress mechanism For tin whiskers compressive stress formation mechanism exists many models. The primary is compressive stress model which consider the compressive stress cause tin whiskers [1].This sort of compressive stress exists in the tin layer and tin layer surface and which include internal compressive stress and external one. The compressive stress cause the tin atom diffuse along crystal boundary, the behavior form lattice defect, the defect leads to tin whiskers formation during the procedure of tin atom diffusion at the threading dislocation. Therefore, the compressive stress is the catalyst of tin whiskers, also this is a form of relieve stress way. The dominant induce factor to compressive stress as shown: 1) Intermetallic compound After the tin electrodeposited to the copper-based surface, the copper atom and tin atom will diffuse to each other, and react to generate intermetallic compound, as Cu6Sn5 and Cu3Sn etc., but the copper atom diffusion velocity is quicker than tin atom diffusion velocity. Correlational research shows that when the copper atom diffuse to the margin of crystal which comes into being tensile stress region, this diffusion consequence creates internal compressive stress, thereout induces tin whiskers formation. The material easier to diffuse to tin layer the Sn-whiskers easier to form, like as copper and zinc. On the contrary, if material is not easier to diffuse to tin layer, and then not easier to generate Snwhiskers. Here provides a reference for the whiskers growth and formation propensity to different base materials, the strong-to-weak sequence is Brass, phosphor bronze, oxygen-free copper, pure copper, brass plated nickel [2]. Lee B et al., thought that the Cu-Sn compound exists in the tin layer which generates axial direction compressive stress. The compressive stress is shearing force which comes from grain boundary, the shearing force enable oxide layer of tin surface results in shearing strain along the grain boundary direction, tin grains extruded out from the oxide layer of strain, thus causes Snwhiskers formation. In addition, disparity grain direction generates different compressive stress, and then Sn-filaments grow along the homologous direction of compressive stress. 2) Coefficient of thermal expansion 2

Correlational research shows that between various elements can also generate compressive stress because of difference coefficient of thermal expansion. Tin coefficient of thermal expansion is 26.7×10-6/℃, the copper coefficient of thermal expansion is 16.6×10-6/℃. For tin and copper coefficient of thermal expansion is different, which will give rise to different expansion rate during thermal cycle, thus presumably breed enormous compressive stress, therefrom also promote whiskers growth. The affection of coefficient of thermal expansion to Sn-whisker growth was found based on the study of FeNi42 as base materials during thermal cycle experiment, in fact ,the FeNi42 is a meglio material as substrate, but it generates severe Sn-filament after thermal cycle, further study shows the reason is because of various elements coefficient of thermal expansion mismatching, that is to say the FeNi42 alloy coefficient of thermal expansion exists colossal difference compare to Sn, at the time FeNi42 dilates which cause tremendous internal compressive stress, thus promotes whiskers growth. 3) Mechanical stress It is inevitable for mechanical fabricate to cause scratch, collision etc., during yield process, all those elements result in induce whiskers growth. Sn-Whiskers growth velocity and density increase with the compressive stress fortify [3, 4]. If applied force 15N/mm2 to Sn-plated component, the velocity of Sn whisker growth increasing to 2000Å/s compare to the nature status 1Å/s, this is gigantic increment, and the tin whisker growth rate presents linear increment along with compressive stress increasing [5]. Fisher and Carroll et al. collaborated paper proposed the significant compressive stress gradient concept of boosting Sn-whisker growth. When clamping specimen by plier, the deferrable time of crystal whisker growth approached to zero with pressure increasing, and proposed the tin atom moves from region of high stress to low stress area, and also proposed significant factor model of crystal whisker growth mechanism—single crystal. Sequentially, explains the root cause regarding the crystal whisker linear growth speed, spontaneously growth induction period and suddenly ceased of high velocity steady growth. 2.2. Dislocation mechanism Peach firstly proposed crystal whisker model which based on dislocation in 1953. Koonce et al., found the original of Sn-whiskers growth at the crystal whisker root by electron microscope during watched tin whisker continuous growth. Based on this growth characteristic, Eshelby and Frank one after another proposed a new Sn-whiskers growth mechanism based on dislocation movement [6, 7]. Eshelby deemed that the result of the finish surface oxidized to form small bulge, generate minus surface tension along with the surface. Dislocation origin at the bottom of bulge which continuous generates new dislocation rings and slide to coating surface, after every dislocation ring arrived at coating surface, its atom of half atom surface will cluster and form Sn-whiskers. However, Frank deemed that one a certain edge dislocation under finish surface is pinned by spiral dislocation, which enable edge dislocation rotate movement around spiral dislocation to form prism dislocation, the half atom of dislocation cluster at surface to form tin whiskers, and also considered that the crystal whisker axial direction should be parallel with dislocation Burgers vector. The oxidation or activation atmosphere, the small bulge and dislocation (especially spiral dislocation) is the precondition for Sn-whisker growth, as shown in Fig.2. But Amelinckx et al., deemed that spiral dislocation move to finish surface by climbing-mechanism, every dislocation arrived at surface which enable atom layer increase one Burgers vector thickness thus promote Sn-whisker growing.

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Fig.2 Frank dislocation mechanism

2.3. Recrystallization mechanism Ellis was the first researcher proposed the recrystallization theory. The theory is that the crystal whiskers formation and growth could be seen as a sort of particular recrystallization. Because the crystal whiskers growth direction is not all in conformity with dislocation surface, but there still exist no-sliding direction phenomenon, this is difficult to explain by dislocation theory. According to this that draw a conclusion: the Sn-whiskers growth has no inevitable relationship with sliding direction, thus established a new mechanism—recrystallization. Theory hypothesis: crystal whisker dimension must be reach the critical value which is the crystal whiskers growth fundamental condition; the driving force of crystal whiskers growth and Sn-whiskers growth is same, both of their driving force come from strain energy. Also Glazunova and Kudryavtsev proved crystal grains recrystallization of coating is momentous affection to crystal whisker growth by the phenomenon of crystal grain growth was suppressed after the plating thermal treated. 2.4. Oxidized layer broken mechanism Tin is easily oxidized, the oxide layer has the effect of facilitation to Sn-whiskers. Because oxygen atoms enter into Sn-layer along with crystal boundary and occur oxidized reaction to form oxide, which leads to tin layer volume expansion, thus come into being compressive stress in the plating layer. Actually, when the base is just plated, the internal stress is tensile stress which produces from the pure interatomic metallic bond, afterwards, for the reason that copper atom diffuses to tin layer and the intermetallic compound (Cu6Sn5) creation which will cause layer volume expansion to form compressive stress. The oxidation layer generated not only increase internal layer compressive stress but also retarded internal stress to release. When the tin layer exists defection and the internal stress great enough, the stress will break through oxidation layer, the whisker will grow at the layer of weak or breakage to release compressive stress, in other words, at the time that tin atom diffuse from adjacent crystal grain to neighboring crystal grain for stress relief, and then the tin crystal grains will be extruded from breakage area, thus form Sn-whiskers. 2.5. Tin layer thickness mechanism The tin layer thickness factor also has important affection to tin whiskers growth. The copper atoms migrate to tin layer and the intermetallic compound generation result in the layer volume change, and lead to generate compressive stress in the deposition layer. Under the same environment condition, if the location of copper migration and intermetallic compound generation is relative, thus it form the average pressure is also relative, then which will generate larger force at the thin tin layer, on the contrary the stress will generate smaller force for thick layer. So, for the thin tin layer which easier to cause Sn-whisker grow for relieving stress compares to thick tin layer. The tin layer thickness not only affects the difficulty level of Sn-whisker growth but also affects 4

tin whiskers grow speed. Under certain situation for the relative location of the origin of the Snwhisker formation, the Sn-whiskers extruded from thick Sn-layer will need longer time, on some level, the thickness tin layer also provide enough space and time to relieve stress, thereby, it is one certain level suppresses Sn-whiskers growth in advanced, even limits it inside of Sn-layer to not extrude out from Sn-layer surface. For the thin Sn-layer has no this advantage, Sn-whiskers will grow out quickly. Some researchers found it exists linear relationship to the thickness of pure tin and Sn0.7Cu plating with Sn-whisker density, the more Sn-whisker thicker the more lower tin density, as shown in Fig.3 [8].

Fig.3 Sn-layer thickness relationship with Sn-whiskers density

2.6. Temperature and humidity mechanism Some researchers found the environment temperature and humidity are also a significant factors to Sn-whisker growth. All researches showed there are vast Sn-whiskers appeared on the Sn-Cu and pure tin surface if exposed it to humid and hot condition 50℃/85% for two months. There literature reported that the pure tin layer very easier causes to grow Sn-whiskers in the range of temperature from ambient temperature to 60℃, because the temperature is the most beneficial to copper diffusion to tin layer to form intermetallic compound (Cu6Sn5) which causes layer volume expansion and generates compressive stress which further leads to Sn-whisker formation. If the temperature is lower which causes that there is no adequate atoms to diffuse, but if excessive temperature will cause compressive stress release which will mitigate Sn-whiskers growth. There is else literature reports that the 50～70℃ is the most suitable temperature range for Sn-whiskers to grow, because which is the most suitable temperature for atom to diffuse, but if temperature higher than 70℃ that the oxidation will change soft, the stress will reduce, and the Sn-whisker will become hillock, because the oxidation change soft that the Sn-whiskers growth-tend will weaken when it gets through pin hole, eventually the pin hole will be enlarged by the extruded Sn-whiskers. Consequently, the Sn-whiskers diameter become large, in the end that Sn-whiskers become too large while turn to hillock. Hillock is not dangerous as Sn-whiskers, because the hillock is too short to lead to short cut. There else researcher believed that the approximately 50℃ is the most suitable tin whisker growth temperature, because the temperature promotes Cu6Sn5 generation, and the 50～55℃ is also tin recrystallization temperature, at this time the compressive stress relieve lentitude, then the Sn-whiskers have sufficient time to grow. The internal tensile stress change to compressive stress under high temperature, eventually which will be eliminated. While recrystallization cause stress relieve slow which promote Sn-whiskers grow [9]. 2.7. Current density and electromigration mechanism Sn-whiskers grow tardiness at low current density but grow rapidly at high current density, the growth rate comes to 3Å/s, much higher than the applying mechanical force growth rate 0.2Å/s 5

[10]. When the current density overtop, the Joule heating effective will be obvious, the Sn-whiskers will be not thin and long along with temperature increasing but hillock. Maybe because excess temperature cause oxidation layer soft, the surface crack become larger, Sn-whiskers diameter also become larger [11]. Literature reports that when loads current density 1.2×104A/cm2 to pure tin which is easiest to grow Sn-whisker at 50℃ condition. Electromigration is that when the current loaded to conductive material that the atom of conductive material is pushed to the direction of current flowing direction. The compressive stress region will form at anode region because of the atom clustering. That is to say tin atom is bombarded by high speed electron and moves to anode region, while because the cathode lose atom that result in generates massive vacancies which cause to form tensile stress region. The Sn-whisker will grow for relieving compressive stress when the compressive stress large enough to break the tin surface oxidation at anode [10]. The electromigration has three major diffusion mechanisms: Lattice diffusion, crystal boundary diffusion and surface diffusion. But because of the tin surface easier generates oxidation which suppress the diffusion. For copper and tin, their lattice diffusion rate much less than crystal boundary diffusion rate. But for crystal boundary diffusion, copper crystal boundary diffusion rate is higher than tin crystal boundary diffusion rate. So, the crystal boundary diffusion is the main way for electromigration diffusion. Electromigration is one of the factor to form Sn-whiskers, so it is a method to degrade Sn-whiskers growth by decreasing electromigration rate. There are three main methods to decrease electromigration effect: First, to reduce current density, means slow down the momentum transformation between electron force and atom; Second, to change diffusion ability, that is to reduce material itself diffusion coefficient; Third, to change microstructure, this method be able to reduce atom diffusion path. The electron force is that the high-velocity motion electron exchanges impulsive with metal atom, consequently the atom is violently impacted by electron. 2.8. Crystal structure and its orientation It was found that Sn-whisker easily grew on irregular-shaped grains which was a few tenths of a micrometer in size, because the irregular-shaped grains contains dislocation rings which might be formed by clustering of vacancies or interstitial atoms upon electro-plating, and also because accumulation of lattice defects, internal base atom migration, intermetallic compound and tarnish film, all those factors cause internal stress, for relieving stress that irregular-shaped grains would continue grows to occur Sn-whiskers. On the contrary, it was hardly found whiskers grew on wellpolygonized grains. And that Sn-whiskers easily grew on crystalline grains whose size is a few microns, because the size is nearly the same as that of the well-polygonized grains [9]. It was found that the Sn-whiskers of occurrence on Sn-Pb is more harder than on pure tin or Sn-Cu electro-plated coating, because of the Sn-Pb has an equi-axed grain structure which causes the creep of the electrodeposit tends to decrease the compressive stress, thus lower compressive stress lead to the Sn-whiskers happened harder, while the pure tin and the Sn-Cu both has columnar structure which causes the localized creep in form of hillocks and whiskers, means the columnar structure can’t reduce the compressive stress. Compact hillocks formation for the tin deposits because the columnar grain boundaries are mobile, When the columnar grain boundary is impeded which would form contorted hillocks and whiskers to relieve compressive for Sn-Cu deposits [12]. Sn-whiskers growth has relationship with crystal grains orientation and preferred orientation, Sn-whiskers orientation is differ from normal Sn crystal grains. George T.T.Sheng et al., found Snwhiskers growth direction is [001] by transmission electron microscopy and electron diffraction technique [13]. While K.N.Tu and J.C.M.Li et al., found the Sn-whiskers crystal grain microstructure is body-centred tetragonal when in study the Sn-whiskers spontaneous growth mechanism of β-Sn by using electron scanning microscopy (SEM), the lattice constant is “a”= 0.58311nm and ”c”= 0.31817nm, Sn-whiskers growth direction has relationship with its axis which 6

grows along the direction of Sn-whiskers length, the primarily direction is “c” axis direction, but there is also other direction, such as [100] and [311] [14]. 3. The method of suppressing Sn-whiskers 3.1. Add other elements The major ingredient of Sn-Pb solder and Sn-Pb coating is eutectic Sn63Pb37, Sn63Pb37 has lower melting point, its eutectic temperature is 183℃, and also it can provide favorable wettability for welding, weldability, conductivity and lower price etc., factors which be used extensively. While lead-free solder and Sn-plating material are mainly consist of tin by adding Ag, Cu, Zn, Bi, In and Sb etc., other alloying elements, and the plumbum contain should be limited less than 0.1% [15]. So far, there are major five kind of series alloy for using to Pb-free welding and electro-plating: SnCu, SnAg, SnAgCu, SnAgCuSb and SnAgBi. The most attractive is SnAgCu series, and also include Sn0.7Cu, Sn3.55Ag etc. The main purpose of adding alloy elements to decrease the difference of between crystal surface energy and crystal boundary interfacial energy, promoting crystal structure grows in homogenous dispersion way, and also to avoid the huge size of intermetallic compound(such as Cu6Sn5) formation to generates compressive stress to cause Snwhiskers growth. Although, there are lots of alloy had been researched and put into used, but which can’t be directly replaced Sn-Pb alloy effectiveness. 3.2. Intermediate barrier and surface organic substance protection To plate certain kind of material whose dilatation coefficient between substrate and surfacecoating substance, which has the effect of suppressing to prevent base substance diffuse to coating. For copper plated with tin, firstly to plated Nickel and then to plated with tin, thus the Nickel has the function to suppress copper atom to diffuse to Sn layer, which will eliminate or weaken the compressive stress what Sn-whisker growth need, the compressive stress comes from intermetallic compound which cause volume expansion. When Sn diffuse to Nickel that the reaction will happen between the two material, and then generates intermetallic compound Sn3Ni4, its expansion affection to Sn-layer is greatly decrease compares to eutectic Cu-Sn intermetallic compound expansion affection, so Sn3Ni4 reduces Sn-layer compressive stress. Generally speaking that use nickel material to plate on the copper as intermediate layer and to plate with matte tin over nickel as surface-coating which will greatly improve the ability to confront Sn-whiskers. Beyond that, coating organic materials on the tin surface is also a method to suppress Sn-whiskers growth, such as silicon-ketone, material-parylene, polyurethane, acrylic resin and epoxy resin etc., which can package Sn-whiskers to retard it growth, and also deter device invalidation because of Sn-whiskers causing. But because of the organic coating easier broken and also existed time effect, means organic coating protection effectiveness will dropping, and then Sn-whiskers will break through the thinner coating to start to grow again. 3.3. Heat-treatment method Heat treatment can change crystal grain structure and reduce crystal boundary area that will decrease contact area with oxygen thus slow down oxidation ratio, thereby reduce the Sn-layer volume expansion because of oxidation generation, and then reduce compressive stress. It was found that the Sn-whiskers grow velocity would conspicuously slowed down and simultaneously Sn-whiskers density also reduced after Sn-layer annealing treatment or reflowing treatment. Because the crystal structure changed after heat treatment, this method relieves the compressive stress of volume expansion generated, and that crystal grains generate equi-axed crystal structure, 7

thereby slow down the diffusion of Sn atom along with crystal boundary, thus suppressed Snwhiskers growth. Kyung-Seob Kim et al., reported that plated 10μm thickness tin over copper, placed it in 125℃ to do annealing treatment, and then placed it in 55℃/85%RH for 1800h, it found the regular-IMC (intermetallic compound) appeared at interfacial region but there was no Snwhiskers growth [16]. And there are other paper indicates there would be perfect effect to relieve stress to suppress Sn-whiskers after treated several seconds at 125℃; Others considered it would be better effect to place the Sn-plated component in 150℃ for 1～4h within less than 24h after annealing treatment, during the process it would generate intermetallic compound Cu3Sn in addition to Cu6Sn5, Cu3Sn has similar performance as nickel, So Cu3Sn became barrier layer between copper and tin which could retard Cu6Sn5 continue generating, thus avoided to cause internal stress, so it repressed Sn-whisker grew. 3.4. Choose appropriate coating thickness Investigation showed that plated pure Sn on copper or on stainless steel with thickness less than 0.5μm or more than 20μm which would greatly lower Sn-whiskers formation. Tin-coated on copper with 2～5μm that Sn-whiskers grows fastest; tin-coated stainless steel with 5～10μm that Snwhiskers grows fastest. But if the Sn-plating is thinner which will lost the expectation function of corrosion prevention, coating associativity and stability etc.; while too thick Sn-coating which easier cause bridge join issue, and it is also difficult to achieve thick Sn-plating for product special structure and location, such as the tip, intersection area etc. Recommend to use the plating thickness within 10μm which can balance the protection function and Sn-whisk issue. So it is very worthy of studying how to choose appropriate tin plating thickness to prevent Sn-whiskers. 3.5. Avoiding bright tin and pure tin plating It found that Sn-whiskers density is the most greater and longest in bright-electroplated tin finishes in which all pure tin finishes. Because there is residual brightener in plating whose expansion coefficient is different compare to tin, so which will cause plating layer volume expansion to generate compressive stress to lead to Sn-whiskers grows. And also, if the electroplating process is not well controlled which will also arouse Sn-whiskers formation. The principal element is brightener concentration, current density, operation temperature and coating surface cleanliness etc. For this purpose, it is worth to study the additive and relevant plating process for reducing Sn-whiskers formation. Nevertheless, it will greatly reduce Sn-whiskers generation by using matte tin and reasonable electro-plating process. Pure tin finishes easily cause Sn-whiskers. Because tin atom migrates to copper is faster than copper atom migrates to tin layer based on copper material, thus react to form Cu6Sn5, the intermetallic compound cause volume expansion and also when copper atom diffuses to tin crystal grain edge to form tensile force region, all these consequence lead to generate compressive stress, thus lead to Sn-whiskers formation. 3.6. Avoiding external mechanical force and coating-polluted If finishes surface damaged which also promotes Sn-whiskers growth because the electrodeposition and thermal diffusion during the plating process and after cooling cause compressive stress, this is the primary inducement, thus for relieve stress Sn-whisker will grow at broken region. But it is difficult to avoid the scratch, bruise and other negative factors during the electronic component assemble, transmission, testing and other actions, these elements will cause Sn-whiskers grow due to physical or chemical reason. So, it must be try to avoid or reduce tin finish surface to be damaged to reduce the possibility of Sn-whiskers growth. 8

Zeng Xu [17] et al., reported that the atmospheric pollution and coating pollution should be considered during assess Sn-whisker growth issue, because this pollution has serious affection under humid and hot condition to tin whisker growth. It found in the experiment that Sn-whisker growth appeared at the edge where there were burrs at the earliest, the reason is that moisture easily condensed at the area which covered by burrs, and then those pollution materials would soluble in it to form strong corrosive substance. The gas as SO2, NH3 and CI2 etc., sediment particles as Na+, K+, Mg+, sulphate etc., all those substance can be absorbed on the coating surface. Especially salts which came from residual electro-plating material, those salts have strong hygroscopicity, and they can corrode coating even in the relative low humidity environment. Because coating corroded, this consequence cause Sn-whisker nucleates and grows in humid and hot environment. The essence is that corrosion break coating surface which provides the way for Sn-whisker growth to realize the final purpose of relieving stress. So, it has great significance to protect coating from polluted. 4. Sn-whiskers evaluation criterion JEDEC (Joint Electron Device Engineering Council) enacts two major international criterions, the two criterions are JESD22A121A and JESD201 which used to assess Sn-whiskers situation. Sn-whiskers generally measured by SEM (Scan Electronic Microscope). JESD22A121.01 (Test Method for Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes) criterion interprets inspection surface as depicted in Fig.4.

Fig.4 JESD22A121.01 Sn-whiskers inspection surface

JESD22A121.01 define Sn-whisker length is total length of Sn-whisker length along axial direction. As shown in Fig.5.

Fig.5 Total axial Sn-whiskers length measurement method

JESD22A121.01 define the measurement instruction as shown in Table 1.

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Table 1 Whisker density ranges that can be determined based on the number of whiskers observed per component or coupon area

5. Sn-whisker case analysis 5.1. Product structure The product as shown in Fig.6. The EON (eye of the needle) appeared Sn-whiskers, EON assembled with PCB (Printed Circuit Board) by interference fit, this kind of matching generates elastic compaction to be fixed with PCB, not welded method. The protection board is floating assembled to EON by interference fit matching before mounted to PCB.

Fig.6 Product assembling relationship

EON material is nickel-copper alloy, intermediate deposit is nickel, Ni-plating thickness is (1.1～2.3)μm, Surface plating is pure tin, Sn-plating thickness is (0.3～1.8)μm. The protection board material is LCP (Liquid Crystal Polymer). 5.2. Sn-whisker issue After EON assembled with protection board which would be stored in ambient temperature and dry surroundings. Sn-whiskers could be found from EON surface after half year or two months and even just one week in varying degrees. As shown in Fig.7.

Fig.7 Product Sn-whisker condition

5.2.1. Sn-whiskers composition testing 10

Use EDS (Energy Dispersive Spectrometer) to test Sn-whisker composition, the composition is tin, as shown in Fig.8.

Fig.8 The EDS Schematic of Sn-whisker composition

5.2.2. Sn-whisker length testing The maximum Sn-whisker length: Lmax. = 0.346mm, the minimum Sn-whisker length: Lmin.= 0.742μm, as shown in Fig.9. This level Sn-whisker length has quite serious damage to high integrate PCB product.

Fig.9 Sn-whisker length schematic

5.3. Sn-whisker formation reason analyzing Based on EON Sn-whisker formation investigation, almost all Sn-whisker grew on the arch surface of EON and also serious, as shown in Fig.7 depicted. 1）EON plated with pure tin, which easily cause Sn-whiskers; 2）EON plating tin thickness is (0.3～1.8)μm, this thickness easily cause Sn-whisker growth, but plated nickel as intermediate barrier which can suppresses Sn-whisker growth; 3）The EON is fabricated by stamping method which easily form stress concentration at arch surface, it easily cause the arch surface Sn-plating cracked, thus Which will induce Sn-whisker grows; 4）The arch surface is very narrow which easily lead to current convergence during electroplating procedure, thereby cause greater current density, which promotes Sn-whisker grow velocity; 5）Because it is interference fit between EON and protection board which will generate pressure (F=3.715N), as shown in Fig.10. After EON assembled with protection board, because interference fit which will generate friction during assembling and extra mechanical pressure at arch surface, means this region oxide layer will be easily broken during assembling, according to oxidized layer broken mechanism, it will induces Sn-whisker growth. Compared the EON after and before assembled with protection board, the former has distinct Sn-whisker growth, but the latter has no Sn-whisker growth, as shown in Fig.11.

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Fig.10 the assemble relationship between EON and protection board

Fig.11 EON assembled with protection board and Un-assembled with protection board Sn-whiskers situation

5.4. Suppressing Sn-whisker method 1) Chang external mechanical pressure Change the interference fit to clearance fit for avoiding external compressive and oxide layer broken to reduce the inducing factor of Sn-whisker growth (F=0N), as shown in Fig.12.

Fig.12 the assemble relationship between EON and protection board

2) Heat treatment method Treating plated EON for 10 seconds in 125℃ high temperature oven. 3) Using Sn-Ag electro-plated replaced pure tin electro-plated for EON.

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The improved specimens are tested by placing in ambient temperature for 1000 hours, the consequence as shown in Fig.13. It is showed that improved specimens Sn-whisker is suppressed by using above methods.

Fig.13 Improved specimen at room temperature 1000 hours

6. Conclusion The investigation of Sn-whisker growth mechanism and inducing factor has great guiding significance according to the academia already researched results and the verification of actual product case. 1) The major factor of generating Sn-whisker is internal compressive stress and external compressive stress: The internal factor main reason is the base atom migrates to plating layer cause generate IMC, internal energy function, thermal transmission, thus lead to plating layer volume expansion thereby generate compressive stress; the external factor main reason is oxygen migrates to Sn-layer react to oxide which lead to volume expansion, thereby which forms compressive stress, and also exists the residual stress which comes from Sn-layer welding and also mechanical loading pressure etc. The Sn-whisker formation rationale: First, the continuous compressive stress is the driving force to maintain Sn-whisker growth; Second, The compressive stress of continuous generating from tin layer need relieve by Sn-whisker growth to keep the energy dynamic equilibrium. Thus it can be seen the force-loading of Sn-layer is the root cause, so if it can be effectively eliminated or reduced, also means the Sn-whisker growth will effectively suppressed. 2) For settling the Sn-whisker issue, first, it ought to locate the Sn-whisker position and then analyze the internal factor and external factor. For the internal factor which should be focus on analyzing the plating composition, tin-layer thickness, substrate types and exist or not intermediate barrier etc. The external factor which should be focus on analyzing Sn-layer surface external mechanical force, temperature-humidity, exist or not protection materials and contaminant etc. And then choose or work out the relevant suppression measurement according to actual Sn-whisker condition. 3) At present, all those research of Sn-whisker is relative decentralized for internal factor and external factor which is not combined together. So, if unify the internal factor and external factor to one model to do analysis by CAE(Computer Aided Engineering), which will benefit to improve analysis efficiency, product reliability and reduce economic loss, and also contribute to environment protection, believe that will have a positive significance. Albeit, Sn-whisker formation is relatively complicated which involves multi-disciplinary fields, but if only focus on the internal and external force-loading research, that must be able to find effective way to solve the Sn-whisker issue. References [1] Bokisa, S George, Bishop. Process for Whisker-free Agueous Electroless Tin Plating [P]. U.S. Patent: 6 361 823, 2002. [2] Osenbach J W,De Lucca J M,Potteiger B D,et al., Lead free packaging and Sn-whiskers, Proceedings of

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