Influence of Cu content on compound formation near the chip side for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump during aging

Sn-Ag-Cu solder is a promising candidate to replace conventional Sn-Pb solder. Interfacial reactions for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder joints were investigated after aging at 150°C. The under bump metallization (UBM) for the Sn-3.0Ag-(0.5 or 1.5)Cu solders on the chip side was an Al/Ni(V)/Cu thin film, while the bond pad for the Sn-3.0Ag-0.5Cu solder on the plastic substrate side was Cu/electroless Ni/immersion Au. In the Sn-3.0Ag-0.5Cu joint, the Cu layer at the chip side dissolved completely into the solder, and the Ni(V) layer dissolved and reacted with the solder to form a (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). For the Sn-3.0Ag-1.5Cu joint, only a portion of the Cu layer dissolved, and the remaining Cu layer reacted with solder to form Cu₆Sn₅ IMC. The Ni in Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. At the plastic substrate side, three interfacial products, (Cu_1−y,Ni_y)₆Sn₅, (Ni_1−x,Cu_x)₃Sn₄, and a P-rich layer, were observed between the solder and the EN layer in both Sn-Ag-Cu joints. The interfacial reaction near the chip side could be related to the Cu concentration in the solder joint. In addition, evolution of the diffusion path near the chip side in Sn-Ag-Cu joints during aging is also discussed herein.     

Mechanism of interfacial reaction for the Sn-Pb solder bump with Ni/Cu under-bump metallization in flip-chip technology

Nickel-based under-bump metallization (UBM) has been widely used in flip-chip technology (FCT) because of its slow reaction rate with Sn. In this study, solder joints after reflows were employed to investigate the mechanism of interfacial reaction between the Ni/Cu UBM and eutectic Sn-Pb solder. After deliberate quantitative analysis with an electron probe microanalyzer (EPMA), the effect of Cu content in solders near the interface of the solder/intermetallic compound (IMC) on the interfacial reaction could be probed. After one reflow, only one layered (Ni_1−x,Cu_x)₃Sn₄ with homogeneous composition was found between the solder bump and UBM. However, after multiple reflows, another type of IMC, (Cu_1−y,Ni_y)₆Sn₅, formed between the solder and (Ni_1−x,Cu_x)₃Sn₄. It was observed that if the concentration of Cu in the solders near the solder/IMC interface was higher than 0.6 wt.%, the (Ni_1−x,Cu_x)₃Sn₄ IMC would transform into the (Cu_1−y,Ni_y)₆Sn₅ IMC. The Cu contents in (Ni_1−x,Cu_x)₃Sn₄ were altered and not uniformly distributed anymore. With the aid of microstructure evolution, quantitative analysis, elemental distribution by x-ray color mapping, and related phase equilibrium of Sn-Ni-Cu, the reaction mechanism of interfacial phase transformation between the Sn-Pb solder and Ni/Cu UBM was proposed.     

Inhibiting the formation of (Au1−xNix)Sn4 and reducing the consumption of Ni metallization in solder joints

The effects of adding a small amount of Cu into eutectic PbSn solder on the interfacial reaction between the solder and the Au/Ni/Cu metallization were studied. Solder balls of two different compositions, 37Pb-63Sn (wt.%) and 36.8Pb-62.7Sn-0.5Cu, were used. The Au layer (1 ± 0.2 μm) and Ni layer (7 ± 1 μm) in the Au/Ni/Cu metallization were deposited by electroplating. After reflow, the solder joints were aged at 160°C for times ranging from 0 h to 2,000 h. For solder joints without Cu added (37Pb-63Sn), a thick layer of (Au_1−xNi_x)Sn₄ was deposited over the Ni₃Sn₄ layer after the aging. This thick layer of (Au_1−xNi_x)Sn₄ can severely weaken the solder joints. However, the addition of 0.5wt.%Cu (36.8Pb-62.7Sn-0.5Cu) completely inhibited the deposition of the (Au_1−xNi_x)Sn₄ layer. Only a layer of (Cu_1-p-qAu_pNi_q)₆Sn₅ formed at the interface of the Cu-doped solder joints. Moreover, it was discovered that the formation of (Cu_1-p-qAu_pNi_q)₆Sn₅ significantly reduced the consumption rate of the Ni layer. This reduction in Ni consumption suggests that a thinner Ni layer can be used in Cu-doped solder joints. Rationalizations for these effects are presented in this paper.     

The effect of intermetallic compound morphology on Cu diffusion in Sn-Ag and Sn-Pb solder bump on the Ni/Cu Under-bump metallization

The eutectic Sn-Ag solder alloy is one of the candidates for the Pb-free solder, and Sn-Pb solder alloys are still widely used in today’s electronic packages. In this tudy, the interfacial reaction in the eutectic Sn-Ag and Sn-Pb solder joints was investigated with an assembly of a solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. In the Sn-3.5Ag solder joints reflowed at 260°C, only the (Ni_1−x,Cu_x)₃Sn₄ intermetallic compound (IMC) formed at the solder/Ni interface. For the Sn-37Pb solder reflowed at 225°C for one to ten cycles, only the (Ni_1−x,Cu_x)₃Sn₄ IMC formed between the solder and the Ni/Cu under-bump metallization (UBM). Nevertheless, the (Cu_1−y,Ni_y)₆Sn₅ IMC was observed in joints reflowed at 245°C after five cycles and at 265°C after three cycles. With the aid of microstructure evolution, quantitative analysis, and elemental distribution between the solder and Ni/Cu UBM, it was revealed that Cu content in the solder near the solder/IMC interface played an important role in the formation of the (Cu_1−y,Ni_y)₆Sn₅ IMC. In addition, the diffusion behavior of Cu in eutectic Sn-Ag and Sn-Pb solders with the Ni/Cu UBM were probed and discussed. The atomic flux of Cu diffused through Ni was evaluated by detailed quantitative analysis in an electron probe microanalyzer (EPMA). During reflow, the atomic flux of Cu was on the order of 10¹⁶−10¹⁷ atoms/cm²sec in both the eutectic Sn-Ag and Sn-Pb systems.     

Interfacial reactions and compound formation in the edge of PbSn flip-chip solder bumps on Ni/Cu under-bump metallization

Flip-chip technology with the layout of ball grid array has been widely used in today’s microelectronics industry. The elemental distribution in the edge of the solder bump is crucial for its correlation with the bump strength. In this study, Ni/Cu under-bump metallization (UBM) was used to evaluate the intermetallic compound (IMC) formation in the edge of the solder bump between the UBM and eutectic Sn-Pb solder in the 63Sn-37Pb/Ni/Cu/Ti/Si₃N₄/Si multilayer structure. During reflows, layered-type (Ni_1−xCu_x)₃Sn₄ and island-like (Cu_1−yNi_y)₆Sn₅ IMCs formed in the interface between the solder and UMB, while only the (Cu_1−yNi_y)₆Sn₅ IMC was observed in the sideway of the Ni/Cu UBM. After high-temperature storage (HTS) at 150°C for 1,000 h, both (Cu_1−yNi_y)₆Sn₅ and (Cu_1−zNi_z)₃Sn were found in the sideway of the Ni/Cu UBM. Two other IMCs, (Ni_1−xCu_x)₃Sn₄ and (Cu_1−yNi_y)₆Sn₅, formed in the interface between the solder and UBM. The growth of the (Cu_1−yNi_y)₆Sn₅ IMC was relatively fast during HTS.     

elemental redistribution and interfacial reaction mechanism for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump with Al/Ni(V)/Cu under-Bump metallization during aging

In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn−Ag−Cu solder is a promising candidate to replace the conventional Sn−Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution, and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). The (Cu_1−y,Ni_y)₆Sn₅ IMC gradually grew with the rate constant of 4.63 × 10⁻⁸ cm/sec^0.5 before 500 h aging had passed. After 500 h aging, the (Cu_1−y,Ni_y)₆Sn₅ IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu₆Sn₅ IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu_1−y,Ni_y)₆Sn₅ IMC was down to 1.74 × 10⁻⁸ cm/sec^0.5 in, the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn−Ag−Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.     

Interactions between solder and metallization during long-term aging of advanced microelectronic packages

The reactions between the eutectic PbSn solder and the Au/Ni/Cu tri-layer metallization in advanced microelectronic packages were studied. In this investigation, reflowed packages were subjected to aging at 160°C for times as long as 4000 h. Immediately after the reflow, all the Au had left the Au/Ni/Cu metallization, forming many (Au_1−xNi_x)Sn₄ particles distributed throughout the whole solderjoint. In addition, there was a thin layer of Ni₃Sn₄ (1.4 μm) at the interface. After 500 h of aging, most of the (Au_1−xNi_x)Sn₄ particles regrouped at the interface as a continuous (Au_0.45Ni_0.55)Sn₄, layer over the Ni₃Sn₄ layer. After 2500 h of aging, nearly all the Ni layer had been consumed. A 15 μm layer of (Au_0.45Ni_0.55) Sn₄ and a 20 μm Ni₃Sn₄ were found over the remaining Ni. At 3000 h, the Cu had started to react with both Ni₃Sn₄ and (Au_1−xNi_x)Sn₄, forming a layer of (Cu_1−p−pAu_pNi_q)₆Sn₅, a layer of (Cu_1−r−sAu₁Ni_s)₆Sn₅, and a layer of Cu₃Sn over the Cu layer. A small amount of Cu (2.7–5.7 at.%) was found to dissolve in this Ni₃Sn₄, forming a ternary compound (Ni_1−yCu_y)₃Sn₄. It was revealed that Au diffused up-hill during the reaction. After aging for 4000 h, all the (Au_1−xNi_x)Sn₄ had disappeared and Au atoms had diffused into the (Cu_1−p−qAu_pNi_q)₆Sn₅ and (Cu_1−r−sAu_rNi_s)₆Sn₅ phases. The practical implications for the above findings were pointed out in this paper.     

Formation and resettlement of (AuxNi1−x)Sn4 in solder joints of ball-grid-array packages with the Au/Ni surface finish

The interfacial reactions between eutectic PbSn solder and the solder ball pads with the Au/Ni surface finish were studied. Solder joints subjected to up to three repeated reflow-and-aging treatments were examined. For the reflow, the peak reflow temperature was 225°C, and the reflow time was 115 s. Each aging process was performed at 160°C for 500 h. After the first reflow, all the Au would disappear from the interface, and formed many (Au_xNi_1−x)Sn₄ particles inside the solder joints. The value of x was between 0.99 and 0.75. In addition, there was a thin layer of Ni₃Sn₄ (1.4 μm) at the interface. After one reflow and one subsequent aging, most of the (Au_xNi_1−x)Sn₄ would relocate from inside the solder joint to the interface, and the value of x for (Au_xNi_1−x)Sn₄ at the interface decreased to 0.45. This (Au_xNi_1−x)Sn₄ resettlement process repeated itself for additional reflow-aging cycles. More reflow-aging treatments, however, made the microstructure of (Au_0.45Ni_0.55)Sn₄ at the interface become more non-planar. It was shown that gravitational effect was not the driving force for the resettlement of (Au_xNi_1−x)Sn₄. It is proposed that the driving force is for (Au_xNi_1−x)Sn₄ to seek Ni at the interface so that it can become more Ni-rich. In other words, the driving force is lowering the Gibbs energy of (Au_xNi_1−x)Sn₄ by dissolving more Ni. A decomposition-diffusion mechanism is proposed to explain what happened. Kinetic rationales for this rapid resettlement of (Au_xNi_1−x)Sn₄ at such a low temperature were also discussed.     

The microstructure characterization of ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallization

The microstructure of the ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallizations (UBMs) was investigated as a function of cooling rate. The ultrasmall eutectic Bi-Sn solder bump, about 50 μm in diameter, was fabricated by using the lift-off method and reflowed at various cooling rates using the rapid thermal annealing system. The microstructure of the solder bump was observed using a backscattered electron (BSE) image and the intermetallic compound was identified using energy dispersive spectroscopy (EDS) and an x-ray diffractometer (XRD). The Bi facet was found at the surface of the ultrasmall Bi-Sn solder bumps on the Au/Cu/Ti UBM in almost all specimens, and the interior microstructure of the bumps was changed with the solidification rate. The faceted and polygonal intermetallic compound was found in the case of the Bi-Sn solder bump on the Au (0.1 μm)/Ni/Ti UBM, and it was confirmed to be the (Au_1−x−yBi_xNi_y)Sn₂ phase by XRD. The intermetallic compounds grown form the Au (0.1 μm)/Ni/Ti UBM interface, and they interrupted the growth of Bi and Sn phases throughout the solder bump. The ultrasmall eutectic Bi-Sn solder bumps on the Au (0.025 μm)/Ni/Ti UBM showed similar microstructures to those on the Au/Cu/Ti UBM.     

Effects of Ni thickness and reflow times on interfacial reactions between Ni/Cu under-bump metallization and eutectic Sn-Pb solder in flip-chip technology

Flip-chip interconnection technology plays a key role in today’s electronics packaging. Understanding the interfacial reactions between the solder and under-bump metallization (UBM) is, thus, essential. In this study, different thicknesses of electroplated Ni were used to evaluate the phase transformation between Ni/Cu under-bump metallurgy and eutectic Sn-Pb solder in the 63Sn-37Pb/Ni/Cu/Ti/Si₃N₄/Si multilayer structure for the flip-chip technology. Interfacial reaction products varied with reflow times. After the first reflow, layered (Ni_1−x,Cu_x)₃Sn₄ was found between solder and Ni. However, there were two interfacial reaction products formed between solders and the UBM after three or more times reflow. The layered (Ni_1−x,Cu_x)₃Sn₄ was next to the Ni/Cu UBM. The islandlike (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC) could be related to the Ni thickness and reflow times. In addition, the influence of Cu contents on phase transformation during reflow was also studied.     

Effect of Cross-Interaction between Ni and Cu on Growth Kinetics of Intermetallic Compounds in Ni/Sn/Cu Diffusion Couples during Aging

The solid-state, cross-interaction between the Ni layer on the component side and the Cu pad on the printed circuit board (PCB) side in ball grid array (BGA) solder joints was investigated by employing Ni(15 μm)/Sn(65 μm)/Cu ternary diffusion couples. The ternary diffusion couples were prepared by sequentially electroplating Sn and Ni on a Cu foil and were aged isothermally at 150, 180, and 200°C. The growth of the intermetallic compound (IMC) layer on the Ni side was coupled with that on the Cu side by the mass flux across the Sn layer that was caused by the difference in the Ni content between the (Cu_1−x Ni _x )₆Sn₅ layer on the Ni side and the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side. As the consequence of the coupling, the growth rate of the (Cu_1−x Ni _x )₆ Sn₅ layer on the Ni side was rapidly accelerated by decreasing Sn layer thickness and increasing aging temperature. Owing to the cross-interaction with the top Ni layer, the growth rate of the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side was accelerated at 150°C and 180°C but was retarded at 200°C, while the growth rate of the Cu₃Sn layer was always retarded. The growth kinetic model proposed in an attempt to interpret the experimental results was able to reproduce qualitatively all of the important experimental observations pertaining to the growth of the IMC layers in the Ni/Sn/Cu diffusion couple.     

Characterizing metallurgical reaction of Sn3.0Ag0.5Cu composite solder by mechanical alloying with electroless Ni-P/Cu under-bump metallization after various reflow cycles

Electroless Ni-P/Cu under-bump metallization (UBM) is widely used in electronics packaging. The Sn3.0Ag0.5Cu lead-free composite solder pastes were produced by a mechanical alloying (MA) process doped with Cu₆Sn₅ nanoparticles. In this study, the detailed interfacial reaction of Sn3.0Ag0.5Cu composite solders with EN(P)/Cu UBM was investigated after reflow. A field-emission scanning electron microscope (FESEM) was employed to analyze the interfacial morphology and microstructure evolution. The intermetallic compounds (IMCs) formed at the interface between the Sn3.0Ag0.5Cu composite solders and EN(P)/Cu UBM after one and three reflows were mainly (Ni_1−x,Cu_x)₃Sn₄ and (Cu_1−y,Ni_y)₆Sn₅. However, only (Ni_1−x,Cu_x)₃Sn₄ IMC was observed after five reflows. The elemental distribution near the interfacial region was evaluated by an electron probe microanalyzer (EPMA) as well as field-emission electron probe microanalyzer (FE-EPMA). Based on the observation and characterization by FESEM, a EPMA, and an FE-EPMA, the reaction mechanism of interfacial phase transformation between Sn3.0Ag0.5Cu composite solders and EN(P)/Cu UBM after various reflow cycles was discussed and proposed.     

Interfacial microstructure of Pb-free and Pb-Sn solder balls in the ball-grid array package

Ball-grid array (BGA) samples were aged at 155°C up to 45 days. The formation and the growth of the intermetallic phases at the solder joints were investigated. The alloy compositions of solder balls included Sn-3.5Ag-0.7Cu, Sn-1.0Ag-0.7Cu, and 63Sn-37Pb. The solder-ball pads were a copper substrate with an Au/Ni surface finish. Microstructural analysis was carried out by electron microprobe. The results show that a ternary phase, (Au,Ni)Sn₄, formed with Ni₃Sn₄ in the 63Sn-37Pb solder alloy and that a quaternary intermetallic phase, (Au,Ni)₂Cu₃Sn₅, formed in the Sn-Ag-Cu solder alloys. The formation mechanism of intermetallic phases was associated with the driving force for Au and Cu atoms to migrate toward the interface during aging.     

Influence of Cu addition to interface microstructure between Sn-Ag solder and Au/Ni-6P plating

The formation and growth of intermetallics at the interface between Sn-Ag-(Cu) alloy balls and Au/Ni-6P plating were experimentally examined as a function of soldering period. Joint strengths were also evaluated by a ball pull test. For the joint with Sn-3.5Ag, the primary reaction product of Ni₃Sn₄ exhibits growth and shrinkage in thickness repeatedly with a passage of reaction time up to 30 min, while the Ni₃SnP reaction layer monotonously increases its thickness without fluctuation. In the cases of the joints with Cu bearing solder, Sn-3Ag-0.5Cu and Sn-3.5Ag-0.8Cu, a single η-(Cu,Ni)₆Sn₅ interface layer grows by fast Cu segregation from liquid solder to the interface layer on soldering. For all the soldered joints, a P-rich layer appears at the surface region of a Ni-6P plating layer by Ni depletion to form those intermetallic compounds at interfaces. The growth rate of a P-rich layer for Sn-3.5Ag is faster by about 4–8 times than those of the Sn-Ag-Cu. The presence of Cu in solder enhances the formation of the Cu₆Sn₅ intermetallic layer at the interface resulting in prevention of Ni diffusion to liquid solder. For all the soldered joints, coarsened reaction interfaces decrease the joint strengths.     

Intermetallic Reactions in Sn-0.4Co-0.7Cu Solder BGA Packages with an ENIG Surface Finish

As-cast Sn-0.4Co-0.7Cu solder contains both (Cu_0.98Co_0.02)₆Sn₅ and (Co_0.85Cu_0.15) Sn₃ intermetallic phases in the matrix. After reflowing, the Au thin film in the electroless Ni/immersion Au (ENIG) surface-finished Sn-0.4Co-0.7Cu solder ball grid array (BGA) packages dissolved rapidly into the solder matrix to form AuSn₄ intermetallics, and a thin layer of (Cu_0.57Ni_0.35Au_0.08)₆Sn₅ intermetallic compound appeared at the solder/pad interface, growing very slowly during aging at 100°C. Increasing the aging temperature to 150°C caused the formation of a new intermetallic layer, (Ni_0.79Cu_0.21)₃Sn₄, at the (Cu_0.57Ni_0.35Au_0.08)₆Sn₅/Ni interface. The reflowed Sn-0.4Co-0.7Cu BGA packages have a ball shear strength of 6.8 N, which decreases to about 5.7 N and 5.5 N after aging at 100°C and 150°C, respectively. The reflowed and aged solder joints fractured across the solder balls with ductile characteristics in ball shear tests.     

Effect of displacement rate on ball shear properties for Sn–37Pb and Sn–3.5Ag BGA solder joints during isothermal aging

The interfacial reactions and ball shear properties of ball grid array (BGA) solder joints aged at 170 °C for up to 21 days were investigated with different displacement rates. Two different kinds of solders, Sn–37Pb and Sn–3.5Ag (all wt.%), and an electroplated Ni/Au BGA substrate were employed in this work. A continuous Ni₃Sn₄ intermetallic compound (IMC) layer was formed at the interfaces between both the Sn–37Pb and Sn–3.5Ag solders and the substrate during reflow. After aging, two different reaction layers, consisting of (Au_xNi_1−x)Sn₄ IMC and Pb-rich phase, were additionally observed between the Sn–37Pb solder and the Ni₃Sn₄ IMC layer. The thicknesses of these interfacial reaction layers increased with increasing aging time. After reflow, all the fractures occurred inside the bulk solder. The fracture location of the Sn–37Pb solder joints was shifted toward the solder/Ni interface with increasing aging time and displacement rate, whereas the fracture of the Sn–3.5Ag solder joints mainly occurred inside the bulk solder, irrespective of the aging time and displacement rate. Consequently, the shear properties of the Sn–37Pb solder joints significantly decreased with increasing aging time, whereas those of the Sn–3.5Ag solder joints slightly decreased. The tendency toward brittle fracture of the Sn–37Pb solder joints was intensified with increasing displacement rate. The shear properties of the ductile solder joints increased with increasing displacement rate, while the displacement until fracture, deformation energy and displacement rate sensitivity of the brittle solder joints significantly decreased with increasing displacement rate.     

Cross-interaction of under-bump metallurgy and surface finish in flip-chip solder joints

The cross-interaction of the under-bump metallurgy (UBM)/solder interface and the solder/surface-finish interface in flip-chip solder joints was investigated. In this study, the UBM on the chip side was a single layer of Cu (8.5 μm), and the surface finish on the substrate side was a 0.2-μm Au layer over 5-μm Ni. It was shown that, after two reflows, the Ni layer of the surface finish had been covered with (Cu_1−xNi_x)₆Sn₅. This shows that the effect of cross-interaction of the two interfaces is important even during the reflow stage. During subsequent solid-state aging at 115°C, 135°C, and 155°C, the formation of (Cu_1−xNi_x)₆Sn₅ over the Ni layer was found to have the effect of reducing the Ni consumption rate. At the same time, the Cu consumption rate of the UBM was accelerated. The results of this study show that the selection of the UBM and the surface finish has to be considered together because the cross-interaction of the two interfaces plays an important role.     

Reflow and burn-in of a Sn-20In-0.8Cu ball grid array package with a Au/Ni/Cu pad

The intermetallic compounds formed after reflow and burn-in testing of a Sn-20In-0.8Cu solder ball grid array (BGA) package are investigated. Along with the formation of the Cu₆(Sn_0.78In_0.22)₅ precipitates (IM1) in the solder matrix, scallop-shaped intermetallic compounds (IM2) with a compositional mixture of Cu₆(Sn_0.87In_0.13)₅ and Ni₃(Sn_0.87In_0.13)₄ appear at the interfaces between the solder balls and Au/Ni/Cu pads. A significant number of intermetallic particles (IM3), with a composition of (Au_0.80Cu_0.20)(In_0.33Sn_0.67)₂, can also be found in the solder matrix. After aging at 115°C for 750 h, an additional intermetallic compound layer (IM4) with a composition of (Ni_0.91Cu_0.09)₃(Sn_0.77In_0.23)₂ is formed at the interface between IM2 and the Ni layer. The ball shear strength of the Sn-20In-0.8Cu BGA solder after reflow is 4.5 N and will rise to maximum values after aging at 75°C and 115°C for 100 h. With a further increase of the aging time at both temperatures, the joint strengths exhibit a tendency to decline linearly at about 1.7×10⁻³ N/h.     

Intermetallic compounds formed during the reflow and aging of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder ball grid array packages

After reflow of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder balls on Au/Ni surface finishes in ball grid array (BGA) packages, scallop-shaped intermetallic compounds (Cu_0.70Ni_0.28Au_0.02)₆Sn₅ (IM1a) and (Cu_0.76Ni_0.24)₆(Sn_0.86In_0.14)₅ (IM1b), respectively, appear at the interfaces. Aging at 100°C and 150°C for Sn-3.8Ag-0.7Cu results in the formation of a new intermetallic phase (Cu_0.70Ni_0.14Au_0.16)₆Sn₅ (IM2a) ahead of the former IM1a intermetallics. The growth of the newly appeared intermetallic compound, IM2a, is governed by a parabolic relation with an increase in aging time, with a slight diminution of the former IM1a intermetallics. After prolonged aging at 150°C, the IM2a intermetallics partially spall off and float into the solder matrix. Throughout the aging of Sn-20In-2Ag-.5Cu solder joints at 75°C and 115°C, partial spalling of the IM1b interfacial intermetallics induces a very slow increase in thickness. During aging at 115°C for 700 h through 1,000 h, the spalled IM1b intermetallics in the solder matrix migrate back to the interfaces and join with the IM1b interfacial intermetallics to react with the Ni layers of the Au/Ni surface finishes, resulting in the formation and rapid growth of a new (Ni_0.85Cu_0.15)(Sn_0.71In_0.29)₂ intermetallic layer (IM2b). From ball shear tests, the strengths of the Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder joints after reflow are ascertained to be 10.4 N and 5.4 N, respectively, which drop to lower values after aging. An erratum to this article is available at .     

Intermetallic formation in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with immersion Ag surface finish

The intermetallic compounds formed in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Ag/Cu pads are investigated. After reflow, scallop-shaped η-Cu₆Sn₅ and continuous planar η-(cu_0.9Ni_0.1)₆Sn₅ intermetallics appear at the interfaces of the Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder joints, respectively. In the case of the Sn3Ag0.5Cu specimens, an additional ε-Cu₃Sn intermetallic layer is formed at the interface between the η-Cu₆Sn₅ and Cu pads after aging at 150°C, while the same type of intermetallic formation is inhibited in the Sn3Ag0.5Cu0.06Ni0.01Ge packages. In addition, the coarsening of Ag₃Sn precipitates also abates in the solder matrix of the Sn3Ag0.5Cu0.06Ni0.01Ge packages, which results in a slightly higher ball shear strength for the specimens.