Reliability of Sn 8 mass%zn 3 mass%bi Lead-Free Solder and Zn Behavior

Transcription

1 Materials Transactions, Vol. 46, No. 11 (2005) pp to 2328 Special Issue on Lead-Free Soldering in Electronics III #2005 The Japan Institute of Metals Reliability of Sn 8 mass%zn 3 mass%bi Lead-Free Solder and Zn Behavior Sun-Yun Cho 1, Young-Woo Lee 1, Kyoo-Seok Kim 1, Young-Jun Moon 2, Ji-Won. Lee 2, Hyun-Joo Han 2, Mi-Jin Kim 2 and Jae-Pil Jung 1 1 Department of Materials Science and Engineering, University of Seoul, Seoul , Korea 2 Mechatronics Center, Samsung Electronics, Suwon , Korea In order to evaluate the reliability of Sn 8 mass%zn 3 mass%bi (hereafter, Sn 8Zn 3Bi) solder, thermal shock tests (temperature range: from 233 to 353 K) and high temperature humidity tests (test condition: 353 K, 85% relative humidity) were conducted. The PCB (Printed Circuit Board) pads were finished by OSP (Organic Solderability Preservative), Sn and Ni/Au. The electric part for test was QFP (Quad Flat Package), and the lead was plated by Sn 3 mass%bi. The joint strength was estimated by pull testing, and Zn-phase behavior in solder joint was examined. The pull strengths of Sn 8Zn 3Bi joints in the as-soldered state were measured to be about 16N, irrespective of the PCB pad coatings. After thermal shock testing (hereafter, TS test) up to 1000 cycles, the pull strengths decreased to around 14N. Meanwhile, after high temperature humidity testing (hereafter, HTH test) up to 1000 h, the pull strengths deceased to around 6N, irrespective of the PCB pad coatings. The reason for the drastic decrease of pull strength after the HTH test was proven to be a crack that was observed along the zinc oxide-phase. The Zn-phase in the Sn 8Zn 3Bi solder changed from a separate acicular shape to a lined-up branched arrangement after HTH tests of 1000 h. (Received May 23, 2005; Accepted October 14, 2005; Published November 15, 2005) Keywords: Sn 8 mass%zinc 3 mass%bismuth, lead free solder, zinc-phase, intermetallic compound, joint strength, high temperature humidity test 1. Introduction The development of lead-free solders has been an important issue for a decade in the electronics packaging industry. Among the many different lead-free solders, the Sn Ag Cu alloy has become the most popular because of its good reliability and sufficient supply. 1) The high melting point and expensive price; however, of Sn Ag Cu are the well known shortcomings of this alloy which can be improved. As one of the alternatives to the Sn Ag Cu alloy, Sn Zn Bi alloy has been investigated by many researchers. Sn 8 mass%zn 3 mass%bi solder has been regarded as a good candidate because it has a lower solidus melting point (e.g. 462 K), good mechanical reliability and is inexpensive. 2) The Zn-phase in a Sn Zn based alloy changed from acicular to spherical after a thermal shock test (hereafter, TS test) between 233 and 398 K for 1000 cycles; 3) other research confirms this change in shape of the Zn-phase. 4) In the case of high temperature aging, Miyazawa et al. reported that the Zn-phase changed from acicular to spherical after holding at 373 K for 1000 h. 5) From the previous results, we can say that Zn-phases in Sn Zn based solder change from acicular to spherical shape after thermal treatments, namely TS and aging. Suzuki et al., 6) and Amita et al. 7) reported that the bond pull strength of Sn 8 mass%zn 3 mass%bi (hereafter, Sn 8Zn 3Bi) was higher than that of Sn 37Pb after TS testing (temperature range: 233/398 K) and high temperature aging at 398 K. Similarly, in the case of QFP leads plated with Sn 3 mass%bi (hereafter Sn 3Bi), Iwanishi et al. 8) demonstrated that Sn Zn Bi had higher strength than that of Sn 37Pb after aging at 398 K for 2000 h. From these results, the mechanical reliability of the Sn Zn Bi alloy is higher than that of Sn 37Pb in the TS test and aging. The high temperaturehumidity test (hereafter, HTH test) reliability of the Sn Zn Bi alloy; however, is not well known. Specifically, the effect of the Zn-phase change after HTH testing has not been found in the literature. In this study, the authors investigated Znphase change and bond strength after HTH test in order to estimate the reliability of the Sn 8Zn 3Bi solder alloy. 2. Experimental 2.1 Experimental materials and soldering process The electronic part for this study was a QFP (Quad Flat Package) which has 208 pins with a 0.5 mm pitch. The QFP lead was copper and the surface was coated with Sn 3Bi with 7.6 mm thickness. PCB (Print Circuit Board) with 1.6 mm thickness was used for the substrate. The metal pads on the PCB were coated with three different materials: OSP (Organic Solderability Preservative), Sn, and Ni/Au from bottom to top. Thickness of OSP and Sn were 0.35 and 0.45 mm, respectively. In the case of Ni/Au plated PCB, the thickness of Ni and Au were 3 and 0.03 mm, respectively. Sn 8Zn 3Bi paste with RMA flux was used as a solder. The Sn 8Zn 3Bi paste was printed on the pads using a metal mask which has a 0.15 mm thickness. The mask hole size was 92% of pad size. The PCB mounted with the QFP was reflow soldered in air using a commercial grade reflow soldering furnace, and the peak temperature for soldering was in the range of K. 2.2 Evaluation method In order to study the reliability of the Sn 8Zn 3Bi reflow joints, the bond strength after TS and HTH tests were evaluated. TS specimens were cycled between 233 and 358 K for 1000 cycles. The isothermal hold time at the minimum and maximum temperatures was 30 min. The bond strength and microstructure of the solder joint were determined every 200 cycles. HTH test conditions were 358 K/85RH (Relative Humidity) for 1000 h. Bond strength and microstructure were examined after 200, 500, and 1000 h. Pull strength of the specimen was evaluated using a Dage-

2 Reliability of Sn 8 mass%zn 3 mass%bi Lead-Free Solder and Zn Behavior µ m/s Fig. 1 Schematic of pull test for the QFP lead. 2000, and Fig. 1 illustrates the pull method. The QFP sample was fixed with a jig and the angle between PCB and pull hook was 45, the hook speed was 200 mm/s. The number of pull tests was 40 (i.e. 10 times on each side) for every specimen. Two QFP samples were tested for each soldering condition; hence, a total 80 leads were pulled in one soldering condition. FESEM (Field Effect Scanning Electron Microscopy) was used for observing the microstructure. The chemical analysis for the solder joints was characterized by EDS (Energy Dispersive Spectrometer) analysis. Intermetallic compounds (hereafter, IMCs) and Zn-phase were identified using EDS. 3. Experimental Result 3.1 Bond strength Figure 2 shows the pull strengths of the leads after TS (233/358 K) and HTH (358 K/85RH) testing. The strengths were derived from an average value of the 80 leads in each condition. A reference value was objectively assigned to the Sn 8Zn 3Bi strength. The reference is an indication of the pull strength of a QFP lead bonded by Sn 37Pb solder on OSP-coated PCB. The lead for the reference was coated by Sn 15 mass%pb. From Fig. 2, bond strength in the as-soldered state was around 16N for Sn 8Zn 3Bi which is 60% higher than that of Sn 37Pb. In the case of Ni/Au-PCB, bond strength after TS gives nearly constant values of about 16N [Fig. 2]. Meanwhile, after 1000 cycles, OSP-PCB bond strength was 14N, which was 12% lower than the as-soldered state, and Sn-PCB was 13N, 18.75% lower than the as-soldered state. Compared to the reference values; however, these values were 4, 3, and 6N higher in each. From these results it was proven the joint strengths of Sn 8Zn 3Bi were higher than those of Sn 37Pb, which agrees with previous results. 6 8) The bond pull strength after HTH testing is given in Fig. 2. The reference values were nearly constant with aging time; however, the strength of Sn 8Zn 3Bi joints decreased drastically. After aging 1000 h the OSP-, Sn-, and Ni/Au-PCB bond pull strength was 5.6, 6.4 and 7.2N, Fig. 2 Pull strength of the QFP leads after TS (thermal shock) test and HTH (high temperature humidity) test, Pull strength after TS test Pull strength after HTH test. respectively, a decrease of 63.76, and 54.38% in each. The pull strength of the Sn 8Zn 3Bi after 1000 h was lower than reference values by about N. The strength decrease was caused by Zn-Phase growth and resultant cracking, and this will be discussed next section. 3.2 Intermetallic compounds Figure 3 shows IMCs observed between pads on PCB/ solder and lead/solder interface after TS of 1000 cycles. Generally, the Cu Sn IMC of Cu 6 Sn 5 is produced between many Sn-based solders and Cu pad. On the other hand, Ni Sn IMC of Ni 3 Sn 4 is formed between most Sn-based solders and Ni-pad. However, in the case of Sn Zn based solders Zn reacts with the pad faster than Sn. Thus, Cu 5 Zn 8 is produced between Cu-pad and solder, and Ni 5 Zn 21 between Ni-pad and solder. 9 11) IMCs produced between various PCBs and Cu-lead in this study were analyzed with EDS. From the analyzed results, the ratio of Cu to Zn was 36:64. Considering the report of Abe 12) and Ikeda 13) and the Cu Zn phase diagram, this IMC is believed to be Cu 5 Zn 8. Meanwhile, in the case of Ni/Auplated PCB, the IMC along the interface was believed to be Ni 5 Zn 21 with reference to the Ni Zn phase diagram and the results of Kim 14) et al. Figure 4 shows IMCs along the PCB/solder and lead/

3 2324 S.-Y. Cho et al. (d) Fig. 3 Intermetallic compounds (IMCs) after TS test (233/358 K, 1000 cycles) on OSP-coated pad, on Sn-plated pad, on Ni/Auplated pad and along Cu-lead (d). (d) Fig. 4 IMCs after HTH test (358 K/85RH, 1000 h) on OSP-coated pad, on Sn-plated pad, on Ni/Au-plated pad and along Cu-lead (d).

4 Reliability of Sn 8 mass%zn 3 mass%bi Lead-Free Solder and Zn Behavior 2325 Fig. 5 Microstructure change in Sn 8Zn 3Bi solder, as soldered, after TS test (233/358 K, 1000 cycles), after HTH test (358 K/ 85RH, 1000 h). Point a Point b Fig. 6 EDS-analyzed points of Zn-phase after HTH test. solder interface for Sn 8Zn 3Bi after HTH testing at 358 K/ 85RH for 1000 h. The IMCs between solder and various PCBs and Cu-lead were similar to those of the TS test results (Fig. 3). The interfaces between solder/pads and leads were free of cracks after TS and THT testing. 3.3 Microstructure of Sn 8Zn 3Bi and cracks Figure 5 gives the microstructure of the Sn 8Zn 3Bi solder after HTH and TS testing. Cracks were not observed at the interface between pad and solder after TS test of 1000

5 2326 S.-Y. Cho et al. cycles and HTH test of 1000 h. In Hisazato s study; 3) however, cracking was observed after TS of 1000 cycles between 233 and 398 K. The difference in results from Hisazato seems to come from a temperature difference, Hisazato had more severe temperature conditions resulting in cracking. Generally, the Zn-phase in Sn Zn Bi solders appears acicular in shape, and through thermal treatment this will be changed into a spherical shape. 3 5) Figure 5 shows the assoldered microstructure, the acicular type Zn-phase was observed with a maximum size of around 20 mm. Figure 5 shows the microstructure after 1000 TS cycles, most of the acicular Zn-phase has changed into a nearly round type. The maximum size of Zn-phase was almost 10 mm in the major diameter in and 5 mm in the minor diameter. The Zn-phase after HTH testing was quite different from the TS result, as shown in Fig. 5. Namely, the Zn-phase does not transform to a spherical shape after HTH test, but it lines up in a branch-like manner. This result is unique compared to the shape from the TS and high temperature aging tests where the Zn-phases appear rounded. In the case of the TS and aging tests, only temperature is considered and humidity excluded. However, in HTH testing, humidity is included in addition to high temperature storage. This additional factor is believed to affect the Zn-phase shape in HTH test microstructure. Since the bond pull strength decreases drastically after HTH test, the branched type of Zn-phase is believed to have a serious harmful effect. Table 1 Results of composition analysis for the Zn-phase after HTH test for 1000 h. Elmt Point a (inside fillet) Element At Elmt Point b (on IMC layer) Element At O O Zn Zn Sn Sn total Total Analysis of Zn-phase and crack To further understand the branching phenomenon of the Zn-phase a point in the fillet and close to the lead ( a and b in Fig. 6), were analyzed by EDS. Chemical compositions of a and b are shown in Table 1. Namely, point a is composed of 44.35% O and 47.40% Zn, and point b is 41.45% O and 40.71% Zn. Since the ratio of Zn to O is almost 1 to 1, the Zn-phase is believed to be ZnO. Some Sn in the analysis may come from the solder matrix, or be included in the Zn-phase. The presence of ZnO is reported in a study on the effects of Bi and Pb on oxidation in humidity for low temperature lead-free solder systems on the Cu pads of the PWBs with Sn and Sn 10Pb plated chip. 16) Ni/Au-, Sn- and OSP-plated Cu-pads using Sn 3Bi plated Cu-lead of QFP were used in this work. Figure 7 shows the growing procedure of the Zn-phase during the HTH test. In the as-soldered state, the Zn-phase exists as an acicular type, as previously mentioned. After 200 h of HTH testing, the acicular shapes have change into Fig. 7 Growth of Zn phase under HTH test, after 200 h, after 500 h, after 1000 h.

6 Reliability of Sn 8 mass%zn 3 mass%bi Lead-Free Solder and Zn Behavior 2327 (d) Fig. 8 Schematic diagram of ZnO growth during HTH test, beginning of Zn-oxidation, growth of ZnO phase, supply of H 2 O and O 2, (d) crack after 1000 h. round types [Fig. 7], and they cluster together in a branched arrangement [Fig. 7]. Finally, after 1000 h, the Zn-phase has lined up in a branched formation from the solder fillet to the lead [Fig. 7]. In order to understand the growth of the Zn-phase, simplified illustration is given in Fig. 8. First, the Zn-phase at the solder surface can be oxidized by reaction with H 2 O and O 2. Gibbs free energy for Zn-oxidation is lower than that of SnO 2 and Cu 2 O. 15) That means, at 358 K for the HTH test, the Gibbs free energy for ZnO was calculated as 74:6 kcal, 33:5 kcal for Cu 2 O and 61:2 kcal for SnO 2. This could contribute to the production of ZnO thermodynamically. Unfortunately, however, the reason why ZnO-phase grows and lines up in a branched arrangement was not been clarified yet. Meanwhile, when Zn is oxidized, the lattice unit volume increases by 50% from 3: to 4: m 3. In theory this lattice volume expansion originates the crack. A number of cracks were observed along the ZnO-phase (Fig. 8). This phenomenon was also observed and discussed in the study on the effects of Bi and Pb on oxidation in humidity for low temperature lead-free solder systems on the Cu pads of the PWBs with Sn and Sn 10Pb plated chip. 16) H 2 O could penetrate into and diffuse along these cracks resulting in further Zn oxidation. Undoubtedly, these cracks deteriorate pull strength of Sn 8Zn 3Bi after HTH testing. 4. Conclusion In order to evaluate the reliability of Sn 8Zn 3Bi solder for HTH and TS testing, Sn 3Bi coated QFP was reflow soldered with Sn-, OSP- and Ni/Au-finished PCB. The soldered leads were pull tested after HTH and TS tests. The results can be summarized as follows; In the case of TS, pull strength of the lead soldered with Sn 8Zn 3Bi was 40% higher than that of Sn 37Pb. However, pull strength of the Sn 8Zn 3Bi solder after HTH test became 36% lower than that of Sn 37Pb. Under TS testing, the Zn-phase in the solder changed from acicular to round type. However, during HTH test, the Znphase grew and lined up in a branch-like manner from the solder fillet to the lead. A number of cracks were observed along the branched Zn-phase, which was proven to be oxidized. The crack formation should be a main reason for the decrease in the pull strength after HTH testing. IMCs after HTH and TS tests were Cu 5 Zn 8 for the OSP and Snfinished PCB, and Ni 5 Zn 21 on the Ni/Au-finished PCB. Acknowledgments The authors would like to thank Samsung Electronics & KOSEF (R ) in Korea for financially support, and also to acknowledge the Ref. 16) provided by the reviewer. REFERENCES 1) K. Suganuma: Curr. Opin. Solids State Mater. Sci. 5 (2001) ) R. A. Islam, B. Y. Wu, M. O. Alam, Y. C. Chan and W. Jillek: J. Alloys Compd. 392 (2005) ) Y. Hisazato, M. Kato, K. Kawakami and Y. Saito: 9th Symposium on

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