Study on low-Ag content Sn–Ag–Zn/Cu solder joints

The eutectic Sn–Ag–Cu solder is the most popular lead free solder. But reliability and cost issues limit its application. On the other hand, Sn–Ag–Zn system has many advantages comparing with Sn–Ag–Cu. In this paper, interfaces of Sn–xAg–1Zn/Cu and Sn–2Ag–xZn/Cu (x = 1, 2, 3), Sn–2Ag–2.5Zn/Cu and Sn–1.5Ag–2Zn/Cu solders joints were studied to understand effects of Ag and Zn contents. Results show that shearing strength of as-reflowed Sn–2Ag–2Zn/Cu and Sn–1.5Ag–2Zn/Cu joints is higher than other joints. Because of the strong Cu–Sn reaction and the formation of Ag₃Sn, the Sn–Ag–Zn series solder joints are not suitable for use above 150 °C temperature. After 250 °C soldering for 4 h, while the Zn content increased from 1 wt% to 2 wt%, the interfacial IMC of Sn–Ag–Zn/Cu altered from Cu₆Sn₅ to Cu₅Zn₈. The Cu₅Zn₈ interface has higher shearing strength than Cu₆Sn₅ interface. Relationships among microstructure, strength and aging condition are discussed.     

Effects of the intermetallic compound microstructure on the tensile behavior of Sn3.0Ag0.5Cu/Cu solder joint under various strain rates

The effects of the intermetallic compound (IMC) microstructure and the strain rate on the tensile strength and failure mode of Pb-free solder joints are investigated. The samples of Sn3.0Ag0.5Cu/Cu solder joints are aged isothermally at 150 °C for 0, 72, 288 and 500 h, and the thickness of the IMC layer and the roughness of the solder/IMC interface are measured and used to characterize the microstructure evolution of the IMC layer. The tensile tests of the aged solder joints are conducted under the strain rates of 2 × 10⁻⁴, 2 × 10⁻² and 2 s⁻¹. The results indicate that both the thickness and roughness of the IMC layer have influence on the strength and failure mode of the solder joint. With the increase of the aging time, the thickness of the IMC layer increases and the roughness of the solder/IMC interface decreases, as a result, the tensile strength of the solder joint decreases and the dominant failure mode migrates from the ductile fracture in the bulk solder to the brittle fracture in the IMC layer. There is a positive correlation between the tensile strength of the solder joint and the stain rate applied during the test. With the increase of the strain rate, the failure mode migrates from the ductile fracture in the bulk solder to the brittle fracture in the IMC layer.     

Interfacial reaction and shear strength of SnAgCu-xNi/Ni solder joints during aging at 150 °C

The interfacial microstructure and shear strength of Sn3.8Ag0.7Cu-xNi (SAC-xNi, x = 0.5, 1, and 2) composite solders on Ni/Au finished Cu pads were investigated in detail after aging at 150 °C for up to 1000 h. The interfacial characteristics of composite solder joints were affected significantly by the weight percentages of added Ni micro-particles and aging time. After aging for 200 h, the solder joints of SAC, SAC-0.5Ni and -1Ni presented duplex intermetallic compound (IMC) layers regardless of the initial interfacial structure on as-reflowed joints, whose upper and lower IMC layers were comprised of (CuNi)₆Sn₅ and (NiCu)₃Sn₄, respectively. Only a single (NiCu)₃Sn₄ IMC layer was ever observed at the SAC-2Ni/Ni interface on whole aging process. Based on the compositional analysis, the amount of Ni within the IMC regions increased as the proportion of Ni addition increased. The IMC (NiCu)₃Sn₄ layer thickness on the interface of SAC and SAC-0.5Ni grew more slowly when compared to that of SAC-1Ni and -2Ni, while for the (CuNi)₆Sn₅ layer the reverse is true. Except the IMCs sizes are increased with increased aging time, the interfacial IMCs tended to transfer their morphologies to polyhedra. In all composite joints testing, the shear strengths were approximately equal to non-composite joints. The fracturing observed during shear testing of composite joints occurred in the bulk solder, indicating that the SAC-xNi/Ni solder joints had a desirable joint reliability.     

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.     

Impact strength of Sn–3.0Ag–0.5Cu solder bumps during isothermal aging

In order to investigate the fracture behavior of Sn–3.0Ag–0.5Cu solder bump, solder balls with the diameter of 0.76 mm were soldered on Cu pad in this study, then high speed impact test and static shear test of solder bumps were carried out to measure the joint strength of the soldering interface. The effect of isothermal aging on joint strength as well as fracture behavior of solder bumps was investigated, and the composition of the fracture surface was identified by means of EPMA. The results indicate that the fracture is inside the bulk solder in low speed shear test regardless of the aging effect, thus the maximum load reflects the solder strength rather than the interfacial strength. It is also found that under 1 m/s impact loading, the crack initiation position is changed from solder/Cu₆Sn₅ interface to Cu₃Sn/Cu interface after long time isothermal aging, and the fracture occurs inside the bulk solder accompanying with intermetallic compound in both of the as-soldered and aged joints. The thickened multiple IMC layers during isothermal aging account for the degraded impact resistance, and the change of the solder matrix is another factor for reduced impact resistance owing to Sn residue on the fracture surface.     

高密度LED焊点微空洞的X射线检测和分析

利用高精度X射线检测设备分别对用Sn37Pb焊膏和Sn3.0Ag0.5Cu焊膏组装的高密度LED灯板进行焊后和老化后的微空洞检测,观察了焊点的微空洞缺陷,并计算微空洞尺寸。结果表明:老化前微空洞面积与焊点面积比在10%~25%的,Sn3.0Ag0.5Cu焊点中约含25.5%,略大于Sn37Pb焊点的23.5%,且明显小于Sn3.0Ag0.5Cu焊点老化后的31.4%。两种焊点老化前后微空洞所占面积比都在<25%的合格范围内,但Sn3.0Ag0.5Cu焊点更易形成微空洞。     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Experimental evaluation of SnAgCu solder joint reliability in 100-μm pitch flip-chip assemblies

The intermetallic compound (IMC) evolution and the thermal–mechanical reliability of Sn–3.0Ag–0.5Cu (SAC305) solder joints were studied using air-to-air thermal shock testing of 100-μm-pitch peripheral-row flip chip assemblies. Flip chips assembled on organic substrates were subjected to air-to-air thermal shock testing between −55 °C and 125 °C, and the samples were removed at regular intervals for cross-sectioning and failure analysis. It was seen that on the die side, after 2000 cycles, all of the (Cu,Ni)₃Sn₄ had transferred to (Cu,Ni)₆Sn₅ due to strong cross-pad interaction between the chip-side Ni pad and substrate-side Cu pad, and thus, there was no premature solder cracking possibly due to the absence of dual IMC structure. On the substrate-side Cu interface, the Cu₃Sn growth was hindered, and thus there was very little increase in Kirkendall voids in the Cu₃Sn after 2000 cycles. Therefore, there was no premature brittle failure in the intermetallic. Failure analysis shows that the cracks in the outermost corner solder joint started to form after 2000 cycles near the chip-side pad, and the cracks propagated in the solder matrix around the IMC like a ring to create solder open. From the experimental data, crack propagation rate equation parameters and characteristic mean life were determined.     

Isothermal aging characteristics of Sn–Pb micro solder bumps

The shear strength of the under bump metallurgy (UBM) structure in both the high-melting solder bump and low-melting solder bump after aging were evaluated. Scanning electron microscopy and transmission electron microscopy were examined in the intermetallic compounds (IMCs) and bump joint profiles at the interface between solder and UBM. In 900 h aging experiments, the maximum shear strength of Sn–97wt.%Pb and Sn–37wt.%Pb decreased by 25% and 20%, respectively. The growth of Cu₆Sn₅ and Cu₃Sn was ascertained by the aging treatment. The crack path changes from the interior of a solder to the IMC interface. Compare with the Cu–Sn IMC, the amount of Ni–Sn IMC was small. The Ni layer is considered as the diffusion barrier.     

Microstructural evolution of a lead-free solder alloy Sn-Bi-Ag-Cu prepared by mechanical alloying during thermal shock and aging

In a previous study, a lead-free solder, Sn-6Bi-2Ag-0.5Cu, was developed by mechanical alloying. The alloy shows great potential as a lead-free solder system. In the present work, the microstructural evolution during thermal shock and aging was examined. In the as-soldered joints small bismuth (1 μm to 2 μm) and Ag₃Sn (1 μm) particles were finely dispersed in a nearly pure tin matrix with a small amount of η-Cu₆Sn₅ phase in the bulk of solder. During thermal shock and aging microstructural evolution occurred with Cu-Sn intermetallic compound (IMC) layer growth at interface, bismuth phase coarsening and Ag₃Sn phase coarsening. The microstructure of the solder appeared to be stable at high temperature. The shear strength of the present solder joint is higher than that of Sn-37Pb and Sn-3.5Ag solders. Shear failure occurred Cu-Sn IMC layer-solder interface and in the bulk of solder.     

Influence of current density on mechanical reliability of Sn-3.5Ag BGA solder joint

In this study, we investigated the effect of the current density on the interfacial reaction and mechanical reliability of an electroless Ni/immersion Au (ENIG) substrate with Sn-3.5Ag solder. We first evaluated the interfacial reactions of the solder joint under aging for up to 800 h and current stressing with current densities of 3 × 10² A/cm² and 5 × 10³ A/cm². Also, we successfully revealed the correlation between the interfacial reaction behavior and mechanical reliability under current stressing. With increasing aging time, the thickness of the Ni₃Sn₄ layer increased. At both low and high current densities, the thickness of the Ni₃Sn₄ layer increased up to 400 h and decreased thereafter at the cathode, while that of the IMC increased up to 800 h at the anode. After the die shear test, the ductile fracture was observed in the as-reflowed joint without current stressing. The fracture mode changed from ductile fracture to brittle fracture when thermal aging and current flow were simultaneously applied. The combination of the current stressing and isothermal aging at high temperature significantly deteriorated the mechanical reliability of the solder joint.     

Sn3.8Ag0.7Cu和Sn37Pb焊点界面显微结构研究

利用扫描电子显微镜（SEM）和透射电子显微镜（TEM）研究了Sn3.8Ag0.7Cu（Sn37Pb）/Cu焊点在时效过程中的界面金属间化合物（IMC）形貌和成份。结果表明：150℃高温时效50、100、200、500h后,Sn3.8Ag0.7Cu（Sn37Pb）/Cu焊点界面IMC尺寸和厚度增加明显,IMC颗粒间的沟槽越来越小。50h时效后界面出现双层IMC结构,靠近焊料的上层为Cu6Sn5,邻近基板的下层为Cu3Sn。之后利用透射电镜观察了Sn37Pb/Ni和Sn3.8Ag0.7Cu/Ni样品焊点界面,结果显示,焊点界面清晰,IMC晶粒明显。     

Investigation of interfacial reaction between Au-Sn solder and Kovar for hermetic sealing application

The microstructural evolution and interfacial reactions of Au/Sn/Au/Au/Ni/Kovar joint were investigated during aging at 180 and 250 °C for up to 1000 h. The Au/Sn combination formed a rapid diffusion system. Even in non-annealed joint, three phases such as AuSn, AuSn₂ and AuSn₄ were formed. After initial aging at 180 °C, the AuSn, AuSn₂, AuSn₄, Au and Sn phases, which were formed after plating, were fully transformed into ζ-phase and δ-phase, and (Ni, Au)₃Sn₂ intermetallic compound (IMC) layer was observed between the ζ-phase and Kovar. As a whole, the microstructure of the joint was stable during aging at 180 °C. On the other hand, the solid-state interfacial reaction was much faster at 250 °C than at 180 °C. During aging at 250 °C, the Ni layer on the Kovar reacted primarily with the δ-phase in the solder, resulting in the formation and growth of the (Au, Ni)Sn IMC layer at the interface. After aging for 48 h, the Fe-Co-Ni-Au-Sn phase was formed underneath the (Au, Ni)Sn IMC layer. Furthermore, cracks were observed inside the interfacial layers after complete consumption of the Ni layer. The study results clearly demonstrate the need for either a thicker Ni layer or an alternative surface finish on Ni, in order to ensure the high temperature reliability of the Au/Sn/Au/Au/Ni/Kovar joint above 250 °C.     

The influence of solder composition on the impact strength of lead-free solder ball grid array joints

This study aims to investigate the shear and tensile impact strength of solder ball attachments. Tests were conducted on Ni-doped and non-Ni-doped Sn-0.7wt.% Cu, Sn-37wt.% Pb and Sn-3.0wt.% Ag-0.7wt.% Cu solder ball grid arrays (BGAs) placed on Cu substrates, which were as-reflowed and aged, over a wide range of displacement rates from 10 to 4000 mm/s in shear and from 1 to 400 mm/s in tensile tests. Ni additions to the Sn-0.7wt.% Cu solders has slowed the growth of the interface intermetallic compounds (IMCs) and made the IMC layer morphology smooth. As-reflowed Ni-doped Sn-0.7wt.% Cu BGA joints show superior properties at high speed shear and tensile impacts compared to the non-Ni-doped Sn-0.7wt.% Cu and Sn-3.0wt.% Ag-0.7wt.% Cu BGAs. Sn-3.0wt.% Ag-0.7wt.% Cu BGAs exhibit the least resistance in both shear and tensile tests among the four compositions of solders, which may result from the cracks in the IMC layers introduced during the reflow processes.     

Interfacial behaviors of Sn-Pb,Sn-Ag-Cu Pb-free and mixed Sn-Ag-Cu/Sn-Pb solder joints during electromigration

SnPb-SnAgCu mixed solder joints with Sn-Pb soldering Sn-Ag-Cu Pb-free components are inevitably occurred in the high reliability applications. In this study, the interfacial behaviors in Sn-37Pb and Sn-3.0Ag-0.5Cu mixed solder joints was addressed and compared with Sn-37Pb solder joints and Sn-3.0Ag-0.5Cu solder joints with the influence from isothermal aging and electromigration. Considering the difference on the melting point between Sn-3.0Ag-0.5Cu and Sn-37Pb solder, two mixed solder joints: partial mixing and full mixing between Sn-Pb and Sn-Ag-Cu solders were reached with the peak reflowing temperature of 190 and 250 °C, respectively. During isothermal aging, the intermetallic compound (IMC) layer increased with aging time and its growth was diffusion controlled. There was also no obvious affect from the solder composition on IMC growth. After electromigration with the current density of 2.0 × 10³ A/cm², Sn-37Pb solder joints showed the shortest lifetime with the cracks observed at the cathode for the stressing time < 250 h. In Sn-3.0Ag-0.5Cu Pb-free solder joints, current stressing promoted the growth of IMC layer at the interfaces, but the growing rate of IMC at the anode interface was far faster than that at the cathode interface. Therefore, there existed an obvious polarity effect on IMC growth in Sn-Ag-Cu Pb-free solder joints. After Sn-37Pb was mixed with Sn-3.0Ag-0.5Cu Pb-free solder, whether the partial mixing or the full mixing between Sn-Pb and Sn-Ag-Cu can obviously depress both the crack formation at the cathode side and the IMC growth at the anode.     

Effects of Cu on the interfacial reactions between Sn–8Zn–3Bi–xCu solders and Cu substrate

The microstructure, thermal property, and interfacial reaction with Cu substrate of Sn–8Zn–3Bi–xCu (x = 0, 0.5, 1) lead-free solders were investigated in this work. Cu–Zn intermetallics formed in the solder matrix and the melting temperature increases slightly with Cu addition. After soldering at 250 °C for 90 s, a flat Cu₅Zn₈ layer and a scallop CuZn₅ layer formed at the interfaces of all samples. The CuZn₅ intermetallic compound (IMC) transformed to Cu₅Zn₈ IMC with longer reaction time due to the diffusion of Cu atoms from Cu substrate. The interfacial IMC layer grew thicker with the reaction time following a parabolic law which suggested the interfacial reactions were diffusion controlled. The calculation results show that the activation energy of IMC growth for Cu-containing solders is larger than that of Sn–8Zn–3Bi solder, which demonstrated that a small amount of Cu addition to the solder can effectively suppressed the growth of the interfacial IMC.     

Electromigration behavior in Cu/Sn-8Zn-3Bi/Cu solder joint

Electromigration behavior in a one-dimensional Cu/Sn-8Zn-3Bi/Cu solder joint structure was investigated in ambient with a current density of 3.5 × 10⁴ A/cm² at 60 °C. Due to the compressive stress induced by volume expansion resulting from Cu-Zn intermetallic compound (IMC) growth, Cu₅Zn₈ IMC layers were squeezed out continuously along IMC/Cu interfaces at both the anode and the cathode with increasing the current stressing time, which was not only driven by the concentration gradient, but also accelerated by the electromigration. And a few voids propagated and formed at the anode and the cathode solder/IMC interfaces during electromigration. Additionally, Sn hillocks occurred in the bulk solder, and Sn hillocks formed at the anode side were larger than those at the cathode side.     

Mechanical reliability evaluation of Sn-37Pb solder joint using high speed lap-shear test

This study utilized a high speed lap-shear test to evaluate the mechanical behavior of Sn-37Pb/Cu under bump metallization (UBM) solder joints under high speed loading and hence the drop reliability. The samples were aged for 120 h at different temperatures (150 °С, 180 °С) and then tested at different displacement rates in the range of 0.01 mm/s to 500 mm/s to examine the effects of aging on the drop reliability. The combination of the stress-strain graphs captured from the shear tests and the fracture morphology analysis discloses that the aging at high temperatures has influenced critically the deformation behavior of the solder joints and the effects appears more significant at high strain rates. This study demonstrates a unique capability of a drop reliability evaluation method that utilizes a high speed lap-shear test.     

Morphology and mechanical properties of intermetallic compounds in SnAgCu solder joints

This paper presents the studies to determine hardness and elastic modulus of intermetallic compound (IMC) layers in lead-free solder joints using nanoindentation technique. Two types of surface finishes, i.e., organic solderability preservative (OSP) and electrolytic Ni/Au on Cu pad, with Sn3.5Ag0.5Cu solder balls of 330 μm in diameter are studied, and the intermetallic layers are identified to be Cu₆Sn5, Cu₃Sn and (Ni_y,Cu_1−y)₃Sn₄. The thicknesses of these IMC layers are only few microns at reflowed conditions (less than 2.3 μm). Therefore, probing mechanical properties of thinner IMCs using nanoindentation techniques poses immense difficulties and challenges. In this study, taper-mounted samples are used rather than standard cross-sectional mounted for solder joints. This taper sample gives a larger area for nanoindentation measurements. The elastic modulus and hardness of IMC layers are determined based on the parameter P/S² (load/stiffness²) as a function of the indentation depth to minimise the effects of underlying UBM or solder materials. The modulus of Cu₆Sn₅, Cu₃Sn, (Cu_x,Ni_1−x)₆Sn₅ and (Ni_y,Cu_1−y)₃Sn₄ layer are found to be 112.0 ± 5.1 GPa, 135.5 ± 4.3 GPa, 165.0 ± 11.3 GPa and 136.8 ± 5.8 GPa; whereas the hardness values are found to be 6.8 ± 0.4 GPa, 6.6 ± 0.5 GPa, 7.2 ± 0.9 GPa and 8.2 ± 1.0 GPa, respectively. Thus, the IMC layers in the order of increasing hardness and modulus are found to be Cu₆Sn₅, Cu₃Sn, (Cu_x,Ni_1−x)₆Sn₅ and (Ni_y,Cu_1−y)₃Sn₄.     

Shear strength and fracture surface analysis of Sn58Bi/Cu solder joints under a wide range of strain rates

The influences of the strain rate on the shear strength and failure mode of Sn58Bi/Cu solder joints were investigated. After reflowing, some Kirkendall voids were observed at the neighborhood of the Cu₃Sn/Cu interface or in the inner Cu₃Sn layer. In addition, another type of void could also be observed inside the Sn58Bi eutectic solders, and its size was much larger than that of Kirkendall voids. Some Bi particles were obviously found to segregate at the interface between the Cu-Sn IMC and the Sn58Bi solder. The single lap shear test results indicated that the strain rate had an important influence on the shear strength and failure mode of Sn58Bi/Cu solder joints. The shear strength of joints demonstrated increment at first and then decrement as the strain rate increased from 3.33 × 10⁻⁴ s⁻¹ to 3.33 s⁻¹. It was observed that all Sn58Bi/Cu solder joints broke in a mixed-type fracture mode under a wide range of strain rates. Additionally, more broken IMC grains were exposed on the fracture face and more fracture occurred within the IMC layer with increasing strain rate. Furthermore, the fracture path gradually moved from the solder side to the inner IMC side as the strain rate increased.