THE EFFECTS OF INTERNAL STRESSRS IN BGA Ni LAYER ON THE STRENGTH OF Sn/Ag/Cu SOLDER JOINT

Transcription

1 THE EFFECTS OF INTERNAL STRESSRS IN BGA Ni LAYER ON THE STRENGTH OF Sn/Ag/Cu SOLDER JOINT C.H. Chien 1, * C.J. Tseng 1,2 T.P. Chen 1,3 1 Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan chchien@mail.nsysu.edu.tw 2 BGA R&D Group, ASE GROUP, Kaohsiung 80424, Taiwan CJ_Tseng@aseglobal.com 3 Department of Electrical Engineering Fortune Institute of Technology, Kaohsiung 84241, Taiwan taiping@center.fjtc.edu.tw ABSTRACT Due to the consideration of environmental protection policy, all electronic products are requested to be lead free. In package field, solder ball is also requested to be lead free and currently the most popular type of solder ball is the Sn/Ag/Cu solder [1]. Sn/Ag/Cu solder has higher melting point and weaker wettability [2-3] during IR re-flow profile than eutectic solder ( Sn/Pb ), therefore the strength of solder joint of Sn/Ag/Cu solder has risk to be worse than Sn/Pb Solder. In this study, the effects of internal stresses in BGA Ni layer on the strength of Sn/Ag/Cu solder joint are investigated. The drop test and peel off test are adopted in order to test the strength according to the standard of JEDEC. The peel off test and drop test results show that the higher tensile stress and compressive internal stresses in the Ni layer have worse effects on the joint strength than lower tensile internal stresses can affect. According to the failure modes analysis, the failure mode is at the intermetallic compound between BGA Ni layer and solder ball.. Introduction With portable and handheld product demand in market, such as cellular phone, PDAs, and MP3, the lighter, thinner and smaller size products become more and more popular. For achieving the demand In industrial field, the line pitch in chip on wafer enhances from 90nm to 65nm, and the package size in assembly is reduced to 5mm*5mm and ball opening size is reduced from 300µm to 275µm. Because of reducing the ball opening size, the solder joint becomes weaker than bigger openings. In addition the portable product has higher opportunity to drop on ground during using in life that will increase the risk of product failed due to solder joint crack. At the same time, the governments in the world make policy to exhibit the element lead using in electrical products due to environmental and human health concerns [4]. The electrical industrial fileds research and develop the new materials to replace the leaded materials and products in order to meet the policy requirement. Eutectic solder ball consists of 63%Tin and 37%Lead and has the lower melting temperature, excellent mechanical properties and wettability. Therefore the eutectic balls were used in electrical products for past many years. In these years, the research and development units develop many kinds of lead free solder, such as Sn/Ag, Sn/Cu, Sn/Ag/Cu, Sn/Bi, and Sn/Zn etc [5-6]. But no one can compatible to eutectic solder. In all kinds of lead-free solder, Sn/Ag/Cu solder is chosen as the one to replace eutectic solder in assembly, SMT process currently. The Sn/Ag/Cu solder has the higher melting temperature 217 than eutectic solder 183 and weaker wettability so that the Sn/Ag/Cu solder is easily to cause poor solder joint in assembly field. For enhancing the solder joint strength, many papers studied the effects of IR-reflow profile, flux type, solder ball type during ball mound process in assembly field [7-8] and the effects of different metal finish, such as OSP, ENIG, Electroless Tin on ball pad surface [9-11]. The metho to detect the solder joint quality in industrial field are peel off test and JEDEC Standard Drop Test [12]. According to the Anand s study [13], the residual stresses in Ni layer maybe affect the solder joint by comparing electroless Ni and electrolytic Ni. The report aims at the effects of internal stresses in BGA electrolytic Ni layer. In BGA manufacturing, electrolytic Ni is the major metal finish on ball pad surface. The Watt s bath is the solution that consists of NiSO 4, NiCl 2 and H 3 BO 3. In Watt s bath solution, Ni concentration, NiCl 2 concentration, PH Value, impurity, current density and additive will affect the internal stresses in Ni layer. Amongst these factors, current density and additive are the two key factors to affect the values of the internal stress. Higher Current density causes the higher tensile stress and additive of

2 reducing stress converts the tensile stress to compressive stress. This study used the Watt s bath as the solution and controled the current density and additive of reducing stress to obtain the 5 levels of internal stress in Ni layer. Peel off test and Drop test were adopted in order to obtain the strength of solder joint. Finally, the dye penetration method was used to analyze the failure mode. Experimental Procedures The experimental procedure included experiment design, BGA substrate manufacturing, PCB test board manufacturing, assembly, SMT process, peel off test, Drop test, and failure mode analysis by dye penetration. Detailed information is described as below. Experiment design Following the JEDEC Standard definition, the 14mm by14mm package with 275 ± 40 µm ball pad opening were chosen for the specimen used in the experiment. Sn95.5%/Ag4.0%/Cu0.5 lead free solder was used in assembly ball mount and SMT process. There were 15 packages on PCB test board for drop test and the solder joint strength was analyzed through Weibull distribution analysis. BGA substrate manufacturing The 14mm * 14mm package size and 409 lead count device were chosen as the specimen. The thickness of the substrate is 0.26mm with 0.5~1.5µm Au thickness, 5~15µm Ni thickness and 275 ± 40 µm ball opening as shown in table 1. In order to obtain the different levels of internal stress in Ni layer, the Watt s Bath was used as the solution, and a spiral contractometer was used to measure the values of internal stress. The principle of spiral contractometer is to calculate the deformation of spiral after plating and transfer the values to internal stress. For the Watt s Bath used, the concentrations of Ni, NiCl 2, and H 3 BO 3 were 70g/L, 35g/L, and 30g/L, respectively, and the value of PH was 3.0 with 55 temperature. The adjustment of the current density and additive saccharin were used to control the levels of internal stress in Ni layer. Table 2 shows the existed internal stresses and the corresponding current densities. The tensile stresses of 188 MPa, 348 MPa and 639 MPa were obtained by adjusting the current density to 0.5 A/dm 2, 1.0 A/dm 2, 3.0 A/dm 2, respectively, and without using the additive saccharin. The compressive stresses of 49MPa and 235 MPa were obtained by adjusting the current density to 0.5 A/dm 2 and 3.0 A/dm 2, respectively, and by using the additive saccharin. Table 1. BGA Substrate specifications and Pre-treatment parameters in Ni solution Table 2. DOE Parameters in Ni solution

3 PCB test board manufacturing Following the JEDEC Standard definition, the size of the PCB test board used is 132 mm by 77 mm by 1 mm with 8 layers of PCB Board were used. The Pad design was chosen as the NSMD (no soldermask define) and OSP(Organic Solderability Preservation) was used as the metal finish. Assembly and SMT process Assembly process included die attach, wire Bond, molding, ball mount and singulation. The Sn95.5%/Ag4.0%/Cu0.5% lead free solder and 260 peak temperature of IR Re-flow profile were used to form the IMC on BGA ball pad surface. The packages on PCB test board were assembled with Sn95.5%/Ag4.0%/Cu0.5% paste as shown in Figure 1. Figure 1. PCB test board after SMT Process Peel off test The packages were separated from PCB test board by manual and the failure mode corresponding to each package was inspected by microscope. There were four possible failure modes which were failed due to PCB ball pad be peeled off, failed at PCB-IMC layer, failed at package-imc layer, and failed at solder. The failure rate was calculated according to failed ball count number. Drop test The peak acceleration of drop test is % with 0.5 ms 30% pulse duration and the failure criterion is 1000ohm detected. According to the definition of JEDEC Standard, the result is passed when there is no failure occurs after 30 drops. The structure of drop test equipment is shown in Figure 2. The PCB test board with face down packages was put on base plate. Then the drop table was pull to the setting height and was released. It is noted that at the moment of the drop table striking at strike surface, the JEDEC Standard requirements described above need to be achieved. Figure 2. The structure of drop test equipment

4 Failure modes analysis The failure modes were checked by using the dye penetration method. The dye penetrated into the crack area and the package was peeled off from PCB test board by pull off machine. The failure mode was inspected by microscope and the failure rate was calculate based on failed ball count number. Four possible failure modes may exist as shown in figure 3, mode A1 is failed due to PCB ball pad be peeled off, mode A2 is failed at PCB-IMC layer, mode B2 is failed at package-imc layer, and mode A4 is failed at Solder. If the failure rate corresponding to the fails at package-imc layer is higher, then the solder joint strength on package is worse. Figure 3. Failure modes definition Results and Discussions Peel off test results Table 3 shows the peel off test results. For each internal stress level, there were 8 packages be peeled off. Only the ball which was failed at package-imc layer was counted. There were 409 ball counts for each package. The failure rate, i.e., the failed ball count percentage, was obtained by using the ball counts which were failed at package-imc layer divided by 409. If failure rate is higher, then the solder joint strength is worse. From the table 3, it can be seen that the failure rates corresponding to internal tensile stresses are lower than the failure rates corresponding to internal compressive stresses. Also, the higher internal tensile stress caused the higher failure rate. So, one can conclude that the lower internal stress in Ni layer can cause the stronger solder joint strength. Table 3. Peel off test results Drop test results Table 4 shows the drop test results and the corresponding Weibull analysis is shown in Figure 4. For each internal stress level, there were 2 PCB test boards be dropped and there were 15 packages be mounted on each PCB test board. It can be

5 seen that only the 2 PCB test boards correspond to 348 MPa internal tensile stress (i.e., Tension_Mid) and the PCB test board 1 corresponds to 188 MPa internal tensile stress(i.e., Tension_Low) can pass the 30 drops criteria, all the rest PCB test boards cannot. It is noted that even though the PCB test board 2 corresponds to 188 MPa internal tensile stress cannot pass the 30 drops criteria, but the first failed package occurred at the 24 th drop, unlike the PCB test boards correspond to internal compressive stresses and high internal tensile stress, the first failed package had been occurred at the 1 st and the 3 rd drop, respectively. In order to check the capability of the packages correspond to 348 MPa internal tensile stress, the drop test was continued to 100 drops, and the results showed that there were 53% of the packages can pass 50 drops, and 26% of the packages can pass the 100 drops. From the weibull distribution analysis shown in figure 4, it can be seen that the characteristic life( ) of 348 MPa internal tensile stress is 60 drops, but characteristic life( ) of the higher internal tensile stress 639 MPa is only 22 drops, and the characteristic lives of compressive stress 49 MPa and 235 MPa are 10 and 6 drops, respectively, only. Again, one can conclude that the lower internal stress in Ni layer can cause the stronger solder joint strength. Table 4. Drop test results Figure 4. Weibull distribution analysis

6 Failure mode analysis results The failure modes of the failed packages from the drop test were examined by using the dye penetration method. The analysis results are shown in Figure 5. The failure modes in the Figure 5 were defined in the previous section already and the mode B2 corresponds to the ball failed at package-imc layer, It is noted that only 48% and 36% of the failed balls of the 188 MPa and 348 Mpa internal tensile stress cases, respectively, are the mode B2 failures. But the percentages of the mode B2 failed balls of the 639 MPa, 49 MPa,and 235 MPa cases are as high as 93%, 74%, and 93%, respectively. Once again, one can conclude that the lower internal stress in Ni layer can cause the stronger solder joint strength. Figure 5. Failure mode analysis results Conclusions In this study, the effects of internal stresses in BGA Ni layer on the strength of Sn/Ag/Cu solder joint were investigated. The peel off test, drop test, and failure mode analysis were adopted in testing the strength of solder joint according to the standard of JEDEC. All the test results have shown that the internal compressive stresses and the higher internal tensile stresses in the Ni layer have worse effects on the joint strength than lower internal tensile stresses can affect. Therefore, the lower internal stresses in Ni layer can cause the stronger Sn/Ag/Cu solder joint strength. References 1. Puttlitz, K. J. and Stalter, K. A., Handbook of Lead-Free Solder Technology for Microelectronic Assemblies. pp , Selvaduray, A. G., Lead-Free Solders in Microelectronics, Materials Science and Engineering R, Vol.27, pp , Fan, J. W., Kuo, C. T., and Yip, M. C., Mechanical Characterization of Board-Level 63Sn37Pb and Lead-free Solder Sphere Attachment on Cu-Pad/Ni/Au Surface Finish Substrate, 2003 Electronics Packaging Technology Conference, Miric, A. Z. and Grusd, A., Lead-Free Alloys, Soldering and Surface Mount Technology, Vol.10, pp.19-25, McCormack, M., Jin, S., Kammlott, G. W., and Chen, H. S., New Pb-Free Solder Alloy with Surperior Mechanical Properties, Vol.63, pp.15-17, Puttlitz, K. J. and Stalter, K. A., Handbook of Lead-Free Solder Technology for Microelectronic Assembly, pp , 2004

7 7. Chong, D. Y. R., Toh, H. J., Lim, B. K., and Low, P. T. H., Drop Reliability Performance Assessment for PCB Assemblies of Chip Scale Packages (CSP), 2005 Electronics Packaging Technology Conference, Lee, M., Hwagn, Y., Pecht, M., Park, J., and Liu, W., Study of Intermetallic Growth on PWBs Soldered with Sn3.0Ag0.5Cu Solder, 2004 Electronics Packaging Technology Conference, Amagai, M., Toyoda, Y., Ohnishi, T., and Akita, S., High Drop Test Reliability : Lead-free Solder, 2004 Electronic Components and Technology Conference, Ngoh, S. L., Zhou, W., Pang, H. L., Spowageand A. C., and Li, J., Stress Effect on Growth of IMCs at Interfaces Between Sn-Ag-Cu Lead Free Solder and Cu Substrate, 2005 Electronics Packaging Technology Conference, Chong, D. Y. R., Ng, K., Tan, J. Y. N., and Low, P. T. H., Drop Test Reliability Assessment of Leaded and Lead-Free Solder Joints for IC Package, 2004 Electronics Packaging Technology Conference, Board Level Drop Test Method of Components for Handheld Electronic Products, JESD22-B111, JEDEC Solid State Technology Association, Anand, A., Mui, Y. C., Weidler, J., and Diaz, N., Impact of substrate finish on Sn/Ag/Cu alloy solder joint, 2004 Electronics Packaging Technology Conference, 2004.