Interfacial reaction and mechanical evaluation in multi-level assembly joints with ENEPIG under bump metallization via drop and high speed impact test
Introduction
Functional electronic device nowadays has been widely used due to the convenient properties. In flip chip packaging, high input/output density and its small package size provides mechanical attachments and electrical interconnections simultaneously on chips, interposers, and printed circuit boards (PCBs). Because of the rapidly communication and globally international networking, the portable electronic devices are provided and handled everywhere and every time. Hence, a great importance on mechanical performance of handheld devices under environmental heating should be much considered and investigated. Lots of doping elements exhibit the advantage for the slower IMC growth and retards the transformation of phases [1], [2], [3], [4]. Pd additions reveal many benefits in the solder joints [5], [6], [7], [8], [9], [10], [11], [12]. The hindrance of intermetallic compounds (IMCs) growth at interface was attributed to the lower Cu supply by Pd trapping in solder matrix and the grain refinement of Pd doping [8], [9], [12]. In addition, the need-like (Cu, Ni, Pd)6Sn5 growth enhanced the mechanical strength solder/ENEPIG attachment joints in the previous study [11]. However, there are not standardized criterion for PCB industry between the total energy from high speed impact test and the Weibull plot meaning from drop performance. The conventional electroless Ni/immersion Au (ENIG) and a new alternative surface finish, electroless Ni/electroless Pd/immersion Au (ENEPIG) have been used to correlate these two kinds of mechanical testing. Recently, the faster and more convenient methodology of high speed pendulum impact test has been developed to evaluate the reliability of attachment solder joints. After the evaluation of impact strength in the solder joints with various surface finishes attached from energy viewpoints, the real assembly solder joints were proposed and fabricated by Kinsus Interconnect Technology Corporation. The drop testing was proceeded in these assembly solder joints by the JEDEC standard (JESD22-B111) [13], [14], [15], [16]. According to the mechanical reliability of Weibull plot, the statistical probability of various assembly solder joints could be identified and compared. Considering the IMCs layer to be the weakest region within the solder joint, resulting in the brittle failure of the joint, the evaluation of IMCs strength is very important in this study. A complete interfacial study and investigation of cracks initiation and propagation were addressed. This study combined the mechanical reliability of the conventional drop testing via the JEDEC standard (JESD22-B111) with the newly developed high speed pendulum impact testing to systematically evaluate the assembly samples and attachment joints with various thickness of Pd layer deposit. The correlations between the extrinsic performance (mechanical reliability) and the intrinsic properties (systematic Pd-containing solder joints) were established and demonstrated. The major contribution of this research is to set up the mechanical reliability methods for the systematic samples from attachment joints by impact testing and from assembly joints by drop testing. Although the correlation of these vehicles is not yet complete for the timing being, the data base in the preliminary evaluation before the assembly joints fabrication should be considered for the reliability and cost issues in electronic industry.
Section snippets
Experimental procedures
The distinct under bump metallization (UBM) were prepared and provided by Kinsus Interconnect Technology Corporation to act as a surface finish in flip chip technology with solder joints interconnection. Organic solderability preservative (OSP), Electrolytic Ni/Au (E-Ni/Au, GPEG), electroless nickel immersion gold (ENIG) and electroless Ni–P/electroless Pd/immersion Au (ENEPIG) substrates with various thickness of Pd layer were constructed. The thickness of Pd layer in the ENEPIG joints were
High speed impact behavior and phase evaluation of attachment joints
The first study of attachment solder joints started from the interfacial reaction images of SAC305 solder attached with the ENEPIG surface finishes with various Pd deposit as shown in Fig. 1. The intermetallic compound (IMC) phase formation and growth were obviously realized and identified. Only the (Cu, Ni, Pd)6Sn5 phase formed in the SAC305/ENEPIG (Pd 0.06 μm) solder joints after reflow process for 1, 5, 10 min, as shown in Fig. 1(a), (d) and (g). In contrast to the solder joints with thinnest
Conclusion
This study aimed to investigate the correlation between distinct mechanical testing, high speed impact testing and drop testing. The Pd influence was focused in both attachment and assembly solder joints with various surface finish. The results of high speed impact test revealed good impact strength in SAC305/ENEPIG (Pd 0.06) attachment solder joints due to the thinnest IMC growth rate and single phase formation even after the acute reflow process for 5 and 10 min. The total energy of
Acknowledgments
The authors thank Kinsus Interconnect Technology Corp. for material supplies, and National Science Council Taiwan, for financial support under Contract No. NSC102-2622-E-007-002-CC1.
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Bi solder and thin electroless Ni-electroless Pd-immersion Au (ENEPIG)-finished Cu pads. Laser soldering was performed using various laser powers (130, 150, and 170 W) and times (2 and 4 s). Furthermore, an aging test was performed at 110 °C for 2000 h to evaluate the long-term reliability of the soldered joints. The mechanical properties, including the top and cross-sectional views and fracture surfaces, of the soldered joints were analyzed by conducting shear tests after aging. During laser soldering, various intermetallic compounds (IMCs) were formed at the joints depending on the applied energy. The metallization layer and Cu reacted with Sn in the solder after different aging durations, and additional IMCs were formed and grown. After aging for 2000 h, the shear strength decreased, and the interfacial IMC thickness increased. As the aging time increased, the fracture mode changed from an initial ductile fracture to brittle fracture (between the solder and IMCs and/or between IMCs and the Cu pad). The reflow soldered joints exhibited stable shear strength, resulting in ductile fracture until aging for 500 h. However, the shear strength decreased sharply after aging for 1000 and 2000 h, and Bi-segregation was observed after aging for 1000 h, resulting in inferior long-term reliability. After laser soldering at 150 and 170 W for 4 s, the strength of the samples decreased sharply after aging for 1000 and 250 h, respectively, and Bi-segregation was observed after aging for 2000 h. The shear strength of the sample laser soldered at 170 W for 2 s gradually decreased with increasing aging time and maintained a stable shear strength until aging for 2000 h. Therefore, laser soldering at 170 W for 2 s was considered as the optimal condition for long-term reliability.