ENEPIG Surface Finish Research Articles

Thermo-Compression Bonding of Cu/SnAg Pillar Bumps with Electroless Palladium Immersion Gold (EPIG) Surface Finish

Thermo-compression bonding (TCB) properties of Cu/SnAg pillar bumps on electroless palladium immersion gold (EPIG) were evaluated in this study. A test chip with Cu/SnAg pillar bumps was bonded on the surface-finished Cu pads with the TCB method. The surface roughness of the EPIG was 82 nm, which was 1.6 times higher than that of the ENEPIG surface finish because the EPIG was so thin that it could not flatten rough bare Cu pads. From the cross-sectional SEM micrographs, the filler trapping of the TC-bonded EPIG was much higher than that of the ENEPIG sample. The high filler trapping of the EPIG sample was due to the high surface roughness of the EPIG surface finish. The contact resistance increased as the thermal cycle time increased. The increase of the contact resistance with 1500 cycles of the thermal cycle test was 26% higher for the EPIG sample than for the ENEPIG sample.

In-situ Observation and Modeling of Intermetallic Compound Growth Induced by Electromigration in Sn-2.5Ag Solder Joints with OSP and ENEPIG Surface Finish

In-situ Observation and Modeling of Intermetallic Compound Growth Induced by Electromigration in Sn-2.5Ag Solder Joints with OSP and ENEPIG Surface Finish

Intermetallic Compound Growth Induced by Electromigration in Sn-2.5Ag Solder Joints with ENEPIG Surface Finish

The equation for predicting the growth behavior of intermetallic compounds (IMC) by electromigration (EM) of the Cu/ENEPIG/Sn-2.5Ag/Cu solder joint was modeled, and the actual behavior observed through experiments and the predicted behavior were compared. After reflow, (Cu, Ni)<sub>6</sub>Sn<sub>5</sub> was formed near the ENEPIG/solder interface, and Cu<sub>6</sub>Sn<sub>5</sub> was produced near the solder/Cu interface. Furthermore, Cu<sub>6</sub>Sn<sub>5</sub> islands and Ag<sub>3</sub>Sn were formed with the β-Sn matrix in the solder. The mobility of Cu, Ni, and Sn atoms at the ENEPIG/IMC/solder interface was calculated to derive a thickness variation equation of the IMC with respect to the current application time. The modeling predicted that if the current density was maintained for 250 h at 10 kA/cm<sup>2</sup>, the IMC thickness increased by 4.2 ㎛. As a result of the EM experiment, the IMC at the ENEPIG/solder interface grew by 4.2 ㎛; this exactly matched the prediction. A comparison of the thickness of the IMC layer indicated that the OSP/solder interface produced approximately 9 ㎛, and the ENEPIG/solder interface grew by approximately 4.2 ㎛. Therefore, the Ni plating layer of the ENEPIG surface treatment prevented the diffusion of Cu and suppressed the growth of IMC by approximately 50 %.

Effect of Ni(P) Layer Thickness on Interface Reaction and Reliability of Ultrathin ENEPIG Surface Finish.

Electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) is a common surface finish in electronic packaging, while the Ni(P) layer increases the impedance of solder joints and leads to signal quality degradation in high-frequency circuits. Reducing the thickness of the Ni(P) layer can balance the high impedance and weldability. In this paper, the interfacial reaction process between ultrathin ENEPIG substrates with different Ni layer thicknesses (0.112 and 0.185 μm) and Sn–3.0Ag–0.5Cu (SAC305) solder during reflow and aging was studied. The bonding ability and reliability of solder joints with different surface finishes were evaluated based on solder ball shear test, drop test and temperature cycle test (TCT), and the failure mechanism was analyzed from the perspective of intermetallic compound (IMC) interface growth. The results showed that the Ni–Sn–P layer generated by ultrathin ENEPIG can inhibit the growth of brittle IMC so that the solder joints maintain high shear strength. Ultrathin ENEPIG with a Ni layer thickness of 0.185 μm had no failure cracks under thermal cycling and drop impact, which can meet actual reliability standards. Therefore, ultrathin ENEPIG has broad prospects and important significance in the field of high-frequency chip substrate design and manufacturing.

Flip Chip - Chip Scale Package Bonding Technology with Type 7 Solder Paste Printing

In this study, thermo-compression(TC) and vacuum reflow(VR) bonding processes of fine pitch interconnection were optimized using Type 7 Sn-3.0Ag-0.5Cu solder paste. A chip with Cu pillar Sn-2.5Ag bumps and BT substrate with ENEPIG surface finish were used, and the thickness and the aperture ratio of the metal mask were 30 ㎛/50 ㎛ and 70%/100%, respectively. As a result of optimizing the printing process, a metal mask with a thickness of 30 ㎛ and an aperture ratio of 100% was selected. As a result of the TC and VR bonding processes optimization, Cu6Sn5 and (Cu,Ni)6Sn5 IMC layers were formed on the top and bottom of the solder joint, respectively. In the conventional TC process sample, the thickness of the solder was measured to be 4.1 ㎛, and the IMC thickness ratio was calculated to be 40%. Also, in the TC and VR process samples with solder paste, the thickness of the solder was measured to be 6.5 ㎛ and 10.3 ㎛, respectively, and the ratio of the IMC thickness was calculated to be about 32% and 43%. It was expected that it would be beneficial to reliability by reducing the occurrence and propagation of cracks in the solder joint due to the effect of lowering the IMC thickness ratio as the volume of the solder becomes larger than that of the conventional TC process.

Mechanical Properties of Cu-Core Solder Balls with ENEPIG Surface Finish

The development of ball grid array (BGA) packages, such as the chip scale package, wafer level package and package on package, has focused on creating electronics packages that are smaller, thinner, higher-performance and capable of higher functionality, among other desirable traits. Among the interconnection materials used in BGA packages, the Cu-core solder ball (CCSB) has many advantages, such as the use of finer pitch, improved electrical conductivity and better controllability of the coplanarity of the chip. In this study, we evaluated the mechanical properties of the CCSB and Sn-3.0Ag-0.5Cu (SAC) by low-speed shear tests, and the von Mises stress distribution and plastic strain distribution were simulated using a finite element method. The diameter of each solder ball was 280 μm, and the outer layer of the CCSB was a plated SAC layer. The shear strength of the CCSB was about 10% greater than that of SAC. The maximum value of the simulated von Mises stress for the CCSB was higher than that of SAC because the Cu-core is stiffer than SAC. The fracture energy of the CCSB decreased by about 50% compared to that of SAC. The maximum value of simulated plastic strain, which is associated with fracture surfaces, was higher with the CCSB than with SAC. We can thus conclude that the Cu-core in the CCSB affects the shear strength and fracture behavior of solder joints.

Solid-state growth kinetics of intermetallic compounds in Cu pillar solder flip chip with ENEPIG surface finish under isothermal aging

Electroless Ni/electroless Pd/immersion Au (ENEPIG) constitutes a promising metallization to replace the conventional ENIG or Ni/Au for 3D IC integrated module and optoelectronic packaging. Different ENEPIG plating thicknesses with copper pillar solder joints are subjected to thermal aging at 150 °C to investigate the intermetallic compounds (IMCs) formation and growth. Due to low temperature solid-state bonding, both Pd and Au layers do not completely diffused into Sn matrix and participated in the interfacial reactions to form (Pd,Au)Sn4 IMCs phase. (Pd,Au)Sn4 IMCs based on PdSn4 phase with dissolved Au, exhibits high growth rate and substantial consumption of Sn from solder. Upon increasing the aging time, it is found that (Cu,Ni)6Sn5 IMCs and (Pd,Cu,Au)Sn4 phase form at interface and follows a diffusion control mechanism, which weakened the solder joint and caused brittle fracture in a die peel test. (Cu,Ni)6Sn5 IMCs growth rate increases with decreases Pd thickness. Therefore, the types of IMCs formation, growth rate, and reliability of Cu pillar joint strongly depend on bonding temperature, Au and Pd thicknesses, and solder volume. Based on this study, the authors recommend suitable ranges of Au and Pd layer thicknesses for reliable Cu pillar solder joints.

Comparison between SAC405 Lead-Free Solders and EN(P)EPIG and EN(B)EPIG Surface Finishes

In electronics industries, most of them had to shifted their solder materials from leaded solders into lead-free solders due to the environmental concerns and follow the legislation of Restriction of use Hazardous Substances (RoHS). Thus, Sn-Ag-Cu solder is one of the choices that can replace the leaded solder and also offer better properties. This study investigates the comparison between Sn-4.0Ag-0.5Cu (SAC405) and EN(P)EPIG and EN(B)EPIG surface finishes. Reliability of solder joint has been assessed by performing solid state isothermal aging at 150oC for 250 up to 2000 hours. After reflow soldering process, (Cu,Ni)6Sn5intermetallic compound (IMC) is dominated at near centre of solder meanwhile (Ni,Cu)3Sn4IMC is dominated at near outside of solder ball.Moreover, aging time resulted in an increase in thickness and changed the morphology into more spherical, dense and large grain size. Analysis by optical microscope revealed that the IMC thickness of EN(B)EPIG produced thicker IMC compared to EN(P)EPIG surface finish during reflow as well as isothermal aging.

Interfacial reaction and mechanical evaluation in multi-level assembly joints with ENEPIG under bump metallization via drop and high speed impact test

The criteria of mechanical reliability in solder joints can be identified and described by comparative evaluation via drop test and high speed pendulum impact test. Systematic samples of assembly and attachment joints with various Pd additions were employed and investigated in this study. The statistical values of mechanical performances were calculated and compared. Better high speed impact performance of SAC305/ENEPIG attachment joints with 0.06μm Pd layers was confirmed owing to the single Cu6Sn5 phase growth. However, the comparative measurement of the better performance on drop testing exhibited in ENEPIG/SAC305/immersion Sn assembly joints with 0.1μm Pd layers deposit resulted from the thinner and layer-type IMC growth. The correlation between the cracks propagation and Pd addition was established on the basis of the elemental X-ray color mapping via Field-Emission Electron Probe Microanalyzer (FE-EPMA). It is expected that through comparison between impact and drop test in mechanical reliability, a criterion of joints reliability can be established. Besides, the optimal Pd layer deposit for the ENEPIG surface finish in the attachment and assembly solder joints was demonstrated and confirmed.

Effects of Pd layer thickness on mechanical properties and microstructures of Sn-3.0 Ag-0.5 Cu solder joints

In order to investigate effects of Pd layer thickness on the mechanical properties and microstructures of Sn-3.0 Ag-0.5 Cu solder joints, three different Pd layers, i.e., 0.05, 0.1, 0.15 µm, were employed on the electroless nickel immersion gold [ENIG, Ni-P(5 µm )/Au(0.08 µm)] surface finished flame retardant type 4 (FR4) printed circuit board (PCB) substrates. Our studies showed that the addition of Pd layer in ENIG treatment, i.e., ENEPIG, enhanced the bonding strength of solder joints by reducing the formation of intermetallic compounds (IMCs), (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4. The different failure modes were also observed – the crack propagation occurred along the interface between P-rich Ni layer and Cu layer in 0.05 µm Pd specimens while specimens with 0.1 µm and 0.15 µm Pd layers showed the crack propagation proceeded mostly inside solders. In this study the effects of Pd layer thickness in ENEPIG surface finish on the bonding strength of solder joints were investigated by means of drop test and ball shear test, followed by observation of microstructures and interfaces using a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS).

Effect of Solder Volume on Interfacial Reaction between SAC405 Solders and EN(B)EPIG Surface Finish

The electronic packaging industry is now being driven towards smaller, lighter, and thinner electronic products but with higher performance and more functions. Thus, smaller solder ball sizes are needed for fine solder joint interconnections to fulfill these requirements. This study investigates the interfacial reactions during reflow soldering and isothermal aging between Sn-4.0Ag-0.5Cu (SAC405) and electroless nickel (boron)/ immersion palladium/immersion gold (EN(B)EPIG). Reliability of solder joint has also been investigated by performing solid state isothermal aging at 125 °C for up to 2000 hours. The results revealed that after reflow soldering, (Cu, Ni)6Sn5 IMC is formed between solder and substrate while after aging treatment another IMC was found between (Cu, Ni)6Sn5 and substrate known as (Ni, Cu)3Sn4. Aging time of solder joints resulted in an increase in IMC thickness and a change in morphology into more spherical, dense and with larger grain size. By using optical microscope, the average thickness of the intermetallics was measured and it found that the larger solder balls produced thicker IMC than the smaller solder balls during reflow soldering. However, after aging the smaller solders produced thicker IMC than the larger solders.

Correlation between microstructure evolution and mechanical strength in the Sn–3.0Ag–0.5Cu/ENEPIG solder joint

This study investigated the interfacial reaction and mechanical strength of Sn–3.0Ag–0.5Cu solder jointed with electroless Ni–P/immersion Au (ENIG) and electroless Ni–P/electroless Pd/immersion Au (ENEPIG) surface finishes under various reflow times. The morphology of interfacial (Cu,Ni,Pd)6Sn5 was refined to needle-like by the Pd insertion in the ENEPIG surface finish. The impact energy of solder joints with Pd doped declined slower than that without Pd-doped after prolonged reflow. By inspecting the fracture surface, the cleavage of interfacial (Cu,Ni)6Sn5 was frequently observed in the ENIG joints after prolonged soldering, whereas ductile dimple-like failure dominated the fracture surface of ENEPIG joints. The enhanced impact strength and the transition of failure mode in the ENEPIG joints was attributed to the needle-like morphology of (Cu,Ni,Pd)6Sn5. The detailed mechanism of improved mechanical strength for solder joints with Pd dissolved will be deliberately addressed and discussed regarding the distinct microstructural evolution in the ENEPIG joint.

Microstructural variation and high-speed impact responses of Sn–3.0Ag–0.5Cu/ENEPIG solder joints with ultra-thin Ni–P deposit

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 μm Ni–P deposit. ENEPIG with Ni–P layer of 0.3 μm exhibited the eutectic structure dispreading in the solder alloys and layer-type P-rich IMCs at solder/metallization interface, while there was (Cu,Ni)6Sn5 precipitation in the solder but no P-rich IMCs layer formed in ENEPIG with 0.05 μm Ni–P layer. Slower interfacial reaction rate in ENEPIG with 0.3 μm 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 μm Ni–P deposit. Moreover, breach in P-rich IMCs and underneath (Cu,Ni)6Sn5 patch were observed in ENEPIG with 0.3 μm Ni–P layer. The growth mechanism was closely related to the Ni diffusion from surface finish and element redistribution.