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ENEG and ENEPIG surface finish for long term solderability
Abstract
The long-term reliability of flip chip and boardlevel solder joint is significantly affected by the properties of the surface finish. Various surface finishes such as Immersion Sn, Immersion Ag, Organic solderability preservatives (OSP), Electrolytic Ni/Au (ENEG), and electroless Ni/immersion Au (ENIG) surface finish have been widely used on based Cu pads and can be compliant with lead free SAC solder alloys. Recently Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) is being offered as an alternative surface finish with high solder joint quality and wire bondability. It is claimed to be more cost effective as an Au layer of lower thickness can be used. To evaluate the performance of solder joints upon ENEG and ENEPIG surface finishes, extended reflow tests at 245°C were conducted. In the case of ENEPIG surface finish, micro-porosities are found to be present on the Au plating surface. PdO oxide forms as a result of Pd exposure, which causes deterioration of solderability as compared with ENEG plating. Following extended reflow, it is found that columnar Cu-Ni-Sn IMCs with small amount of Pd and P-rich Ni layer have formed at the interface of ENEPIG /solder system. In contrast, layer-type Cu-Ni-Sn IMCs formed at the interface of ENEG/solder system in the absence of Pd. While Ni and Ni(P) layers act as barrier to diffusion, in some pad of the ENEPIG samples, the Ni(P) layer is found to be less than 1um after 120 minutes reflow, which is ineffective as a diffusion barrier. By comparison, ENEG surface finish remains effective with 3.5um of Ni barrier layer remaining on Cu after 120minutes reflow. This serves as a good barrier to prevent the diffusion of Cu atoms from the base Cu underneath. Therefore, it has been demonstrated that solder joint on ENEG surface finish is more reliable and suitable to be used for long-term reliability of electronic products.
... To ensure the reliability of the interconnection, surface finishing methods were introduced for the pad of the PCB substrate. One of the most extensively employed methods has been the gold (Au) -based surfaces, which included electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG) [6]. Owing to its high electrical conductivity and chemical resistance, Au is a preferable noble metal in the electronics industry, especially in semiconductor fabrication, which requires high reliability [7], [8]. ...
In this paper, we present the concept of traceback analysis to identify the origins and mechanisms of non-wet defects caused by hydrophobic unknown nanoparticles in the semiconductor packaging process. Traceback analysis consists of five steps, i.e., practical observation of failure mode, detailed physical and chemical analysis, investigation of process, hypothesis establishment, and reproduction of failure with comparison. Firstly, morphology and composition of unknown nanoparticles were analyzed by OM, FE-SEM, EDS, and FT-IR analysis. As a result, spherical unknown nanoparticles with size of ca. 500 nm were consisted of organic imidazole-derived materials, which is similar to the OSP composition. To identify the origin, PCB manufacturing processes were reviewed in step-by-step manner. With the traceback analysis, it was identified that PGMEA cleaning solvent can form spherical emulsion in the DI water during the cleaning process. Specifically, PGMEA and DI water are able to mix in the cleaning bath or recycling pipes. Other possibility is that leftover PGMEA or DI water can remain on the PCB and mixed during the next cleaning process. These PGMEA/DI water mixed solvents result as the formation of emulsions owing to the difference in polarities of hydrophobic PGMEA and hydrophilic DI water and remain in the surface of PCB. Finally, organic nanoparticles were grown in the PGMEA emulsion on Cu pad with the OSP application for the surface finishing purpose. The traceback analysis described in this study, involving five-detailed steps, can be applied not only in semiconductor packaging processes but also to investigate failures across various industries. INDEX TERMS Non-wet, OSP, traceback analysis, emulsion, PCB.
... The thickness of the IMCs is much smaller on the CPB side with the electroplated Ni compared to the side with the electroless Ni(P). Similar results were reported before in [3,4]. The appearance of the two Ni barrier layers after dry heat (2016 h at 150 °C) is significantly different. ...
Surface Acoustic Wave (SAW) RF-filters are essential components for wireless communication. Miniaturization and cost reduction are the main driving forces for the development of these components. Up to now the dominant technology to connect the miniaturized filters are solder bumps. For further miniaturization a technology to fabricate copper pillar bumps (CPB) on SAW RF-filters was developed and the second level reliability of the components was tested. In this paper the physical analysis of CPB interconnects with a high resolution FIB SEM and EDX will be explained in detail. We will also show results of this analysis including the formation of the intermetallic phases and the structure and the degradation of the Ni barrier layers.
... The surface finish was responsible for improved joint reliability. It is germane that surface finish could prevent oxidizing, contamination (Sona and Prabhu, 2014) and act as a diffusion barrier to prevent rapid reaction between the solder and Cu substrate (Rohan et al., 2002;Ho et al., 2013;Pun et al., 2014;Siti Rabiatul Aisha et al., 2015). From the foregoing, it is crucial to seek a reliable surface finish for the Cu substrate applications. ...
Purpose
The surface finish is an essential step in printed circuit boards design because it provides a solderable surface for electronic components. The purpose of this study to investigate the effects of different surface finishes during the soldering and ageing process.
Design/methodology/approach
The solder joints of Sn-4.0Ag-0.5Cu/Cu and Sn-4.0Ag-0.5Cu/electroless nickel/immersion silver (ENImAg) were investigated in terms of intermetallic (IMC) thickness, morphology and shear strength. The microstructure and compositions of solder joints are observed, and analyzed by using scanning electron microscopy (SEM-EDX) and optical microscope (OM).
Findings
Compounds of Cu 6 Sn 5 and (Cu, Ni) 6 Sn 5 IMC were formed in SAC405/Cu and SAC405/ENImAg, respectively, as-reflowed. When the sample was exposed to ageing, new layers of Cu 3 Sn and (Ni, Cu) 3 Sn 5 were observed at the interface. Analogous growth in the thickness of the IMC layer and increased grains size commensurate with ageing time. The results equally revealed an increase in shear strength of SAC405/ENImAg because of the thin layer of IMC and surface finish used compared to SAC405/Cu. Hence, a ductile fracture was observed at the bulk solder. Overall, the ENImAg surface finish showed excellent performance of solder joints than that of bare Cu.
Originality/value
The novel surface finish (ENImAg) has been developed and optimized. This alternative lead-free surface finish solved the challenges in electroless nickel/immersion gold and reduced cost without affecting the performance.
... Besides that, during interfacial reaction between solder and Ni-P, Ni diffuses towards the solder side to form Ni 3 Sn 4 intermetallic (as confirmed by EDX result) with chunky and needle type IMC as reported in previous researcher study [2][3]. Pun et al. and Chen et al. reported on ternary layer formation at 1000 h sample aging test, where original Ni-P was fully transferred into Ni 3 P and self partially transformed into Ni-Sn-P after reflow process but under 60 s reflow during sample preparation, no ternary layer was observed [5,6]. Figure 4 shows samples' shear strength were directly proportional with the Ni-P thickness, but varied at different carrierbump materials combinations. ...
Long-term reliability of board level solder joint is significantly affected by the properties of surface finish. Recently, Electroless Nickel Immersion Gold (ENIG) was offered as alternative surface finish with high solder joint strength to reduce cost. However, the use of electroless Ni-P deposition produces intermetallic compound (IMC) that exists between solder Ni-P alloy boundaries after thermal aging process. In this study, the effect of various Ni-P deposit thickness and different substrate on its failure mode is studied by measuring its shear strength and observing shear morphology after thermal aging. The Ni-P alloy deposit thickness is set to 0.5, 2.5 and 4.5 μm. The substrate used EFTECH 64-Ni, EFTECH 64-Cu and C194-Ni bumps. From the results, the use of EFTECH 64-Cu substrate produce fracture failure at both Ni-P — Substrate and Ni-P — IMC compare to other substrates. Aside that, C194-Ni alloy produces consistence shear strength but unable to produce consistence standard deviation. As a conclusion, the EFTECH 64-Ni bump is consider as suitable substrate material due no fracture was observed between the substrate boundaries and the IMC boundaries.
Due to manufacturing requirements, surface finishes have become a necessity in printed circuit board design. These finishes have significant effects on the RF performance of the transmission lines. In this paper, a filament modeling approach is used to model skin, proximity, and surface roughness effects in transmission lines with surface finishes up to 70 GHz. The approach shows a high accuracy compared with measurements. The model also gives an insight into how the current distributes itself by showing the frequency dependent proportion of the current that flows in each surface finish layer. In the case of NiP-Au or Ni-Au surface finishes, current migrates increasingly into gold at high frequencies and reaches a maximum in the Ni or NiP at around 3.5 GHz, and then declines. The distribution of the current in different materials can also be explained as the decay of an electromagnetic wave at the surface of the conductor. This approach shows that the evanescent wave in the cross section of the conductor can be analyzed as analog to a transmission-reflection problem, what we will call the surface finish effect. This effect brings into question the accuracy of the traditional skin-depth value, delta, and the models that depend on it, such as most surface roughness correction factors, for structures where different metals are layered in thicknesses that are not much larger than delta.
In this study, a ductile electroless Ni-P coating on the flexible printed circuit board (FPCB) was prepared in an acidic nickel plating bath. The addition of dipropylamine (DPA) in electroless plating not only improves the ductility of the Ni-P coating, but also enhances the corrosion resistance. The further analysis reveals that the ductility improvement and enhancement of corrosion resistance for the Ni-P coating may be due to the fact that the addition of DPA significantly refine the volume of columnar nodule and reduce the porosity, thus leading to the released internal stress. In addition, it was found that the nodule within the Ni-P coating grew into a columnar structure, which may be also contribute to the improvement of ductility.
Electroless Ni-P/electroless Pd/immersion Au (ENEPIG) with ultra-thin Ni-P deposit serve as a potential replacement of traditional ENEPIG surface finish because of its superior electrical performance in flip chip solder joints interconnection. However, the interfacial reaction and mechanical reliability of solder joints in ENEPIG with ultra-thin Ni-P layer is not yet well evaluated. In this study, we investigated the characteristic microstructure of interfacial intermetallic compounds and high-speed impact responses of Sn-3.0Ag-0.5Cu/ENEPIG attachments with 4.8, 0.3, and 0.05 mu m Ni-P deposit. ENEPIG with Ni-P layer of 0.3 mu m exhibited the eutectic structure dispreading in the solder alloys and layer-type P-rich IMCs at solder/metallization interface, while there was (Cu,Ni)(6)Sn-5 precipitation in the solder but no P-rich IMCs layer formed in ENEPIG with 0.05 mu m Ni-P layer. Slower interfacial reaction rate in ENEPIG with 0.3 mu m Ni-P layer was attributed to the effect of electroless Ni-P diffusion barrier layer, which would further provide better impact resistivity than that of ENEPIG with 0.05 mu m Ni-P deposit. Moreover, breach in P-rich IMCs and underneath (Cu,Ni)(6)Sn-5 patch were observed in ENEPIG with 0.3 mu m Ni-P layer. The growth mechanism was closely related to the Ni diffusion from surface finish and element redistribution.
Evolution of interfacial phase formation in Sn-3.0 Ag-0.5Cu/Cu (wt%), Sn-3.0Ag-0.5Cu-0.1Ni/Cu, Sn-3.0 Ag-0.5Cu/Cu-15Zn, and Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints are investigated. Doping Ni in the solder joint can suppress the growth of Cu3Sn and alter the morphology of the interfacial intermetallic compounds (IMCs), however it shows rapid growth of (Cu,Ni)(6)Sn-5 at the Sn-3.0Ag-0.5Cu-0.1Ni/Cu interface. In comparison with the Cu substrates, the Cu-Zn substrates effectively suppress the formation of Cu-Sn IMCs. Among these four solder joints, the Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joint exhibits the thinnest IMC, and only (Cu,Ni)(6)(Sn,Zn)(5) formed at the interface after aging. It is revealed that the presence of Ni acts to enhance the effect of Zn on the suppression of Cu-Sn IMCs in the SAC305-0.1Ni/Cu-15Zn solder joint. The limited formation of IMCs is related to the elemental redistribution at the joint interfaces during aging. The Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn joint can act as a stabilized interconnection due to the effective suppression of interfacial reaction.
Intermetallic compound formation at the interface between Sn-3.0Ag-0.5Cu (SAC) solders and electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish and the mechanical strength of the solder joints were investigated at various Pd thicknesses (0 μm to 0.5 μm). The solder joints were fabricated on the ENEPIG surface finish with SAC solder via reflow soldering under various conditions. The (Cu,Ni)6Sn5 phase formed at the SAC/ENEPIG interface after reflow in all samples. When samples were reflowed at 260°C for 5 s, only (Cu,Ni)6Sn5 was observed at the solder interfaces in samples with Pd thicknesses of 0.05 μm or less. However, the (Pd,Ni)Sn4 phase formed on (Cu,Ni)6Sn5 when the Pd thickness increased to 0.1 μm or greater. A thick and continuous (Pd,Ni)Sn4 layer formed over the (Cu,Ni)6Sn5 layer, especially when the Pd thickness was 0.3 μm or greater. High-speed ball shear test results showed that the interfacial strengths of the SAC/ENEPIG solder joints decreased under high strain rate due to weak interfacial fracture between (Pd,Ni)Sn4 and (Cu,Ni)6Sn5 interfaces when the Pd thickness was greater than 0.3 μm. In the samples reflowed at 260°C for 20 s, only (Cu,Ni)6Sn5 formed at the solder interfaces and the (Pd,Ni)Sn4 phase was not observed in the solder interfaces, regardless of Pd thickness. The shear strength of the SAC/ENIG solder joints was the lowest of the joints, and the mechanical strength of the SAC/ENEPIG solder joints was enhanced as the Pd thickness increased to 0.1 μm and maintained a nearly constant value when the Pd thickness was greater than 0.1 μm. No adverse effect on the shear strength values was observed due to the interfacial fracture between (Pd,Ni)Sn4 and (Cu,Ni)6Sn5 since the (Pd,Ni)Sn4 phase was already separated from the (Cu,Ni)6Sn5 interface. These results indicate that the interfacial microstructures and mechanical strength of solder joints strongly depend on the Pd thickness and reflow conditions.