J Fail. Anal. and Preven. (2010)10: 399-407 DOI10.1007/sll668-01093592 TECHNICAL ARTICLE-PEER-REVIEWED Hydrogen Embrittlement on High-Speed Stainless Steel Belts Used for Tin Plating Chip Lead Frame un-song Gru Gong· Zhen-Guo Yang Submitted 010/in revised form: 30 May 2010/Published online: 9 June 2010 C ASM In Abstract The 300 series stainless steels generally exhibit appliances, and so on. Actually, the chip lead frame which good corrosion resistance in common use. However, a pre- is installed around the chip for the purposes of supporting mature fracture event caused by hydrogen embrittlement was the chip, dissipating heat, and connecting exterior circuit, is encountered on 300 series stainless steels which was used as an important component in IC. To provide for a good belt hanging chips in a tin plating process for the chip lead weldability between chips and chip lead frames or between frame. The cause of the fracture was carefully studied. a the gold wires on frame, electric plating the effective metallurgical microscope and photoelectric direct reading of chip lead frame is a significant procedure in mar pectrometer were used to examine the metallographic turing process. However, equipment used in the structures and chemical compositions of the matrix material. production line, particularly the stainless steel belts that are A scanning electron microscope and energy disperse spec- fixed on a rotational disk to hang the chip lead frames in troscope were also applied to analyze the micro morphologies plating process, is frequently subjected to failure events and micro-area composition of the fracture. Meanwhile, the due to the severe service conditions chemistry and hydrogen content of the process media were The main activities of the tin plating process for chip inspected by ion chromatography and hydrogen analyzer In lead frames are commonly divided into six steps, including addition, the finite element method was employed to simulate electrolytic deburring, deoxidation, activation, electroplat the effect on the belt from the service conditions. The analysis ing, neutralization, and deplating, seen in Fig. 1. These results revealed that the unqualified material selection, the activities include the use of aggressive solutions and large aggressive media, and the inappropriate technological density currents in the deoxidation and/or activation steps parameters were the main causes of the failure. Furthermore, Failure events are typically caused by the coupling of the the mechanisms of hydrogen embrittlement were discussed, solutions and currents with inappropriate materials selec- and countermeasures and suggestions were put forward. tion for the belts. Thus, solution chemistry, current densities, and material choices are the three most serious Keywords Stainless steel belt. Hydrogen embrittlement. factors associated with failure and can cause hydrogen ure analvsis embrittlement failure events on the stainless steel belts The high-pressure washing water(nearly 20 MPa)imposed in the second substep after electrolytic deburring is also an Introduction important failure causing process and all six successive and Integrated circuits (IC) have wide applications in our daily repetitive steps may aggravate any failure extent. In this case, a failure event was reported occurring on the life, including our use of mobile phones, laptops, electrical stainless steel belts used for hanging the chip lead frames of a tin plating production line in one chip manufacturing Y.S.u·Y.Gong·Z.-G.Yang(國) works in Shanghai. Some stainless steel belts, made of the Department of Materials Science, Fudan University No 220 Handan Road Shanghai 200433 so-called '304 stainless steel (according to the manufac People's Republic of China turer), failed within one month. This lifetime is much shorter e-mail:zgyang@fudan.edu.cn han the expected life of longer than six months. Thus,a Spring
TECHNICAL ARTICLE—PEER-REVIEWED Hydrogen Embrittlement on High-Speed Stainless Steel Belts Used for Tin Plating Chip Lead Frame Yun-Song Gu • Yi Gong • Zhen-Guo Yang Submitted: 27 April 2010 / in revised form: 30 May 2010 / Published online: 9 June 2010 ASM International 2010 Abstract The 300 series stainless steels generally exhibit good corrosion resistance in common use. However, a premature fracture event caused by hydrogen embrittlement was encountered on 300 series stainless steels which was used as belt hanging chips in a tin plating process for the chip lead frame. The cause of the fracture was carefully studied. A metallurgical microscope and photoelectric direct reading spectrometer were used to examine the metallographic structures and chemical compositions of the matrix material. A scanning electron microscope and energy disperse spectroscope were also applied to analyze the micro morphologies and micro-area composition of the fracture. Meanwhile, the chemistry and hydrogen content of the process media were inspected by ion chromatography and hydrogen analyzer. In addition, the finite element method was employed to simulate the effect on the belt from the service conditions. The analysis results revealed that the unqualified material selection, the aggressive media, and the inappropriate technological parameters were the main causes of the failure. Furthermore, the mechanisms of hydrogen embrittlement were discussed, and countermeasures and suggestions were put forward. Keywords Stainless steel belt Hydrogen embrittlement Failure analysis Introduction Integrated circuits (IC) have wide applications in our daily life, including our use of mobile phones, laptops, electrical appliances, and so on. Actually, the chip lead frame which is installed around the chip for the purposes of supporting the chip, dissipating heat, and connecting exterior circuit, is an important component in IC. To provide for a good weldability between chips and chip lead frames or between the gold wires on frame, electric plating the effective area of chip lead frame is a significant procedure in manufacturing process. However, equipment used in the entire production line, particularly the stainless steel belts that are fixed on a rotational disk to hang the chip lead frames in tin plating process, is frequently subjected to failure events due to the severe service conditions. The main activities of the tin plating process for chip lead frames are commonly divided into six steps, including electrolytic deburring, deoxidation, activation, electroplating, neutralization, and deplating, seen in Fig. 1. These activities include the use of aggressive solutions and largedensity currents in the deoxidation and/or activation steps. Failure events are typically caused by the coupling of the solutions and currents with inappropriate materials selection for the belts. Thus, solution chemistry, current densities, and material choices are the three most serious factors associated with failure and can cause hydrogen embrittlement failure events on the stainless steel belts. The high-pressure washing water (nearly 20 MPa) imposed in the second substep after electrolytic deburring is also an important failure causing process and all six successive and repetitive steps may aggravate any failure extent. In this case, a failure event was reported occurring on the stainless steel belts used for hanging the chip lead frames of a tin plating production line in one chip manufacturing works in Shanghai. Some stainless steel belts, made of the so-called ‘304’ stainless steel (according to the manufacturer), failed within one month. This lifetime is much shorter than the expected life of longer than six months. Thus, a Y.-S. Gu Y. Gong Z.-G. Yang (&) Department of Materials Science, Fudan University, No. 220 Handan Road, Shanghai 200433, People’s Republic of China e-mail: zgyang@fudan.edu.cn 123 J Fail. Anal. and Preven. (2010) 10:399–407 DOI 10.1007/s11668-010-9359-2
J Fail. Anal and Preven.(2010)10: 399-407 Fig. 1 Process, medium, and concentration of tin plating Electrolytic Deburring tration approximate 70-80p/L) ( flumes filled by acidic solution containing fluoride anion Activation Once water spray concentration approximate 95-120B/L) flumes acid and acidity cor proximate 95-120g/L (Alkali concentration approximate 45-50g/L) M Hot-water cleaning Once water spray 3 times blow drying planing (5 bathes Methanesulfonic acid and thorough survey through field investigations and sampling respectively. Figure 2b indicates that the failed position is nalyses on the causes and the mechanisms failure the bottom bracket of the belt moreover the fat fracture is was required to reduce the downtime and resulting increase a sign of macroscopically brittle failure processes, seen in in production costs. Consequently, ion chromatography (IC) Fig. 2c was used for determining chemical composition of plating solution, while hydrogen analyzer, photoelectric direct Matrix Materials Examination reading spectrometer, metallurgical microscope (MM) scanning electronic microscope(SEM) were applied to Chemical compositions of three types of the stainless steel detect the chemical compositions, metallographic structures belt samples, i.e., the original one, the failed one which used and macro micro morphologies of the failured stainless only one month, and the normal one which used longer than steel belts, respectively. Furthermore, finite element method 4 months, were inspected by photoelectric direct reading (FEM) as an auxiliary method was also employed to simu- spectrometer. The main content in matrix material is listed late the stress distribution on the steel belts under high in Table 1. It is found that the matrix material of the failure pressure washing water. The analysis results showed that the steel belt was 301 stainless steel which has higher carbon main cause of this failure was hydrogen embrittlement, content and less chromium than the 304 stainless steel which was introduced from the solutions used in deoxidation required from design. This contrasts the matrix of the belt and electroplating steps. Finally, mechanisms of the failure used for 4 months which was 316L stainless steel. The 316L were discussed and suggestions were proposed, which have steel has an excellent corrosion resistance but a relatively significant importance not only in failure prevention for steel higher cost. Commonly, as metastable austenitic stainless belts used under similar service conditions but also in steels [1], both 301 and 304 exhibit a severer hydrogen developing a better understanding of hydrogen embrittle- embrittlement aptitude than stable austenitic stainless steels ment in engineering practice. The service life of stainless [2]. This observation suggests that inappropriate materials steel belts was extended to the normal level by accepting selection may be one of the failure causes. hese suggestions. In order to judge whether the steel belts were given a surface treatment to increase of corrosion resistance chemical compositions of the polished original stainless Experimental Methods and Results steel belts, and the unpolished one were measured and are listed in Table 1 [3. It's obvious that both surfaces have Visual observation the similar chemical compositions, demonstrating that no surface treatment was conducted. Thus. the lack of The different macro morphologies of the original and required surface treatment to the steel belts is another ailed stainless steel belts are shown in Fig 2a and b. factor of the failure
thorough survey through field investigations and sampling analyses on the causes and the mechanisms of this failure was required to reduce the downtime and resulting increase in production costs. Consequently, ion chromatography (IC) was used for determining chemical composition of plating solution, while hydrogen analyzer, photoelectric direct reading spectrometer, metallurgical microscope (MM), scanning electronic microscope (SEM) were applied to detect the chemical compositions, metallographic structures and macro & micro morphologies of the failured stainless steel belts, respectively. Furthermore, finite element method (FEM) as an auxiliary method was also employed to simulate the stress distribution on the steel belts under high pressure washing water. The analysis results showed that the main cause of this failure was hydrogen embrittlement, which was introduced from the solutions used in deoxidation and electroplating steps. Finally, mechanisms of the failure were discussed and suggestions were proposed, which have significant importance not only in failure prevention for steel belts used under similar service conditions but also in developing a better understanding of hydrogen embrittlement in engineering practice. The service life of stainless steel belts was extended to the normal level by accepting these suggestions. Experimental Methods and Results Visual Observation The different macro morphologies of the original and the failed stainless steel belts are shown in Fig. 2a and b, respectively. Figure 2b indicates that the failed position is the bottom bracket of the belt. Moreover, the flat fracture is a sign of macroscopically brittle failure processes, seen in Fig. 2c. Matrix Materials Examination Chemical compositions of three types of the stainless steel belt samples, i.e., the original one, the failed one which used only one month, and the normal one which used longer than 4 months, were inspected by photoelectric direct reading spectrometer. The main content in matrix material is listed in Table 1. It is found that the matrix material of the failure steel belt was 301 stainless steel which has higher carbon content and less chromium than the 304 stainless steel required from design. This contrasts the matrix of the belt used for 4 months which was 316L stainless steel. The 316L steel has an excellent corrosion resistance but a relatively higher cost. Commonly, as metastable austenitic stainless steels [1], both 301 and 304 exhibit a severer hydrogen embrittlement aptitude than stable austenitic stainless steels [2]. This observation suggests that inappropriate materials selection may be one of the failure causes. In order to judge whether the steel belts were given a surface treatment to increase of corrosion resistance, chemical compositions of the polished original stainless steel belts, and the unpolished one were measured and are listed in Table 1 [3]. It’s obvious that both surfaces have the similar chemical compositions, demonstrating that no surface treatment was conducted. Thus, the lack of a required surface treatment to the steel belts is another factor of the failure. Fig. 1 Process, medium, and concentration of tin plating technics 400 J Fail. Anal. and Preven. (2010) 10:399–407 123
J Fail. Anal. and Preven. (2010)10: 399-407 (b) (c) L SOumI Fig 2 Macroscopic nce of stainless steel belt: (a) original one.(b) fractured or Fig 3 Metallographic structure of fractured stainless steel belts cross section: (a) cross section 100x,(b)center of cross sec tion 500x, and(e)surface of stainless steel belt Table 1 Chemical compositions of original and fractured stainless steel belts (wt % Ele AISI 301 <0.15 2.00 <0.045<003016.0~18.0 6.00~8.00 AISI 304 <0.045<0.03018.0~20.0 8.00~10.50 AISI 316L <0.03 ≤1 <0.045<0.03016.0~1802.00~3.001200~15.00 0.11050.7246 l.0432 0 031717.2484 0.1585 6.7737 Original one(polished) 0.1581 6.783 Original one(unpolished) 0.10920.7633 02120.00070.026717.03540.1602 Normal one after 4-month use 0.0 0.719 0.00340.00416 It is well known that 300 series stainless steels com- section of the failed stainless steel belt is a mixture monly exhibit austenitic microstructures. However, as it is microstructure of both austenite and martensite which has a shown in Fig 3, metallographic structure of the cross twisted fibrous morphology. Generally speaking, the Spring
It is well known that 300 series stainless steels commonly exhibit austenitic microstructures. However, as it is shown in Fig. 3, metallographic structure of the cross section of the failed stainless steel belt is a mixture microstructure of both austenite and martensite which has a twisted fibrous morphology. Generally speaking, the Fig. 2 Macroscopic appearance of stainless steel belt: (a) original one, (b) fractured one, and (c) fracture Table 1 Chemical compositions of original and fractured stainless steel belts (wt.%) Element C Si Mn P S Cr Mo Ni AISI 301 B0.15 B0.75 B2.00 B0.045 B0.030 16.0*18.0 / 6.00*8.00 AISI 304 B0.08 B0.75 B2.00 B0.045 B0.030 18.0*20.0 / 8.00*10.50 AISI 316L B0.03 B1.00 B2.00 B0.045 B0.030 16.0*18.0 2.00*3.00 12.00*15.00 Failure one (polished) 0.1105 0.7246 1.0432 0.0026 0.0317 17.2484 0.1585 6.7737 Original one (polished) 0.1103 0.7223 1.0411 0.0027 0.0320 17.2311 0.1581 6.7831 Original one (unpolished) 0.1092 0.7633 1.0212 0.0007 0.0267 17.0354 0.1602 6.4379 Normal one after 4-month use 0.019 0.719 0.94 0.0034 0.004 16.40 2.17 10.05 Fig. 3 Metallographic structure of fractured stainless steel belt’s cross section: (a) cross section 1009, (b) center of cross section 5009, and (c) surface of stainless steel belt J Fail. Anal. and Preven. (2010) 10:399–407 401 123
J Fail. Anal and Preven.(2010)10: 399-407 existence of martensite, which occurs at the inner part of known inducing corrosion on metals, especially pitting austenite by means of strain-induced martensitic phase corrosion. However, it has been reported that a relatively hydrogen diffusion coefficient and permeability of mar- to the pitting corrosion of carbon steels [6]. In thUrs effect transformations, increases the hydrogen uptake because the high concentration of F will provide an inhibitor tensite is higher than austenite [4]. Therefore, the concentration of F is 6.4 g/ and may explain why martensitic transformation products could act as a suitable was no obvious evidence of pitting corrosion on the medium for hydrogen to entry and transport in the stainless stainless steel belt. steel [5] lon Chromatography Hydrogen Absorption lon chromatography was used to semi-quantitatively ana According to the technological parameters, hydroge lyze the main anions in the solutions concentration in the original 304 stainless steel belt is encounter,i.e, the deoxidation solution, the electrolytic 6 ppm, whereas the value for the failed one which used deburring solution, and the deplating solution. In addition only one month exceeds 17 ppm. The hydrogen absorption to the sulfate SO4- which was detected in all three solu- analysis thus showed the use had increased the hydroger imes of the normal tions, the fiuoride anion F was particularly found in the content to nearly thre deoxidation solution, as seen in Fig. 4. Commonly, So, 2- increase in hydrogen may result from hydrogen evolution is a main component in the electroplating solution and is reactions during the engineering production process and present as sulfuric acid. The F is a halide ion that is will favor the onset of hydrogen embrittlement SEM and EDS Analysis Surface of failed Steel Belts Figure 5 displays the SEM micrographs of the surface of the normal surface morphology. Regularly distributed micro cracks and pits can be clearly found in Fig. 5b and c 02040608.010.012014 and are absent in Fig. 5a. Additionally, there are linear Fig 4 Chromatographic analysis result of deoxidation solution cracks parallel to the fracture, and some of these cracks Fig§ SEM micrograph of defects in fractured belt bracket surface: (a) morphology of belt bracket surface, (b) enlarged local region, and (e) enlarged ()三
existence of martensite, which occurs at the inner part of austenite by means of strain-induced martensitic phase transformations, increases the hydrogen uptake because the hydrogen diffusion coefficient and permeability of martensite is higher than austenite [4]. Therefore, the martensitic transformation products could act as a suitable medium for hydrogen to entry and transport in the stainless steel [5]. Ion Chromatography Ion chromatography was used to semi-quantitatively analyze the main anions in the solutions that the steel belts encounter, i.e., the deoxidation solution, the electrolytic deburring solution, and the deplating solution. In addition to the sulfate SO4 2 which was detected in all three solutions, the fluoride anion F was particularly found in the deoxidation solution, as seen in Fig. 4. Commonly, SO4 2 is a main component in the electroplating solution and is present as sulfuric acid. The F is a halide ion that is known inducing corrosion on metals, especially pitting corrosion. However, it has been reported that a relatively high concentration of F will provide an inhibiting effect to the pitting corrosion of carbon steels [6]. In this case, the concentration of F is 6.4 g/l and may explain why there was no obvious evidence of pitting corrosion on the stainless steel belt. Hydrogen Absorption According to the technological parameters, hydrogen concentration in the original 304 stainless steel belt is 6 ppm, whereas the value for the failed one which used only one month exceeds 17 ppm. The hydrogen absorption analysis thus showed the use had increased the hydrogen content to nearly three times of the normal value. Such an increase in hydrogen may result from hydrogen evolution reactions during the engineering production process and will favor the onset of hydrogen embrittlement. SEM and EDS Analysis Surface of Failed Steel Belts Figure 5 displays the SEM micrographs of the surface of the failed stainless steel belts and compares that surface to the normal surface morphology. Regularly distributed micro cracks and pits can be clearly found in Fig. 5b and c and are absent in Fig. 5a. Additionally, there are linear Fig. 4 Chromatographic analysis result of deoxidation solution cracks parallel to the fracture, and some of these cracks Fig. 5 SEM micrograph of defects in fractured belt bracket surface: (a) morphology of belt bracket surface, (b) enlarged local region, and (c) enlarged side of fracture 402 J Fail. Anal. and Preven. (2010) 10:399–407 123
J Fail. Anal. and Preven. (2010)10: 399-407 have connected to form a larger one. Moreover, a peculiar Fractograph of Failed Steel Belts morphology of step-shaped crack group found on the sur face is the typical feature of hydrogen-induced stepwise In Fig. 7a, a flat cross section can be observed on the cracking(HISC). According to the EDS result shown in fracture. This fracture topography is generally a sign of Fig. 6, it can be seen that no chlorine and fluorine elements macroscopically brittle fracture and is consistent with the were found on the surface of the failed steel belts, which failure mechanism of hydrogen embrittlement. Addition- indicates that halide-induced pitting corrosion should not ally, a long dark strip with even smoother surface is seen be blamed for the emergence of the pits on the surface. on the bottom of the fracture, Fig. 7b. This is a typical Hence, it can be further identified that pits around the morphology of erosion, and then it can be inferred that the micro cracks were caused by hydrogen blistering(HB). To ultimate fracture of the steel belts may also involve the sum up, all the above observations are relevant to hydrogen high-pressure water washing procedure embrittlement of the stainless steel belt which reduced its Both obvious cleavage steps and dimples can be found properties and eventually resulted in fracture [71 n Fig 8a. However, the cleavage steps cover most of the cross section, while the dimples were found only in the part of the cross section, seen in Fig. 8b. This phenomenon is consistent with fracture of the steel belts by hydrogen embrittlement with the embrittlement process being initiated from the outer surfaces of the belt In order to qualitatively analyze the effect from the high-pressure washing water (20 MPa) imposed on the stainless steel belt, finite element method(FEM) software was employed to simulate the stress distribution on the belt fter washing. This is actually a two-dimensional (2-D transient elastic-dynamic analysis [8-10], and the meshed FEM model with element of PLANE 82 is presented in Fig. 9. The fractured part was further refined for accuracy Thickness of the belt is 0.5 mm. Poisson ratio and Youngs modulus were set as 0.3 and 2.06X 10- MPa, respectively Fig. 6 Result of EDs Displacement of the left and right sides(except the bracket) Fig. 7 SEM micrograph of fracture surface of belt bracket: b (a) facture surface of belt bracket and (b) enlarge the left NowY SO Moan set W, Dora Bim Mum stainless steel belt's fracture Spring
have connected to form a larger one. Moreover, a peculiar morphology of step-shaped crack group found on the surface is the typical feature of hydrogen-induced stepwise cracking (HISC). According to the EDS result shown in Fig. 6, it can be seen that no chlorine and fluorine elements were found on the surface of the failed steel belts, which indicates that halide-induced pitting corrosion should not be blamed for the emergence of the pits on the surface. Hence, it can be further identified that pits around the micro cracks were caused by hydrogen blistering (HB). To sum up, all the above observations are relevant to hydrogen embrittlement of the stainless steel belt which reduced its properties and eventually resulted in fracture [7]. Fractograph of Failed Steel Belts In Fig. 7a, a flat cross section can be observed on the fracture. This fracture topography is generally a sign of macroscopically brittle fracture and is consistent with the failure mechanism of hydrogen embrittlement. Additionally, a long dark strip with even smoother surface is seen on the bottom of the fracture, Fig. 7b. This is a typical morphology of erosion, and then it can be inferred that the ultimate fracture of the steel belts may also involve the high-pressure water washing procedure. Both obvious cleavage steps and dimples can be found in Fig. 8a. However, the cleavage steps cover most of the cross section, while the dimples were found only in the middle part of the cross section, seen in Fig. 8b. This phenomenon is consistent with fracture of the steel belts by hydrogen embrittlement with the embrittlement process being initiated from the outer surfaces of the belt. In order to qualitatively analyze the effect from the high-pressure washing water (20 MPa) imposed on the stainless steel belt, finite element method (FEM) software was employed to simulate the stress distribution on the belt after washing. This is actually a two-dimensional (2-D) transient elastic-dynamic analysis [8–10], and the meshed FEM model with element of PLANE 82 is presented in Fig. 9. The fractured part was further refined for accuracy. Thickness of the belt is 0.5 mm, Poisson ratio and Young’s modulus were set as 0.3 and 2.06 9 105 MPa, respectively. Fig. 6 Result of EDS Displacement of the left and right sides (except the bracket) Fig. 7 SEM micrograph of fracture surface of belt bracket: (a) facture surface of belt bracket and (b) enlarge the left part Fig. 8 SEM micrograph of stainless steel belt’s fracture: (a) expended morphology of stainless steel belt’s fracture and (b) dimples showing expended direction J Fail. Anal. and Preven. (2010) 10:399–407 403 123