Thermal Bond Reliability of High Reliability New Palladium-coated Copper Wire

Transcription

1 217 IEEE 67th Electronic Components and Technology Conference Thermal Bond Reliability of High Reliability New Palladium-coated Copper Wire Motoki Eto, Teruo Haibara, Ryo Oishi and Takashi Yamada Nippon Micrometal Corporation 18-1 Sayamagahara, Iruma-city, Saitama , Japan Tomohiro Uno and Tetsuya Oyamada Nippon Steel & Sumitomo Metal Corporation 1 Shintomi, Futtsu-city, Chiba , Japan Abstract Pd coated copper (PCC) wire and Au-Pd coated copper () wire have been widely used in the field of LSI device. Recently, higher bond reliability at high temperature becomes increasingly important for on-vehicle devices. However, it has been reported that conventional PCC wire caused a bond failure at elevated temperatures. On the other hand, new- wire had higher reliability at higher temperature than conventional wire. New- wire has higher concentration of added element than conventional wire. In this paper, failure mechanism of conventional wire and improved mechanism of new- wire at high temperature were shown. New- wire is suitable for onvehicle devices. Keywords- Cu wire bonding; on-vehicle devices; high reliability; high temperature; Pd coated Cu wire I. INTRODUCTION Cu wire has been widely used in the field of LSI devices as a substitute for Au wire. In particular, Pd coated copper (PCC) and Au-Pd coated copper () wires are the majority of the market share, due to excellence in low cost, long-term storage, bonding characteristics and corrosion resistance. Recently, on the background of popularization of electric vehicles and hybrid vehicles, needs for replacing Au wire with Cu wire is increasing also in on-vehicle devices use. On-vehicle devices are required higher bond reliability in severely high-temperature and high-humidity environment than general LSI devices. Especially, hightemperature is a major concern to design and manage long term reliability. It has been widely investigated that Cu ball bond failure mechanism under high-humidity conditions such as pressure cooker test (PCT) or highly accelerated humidity and temperature stress test (HAST). In PCT aging, Cu and Al at the 1st bond inter face mutually diffused, and IMC s were formed. Among the several types of IMC s, Cu 9Al 4 was susceptible to corrosion by halogen and corroded by Cl - dissolved from mold compound. This failure is easier to occur in bare Cu than PCC wire. This is because, Pd was concentrated at the bond interface for PCC after aging. The Pd-enriched layers consisted of Cu-Al-Pd compound and Cu-Pd solid solution. The Pd-enriched layers were helpful for improving the bond reliability under high-humidity by controlling diffusion and IMC formation [1-3]. In the past research, reliability of Cu ball bond under high temperature storage (HTS < 1deg C) test has been less concerned than that under PCT and HAST [4]. As HTS test is in a dry environment, Cl - is difficult to dissolve from mold compound. Further, IMC growth rate of Cu wire is much slower than that of Au wire under Al pad. Therefore, HTS test didn t have significant impact on the failure process for Cu wire. However, more recently, it has been reported that void was formed at ball and stitch bond portions after severely high temperature storage test (HTS > 1deg C). And, the void failure was observed only with PCC wire and not with bare Cu wire. The void failure was caused only by diffusion of Cu through crack of the Pd layer []. On the other hand, other report suggests that the mechanism is the galvanized corrosion by Pd-Cu coupling [6]. In this study, the failure mechanism under severely hightemperature storage test in wire was investigated comparing with bare Cu wire; it is supposed that and PCC wires show the same behavior. A high thermal reliability wire, named new- wire, is also introduced in this study and demonstrated that it is suitable for on-vehicle devices. II. EXPERIMENTAL Three types of bonding wires were used, conventional, new- and 4N bare Cu wire. New- wire has higher concentration of added element than wire. The wire diameter was 18 μm. Wire bondings were carried out using the wire bonder machine (Kulicke & Soffa, ProCu) on 144-pin quad flat package (QFP) with Al-1%Si-.%Cu pads metallization (1.μm thick). After wire bonding, the bonded samples were encapsulated with conventional mold compounds. HTS test conditions were 1, 17 and 2deg C. HTS tests were also conducted in air and in vacuum (< 1Pa) without encapsulation in order to eliminate the influence of impurities in mold compound. Decapsulation process was conducted with laser IC opener (Nippon Scientific, PL11) and mixed acid (fuming nitric acid and concentrated sulfuric acid), followed by acetone washes. The bond strength was measured by 2nd pull test and ball shear test by using Dage 4plus. Cross-sectional samples were prepared by using cross section polisher (JEOL, IB- 191CP) with 6kV Ar-ion. Microstructure observations /17 $ IEEE DOI 1.119/ECTC

2 Pull strength /gf (a) 1deg C (b) 17deg C (c) 2deg C Pull strength /gf Pull strength /gf New- 1 Fig 1. 2nd pull test after HTS test for (a) 1deg C, (b) 17deg C and (c) 2deg C. Black solid line is conventional, blue solid line is new- and black dashed line is bare Cu wire. 1deg C 1μm (a) (b) New- (c) (a) (b) New- 1μm (a) Pd (b) Pd 2deg C Fig 2. SEM images at 2nd bond after 1hr at 1 and 2deg C were performed with conventional FE-SEM (JEOL, JSM- 78F). Some parts of samples were also analyzed using conventional FE-TEM (JEOL, JEM-21F) at an accelerating voltage of 2kV. Analyses of outgassing produced from the mold compound under high temperature were carried out by thermal desorption spectrometry (TDS, TE-36S) and Ion Chromatography (IC). III. RESULTS AND DISCUSSION A. HTS test in molded Fig. 1 shows 2nd pull strength of decapsulated three types of bonded wire (new-, and bare Cu wire) after aging at (a) 1, (b) 17 and (c) 2deg C. For aging at 1deg C, all wires did not decrease in bond strength after 1hr. However, bond strength of wire decreased after 1hr at 17 and 2deg C. Decrease of bond strength at 2deg C was faster than that of 17deg C. Fig. 2 shows SEM images of decapsulated 2nd bond portion after 1hr at 1 and 2deg C. All wires kept good bond condition after 1hr at 1deg C. In the case of higher temperature (2deg C), wire (Fig. 2a) had void (indicated with a white arrow), whereas new- wire (Fig. 2b) and bare Cu wire (Fig. 2c) wire maintained a smooth surface. Fig. 3a and Fig3b shows cross-sectional SEM image and EDS mapping (a) S (b) S Fig 3. Cross-sectional SEM images and EDS mapping at 2nd bond portion after 1hr at 2deg C for (a), (b) New-. of 2nd bond portion of wire and new- wire after 1hr at 2deg C. In case of wire, a large void was observed in Cu exposed portion in which Pd layer was missing. This cracked Pd layer portion is ineluctably formed during wire bonding process. And sulfur was detected at the void portion. It is supposed that this void led to decrease of bond strength. On the other hand, although Cu exposed portion was also observed in new- wire (Fig. 3b), occurrence of void was suppressed. Improved mechanism for new- wire is discussed later. B. HTS test in un-molded In order to clarify the failure mechanism (occurrence of void) for wire under severe HTS condition, it is necessary to classify whether failure is due to the influence of temperature only or also due to the influence of surrounding environment (mold compounds, atmosphere). First, HTS test was conducted in air and vacuum (< 1Pa) in un-molded to eliminate the influence of impurities in mold compound. 1298

3 After aging in air Initial New- (a) (b) (c) 1μm (d) (e) (f) Fig 4. SEM images of wire surface at initial ((a)-(c)) and after 1hr at 2deg C ((d)-(f)) in air with un-molded. (a) and (d) are, (b) and (e) are new-, (c) and (f) are bare Cu wire. (a) 1μm (a) O (a) Cu (a) Pd Fig. 4 shows SEM image of wire surface for before and after HTS test (after 1hr at 2deg C) in air with unmolded. All wires had smooth surface initially (Fig. 4a-c). After HTS test, for wire (Fig. 4d) and new- wire (Fig. 4e), some products (indicated with a white arrow) were locally formed on the wire surface, and in the other portion the initial surface condition was maintained. The size of hemispheric product in wire was bigger than that of new- wire. On the other hand, for bare Cu wire, the hemispheric product was not formed (Fig. 4f), but a wire surface irregularity became noticeable. Fig. a shows crosssectional SEM image of hemispheric product in wire which is observed after HTS test (Fig. 4d). Just below the hemispheric product, gap was seen in Pd layer (EDS (b) (b) O (b) Cu Fig. Cross-sectional SEM images and EDS mapping after 1hr at 2deg C. (a), (b)bare Cu wire. After aging in vacuum (a) 1μm New- (b) (c) Fig 6. SEM images of wire surface after 1hr at 2deg C in vacuum with un-molded. (a), (b) new- and (c) bare Cu wire. mapping in Fig. a Pd) and void was observed in Cu core. The hemispheric product consisted of Cu and O. In case of bare Cu wire, continuous void was formed under surface (Fig. b) and EDS mapping revealed that the layer which consisted of Cu and O film was formed on continuous void. Therefore, although the shapes were different, similar products were formed in both of the and bare Cu wire after HTS test in air with un-molded. Fig. 6 shows SEM image of wire surface after HTS test (after 1hr at 2deg C) in vacuum with un-molded. All wires maintained smooth surface and any hemispheric product was not observed. Even though Fig. 4a-c and Fig. 6a-c had the same heat history, structures of wire surface were greatly different depending on the surrounding environment. C. Failure mechanism after HTS test in un-molded After HTS test in unmolded, such failure as void or hemispheric product occurred only in air and didn t occur in vacuum. In case of wire, if the failure was caused by Cu diffusion through missing Pd portion under high temperature [], the similar failure should occur also in vacuum. That is, it is not intrinsic degradation (diffusion of Cu) of wire under high temperature. It is corrosion induced by surrounding environment (H 2O, O, S, Cl etc.). wire has Pd layer on the Cu core, but there is a gap of Pd layer (i.e. Cu exposed portion) rarely. The gap of Pd layer is formed during wire drawing process and bonding process. In this portion, localized corrosion is easy to occur. At the Cu exposed portion, materials with different electrode potentials, namely Cu and Pd, come into contact in electrolyte containing environment. Cu acts as anode and Pd acts as cathode. Since moisture is always contained in air, it can become electrolyte. In that case, galvanic cell is formed. A large Pd cathode coupled with a small portion of Cu anode results in rapid penetration of Cu, because the galvanic current density at the small Cu anode is very high to keep electric neutrality. Detailed chemical reactions are described as follows. Anodic reaction takes place in Cu exposed potion. Cu Cu e - (1) At cathodic portion, reduction of O takes place. 1299

4 O 2 + 2H 2O + 4e - 4OH - (2) The OH - ions react with Cu 2+ ions. 2Cu OH - 2Cu(OH) 2 (3) Cu hydroxide is oxidized in air. 2Cu(OH) 2 2CuO + 2H 2O (4) In these chemical reactions, H 2O is not consumed, and with the presence of oxygen, the galvanic Cu corrosion can proceed indefinitely. The Cu oxide, the end product, corresponds to hemispheric product (see in Fig. 4d and Fig. a). Furthermore, once Cu oxide is formed on the surface, the difference in the availability of oxygen between wire surface and bottom of the corrosion portion leads to formation of oxygen concentration cells. The more oxygenated portion is cathodic while the less oxygenated portion is anodic. Since cathode reactions involve consumption of oxygen in equation (2), the more oxygenated portion behaves cathodic and the less oxygenated portion behaves anodic. The corrosion caused by oxygen concentration cells is known as differential aeration cell. The void under cracked Pd layer portion after unmolded HTS test is formed in such a process. For wire, localized corrosion has occurred. On the other hand, in case of bare Cu wire, cathodic and anodic reactions take place all over the wire surface, but not simultaneously at the same place, i.e. the both reactions exchange places constantly. Therefore, corrosion layer was formed all over the wire surface (see in Fig. 4f and Fig. b). D. Failure mechanism after HTS test in molded The failure mechanism of wire after HTS test in molded is slightly different from un-molded HTS test. Fig. 7 shows TDS results for mold compound. Outgassing from the mold compound from the room temperature to 2deg C in a vacuum was measured by mass spectrometry. H 2O (m/z = 18) outgassing moderately increased from room temperature and it was increasing further from 18 to 2deg C. O 2 (m/z = 32) outgassing was increased conspicuously above 1deg C. These indicate H 2O and O 2 are present around bonding wire under high temperature (>1deg C), requirements for the progress of the corrosion reaction. Therefore, galvanic corrosion is easy to occur at a gap of Pd layer (i.e. Cu exposed portion) just like the un-molded HTS test in air. Since partial O 2 pressure in case of molded is lower than that of un-molded in air environment, cathode reaction (2) is slow. Therefore, corrosion reaction is also very slow. On the other hand, SO x outgassing from mold compound has an influence on corrosion of Cu at more than 17deg C. Fig. 8 shows SO 4 concentration in absorber collected outgassing from mold compound during 1hr at 1, 17 and 2deg C measured by Ion Chromatography (IC). For this method, among outgassing from mold compound, the SO x dissolved in water were analyzed as SO 4. As temperature was elevating, concentration of SO 4 was increasing. Especially, Spectral intensity per unit mass SO4 concentration /ppm 6.E-9 4.E-9 2.E-9 6E-1 4E-1 2E-1.E Temperature /deg C Fig 7. TDS analysis for mold compound. Blue line is H 2O (m/z =18), black line is O 2 (m/z = 32). 1 1 H 2O concentration of SO 4 at 2deg C was five times higher than that at 1deg C. High concentration SO 4 in an electrolyte leads to decrease of ph value (increase of H + concentration) for this system. At neutral or high ph value, cathodic reaction (2) occurs since the concentration of H + is low. However, as ph value decreases, another cathodic reaction becomes predominant instead of (2). 2H + + 2e - H 2 () Therefore, increase in acidity under higher temperature at corrosion portion promotes even higher corrosion rates, and the process becomes self-sustaining. Hemispheric product was not observed after HTS test with molded. Since partial O 2 pressure in molded is lower than that of un-molded in air, Cu was diffused in mold compound in the state of Cu 2+ during HTS test. E. Improved mechanism for new- after HTS test New- wire also has gap of Pd layer (i.e. Cu exposed portion) as in the conventional wire. But, 2nd pull strength didn t decrease (Fig. 1c) and corrosion was noticeably suppressed after HTS test after 1hr at 2deg C (Fig. 3b). TEM analysis at Cu exposed portion revealed that void due to corrosion was much smaller than that of conventional wire, and it was revealed that added element X was concentrated at the surface of Cu matrix. Furthermore, O was detected at the same portion. Mechanism for improved corrosion suppression for new- wire is described as follow. In the early stage of HTS test, corrosion due to formation of galvanic cell occurs, but corrosion does not progress much more since added element X forms X-enriched layer with O (i.e. passivation layer) at the surface of Cu matrix. Under HTS test condition, Cl - O Temperature /deg. C Fig 8. SO 4 concentration in absorber collected as outgassing from mold compound. 13

5 Shear strength /gf (a) 1deg C (b) 17deg C (c) 2deg C Shear strength /gf dissolved from mold compound which damages passivation layer is low level. Therefore, the passivation layer containing added element X is stabilized in long-term reliability test. New- wire was improved thermal bond reliability. However, failure due to the galvanic cell formation for wire was greatly affected by outgassing from mold compound under high temperature (>17deg. C). In such high temperature condition, it is necessary to take into consideration whether acceleration test reflected the actual usage environment. F. High reliability wire product In this paper, we focused on corrosion at 2nd bond portion of after HTS test. But bonding wire requires total performance including 1st bond reliability, FAB formability and bondability. wire had good performance at 2nd bond portion after HTS test, but 1st bond reliability of bare Cu after HTS test was poor. Fig. 9 shows ball shear strength of decapsulated three bonded wires (new-, and bare Cu wire) after (a) 1, (b) 17 and (c) 2deg C. Bond strength of bare Cu wire decreased after 1hr at 1deg C. On the other hand, and new- wire had good bond strength after 1hr at 17deg C. Fig. 1 shows cross-sectional SEM image of 1st bond portion Fig 9. 1st ball shear test after HTS test for (a) 1deg C, (b) 17deg C and (c) 2deg C Black solid line is conventional, blue solid line is new- and black dashed line is bare Cu wire. New- Fig 1. Cross-sectional SEM images at 1st bond portion after hr at 2deg C. μm Shear strength /gf New- after hr at 2deg C. For bare Cu wire, continuous crack was observed at the bond interface. S was detected at cracked portion. This crack was a significant factor of decrease in bond strength. and new- wire kept good bond interface. Therefore, new- wire has better total bond reliability (1st and 2nd bond) under high temperature. New- wire introduced in this paper is named as EX1R. The high bond reliability wire, EX1R, has the characteristics as follows. 1. HTS test and HAST reliability of EX1R are higher than that of conventional and bare Cu wire. 2. Electrical resistivity of EX1R is 6% higher than conventional wire, but almost same as that of high reliability 2N Au alloy (Pd doped type) wire. 3. To obtain same FAB diameter, EFO time of EX1R is slightly shorter than that of conventional wire under same EFO condition. 4. Bondability at 1st and 2nd bonds is similar with conventional wire. But looping control is slightly different. Loop height of EX1R tends to be higher than that of conventional wire. Details of these results will be published in the future. IV. CONCLUSION 1. Bond reliability of conventional wire was better than that of bare Cu wire after aging at 1deg C. 2. Under high temperature, outgassing from mold compound increased. Some outgassing element influenced 2nd bond reliability for wire. 3. New- wire had better thermal reliability than conventional wire after aging at a higher temperature like 2deg C. 4. We developed high reliability wire named as EX1R. EX1R is suitable for on vehicle devices required for severely high temperature. REFERENCES [1] T. Uno, S. Terashima, and T. Yamada, Surface-enhanced copper bonding wire for LSI, Electronic Components and Technology Conference (ECTC), 29, pp

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