Empirical correlation between package-level ball impact test and board-level drop reliability

Package-level ball impact test and board-level drop test are performed and correlated using a specific chip-scale package with solder joints of different Sn–Ag–Cu solder compositions. A positive correlation is found between characteristics of the impact force profile and reliability from the drop test, which provides a supporting basis for the package-level ball impact test to serve as a substitute of the timely and costly board-level drop test.     

Experimental studies of board-level reliability of chip-scale packages subjected to JEDEC drop test condition

We investigate in this paper board-level drop reliability of chip-scale packages subjected to JEDEC drop test condition B, which features an impact pulse profile with a peak acceleration of 1500G and a pulse duration of 0.5 ms. Effects of Sn–Ag–Cu or Sn–Pb solder joint compositions, fluxes, and substrate pads with Ni/Au surface finish or OSP coating on the drop reliability of the board-level test vehicle are compared. Locations and modes of the failed solder joints are examined using the dye stain test. The results indicate that solder joints with a low Ag weight content and substrate pads with OSP coating both enhance the drop resistance of the board-level test vehicle.     

Ball Impact Responses of Ni- or Ge-Doped Sn-Ag-Cu Solder Joints

In this work, we present ball impact test (BIT) responses and fractographies obtained at an impact velocity of 500 mm/s on Sn-4Ag-0.5Cu, Sn-1Ag-0.5Cu, Sn-1Ag-0.5Cu-0.05Ni, Sn-1.2Ag-0.5Cu-0.05Ni, and Sn-1Ag-0.5Cu-0.05Ge package-level solder joints. The solder joints are bonded on substrate pads of either immersion tin (IT) or direct solder on pad (DSOP) surface finishes. Differences of BIT results with respect to multi-reflow are also reported. Taking the impact energy as an indication of board-level drop reliability of the solder joints, the BIT results indicate that better reliability can be achieved by adopting Sn-Ag-Cu solder alloys with low Ag weight contents as well as IT substrate pad finish rather than DSOP. Moreover, the addition of Ni or Ge to the solder alloy provides a large improvement; Ni alters the interfacial intermetallic compound (IMC) structure while Ge enhances the mechanical behavior of the bulk solder.     

Insights Into Correlation Between Board-Level Drop Reliability and Package-Level Ball Impact Test Characteristics

The ball impact test (BIT) is developed based on the demand of a package-level measure for the board-level drop reliability of solder joints in the sense that it leads to fracturing of solder joints around the intermetallics, similar to that from a board-level drop test. In this paper, both board-level drop test and package-level ball impact test are examined numerically for solder joints of different Sn-Ag-Cu compositions. We propose a stress-based drop reliability index that involves the strength of intermetallics and the maximum interfacial normal stress the solder joints have experienced during the drop impact process. Correlations between the drop reliability index and BIT characteristics are found to be dependent on solder compositions. However, the correlations appear to be universal, i.e., independent of solder compositions, when certain constant multipliers are introduced     

Effects of fourth alloying additive on microstructures and tensile properties of Sn–Ag–Cu alloy and joints with Cu

The effects of the fourth elements, i.e., Fe, Ni, Co, Mn and Ti, on microstructural features, undercooling characteristics, and monotonic tensile properties of Sn–3 wt.%Ag–0.5 wt.%Cu lead-free solder were investigated. All quaternary alloys basically form third intermetallic compounds in addition to fine Ag₃Sn and Cu₆Sn₅ and exhibit improved solder structure. The precipitates of Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Ni, Ti and Mn) alloy are very fine comparing with the other alloys. The effective elements for suppressing undercooling in solidification are Ti, Mn, Co and Ni. All quaternary bulk alloys exhibit similar or slightly larger tensile strengths; especially Mn and Ni can improve elongation without degrading strength. The interfacial phases of Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Fe, Mn and Ti)/Cu joints are typical Cu₆Sn₅ scallops. Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Ni and Co)/Cu joints form very fine Sn–Cu–Ni and Sn–Cu–Co scallops at interface. The Cu/Sn–3Ag–0.5Cu–0.1Ni/Cu joint exhibits improved tensile strength prior to thermal aging at 125 and 150 °C. The fracture surface of Cu/Sn–3Ag–0.5Cu/Cu joint exhibits mixture of ductile and brittle fractures, while Cu/Sn–3Ag–0.5Cu (–0.1X; X=Ni and Co)/Cu joints exhibit only brittle fracture at interface. The Sn–3Ag–0.5Cu–0.1Ni alloy is more reliable solder alloy with improved properties for all tests in the present work.     

Electromigration of Sn–37Pb and Sn–3Ag–1.5Cu/Sn–3Ag–0.5Cu composite flip–chip solder bumps with Ti/Ni(V)/Cu under bump metallurgy

We examine electromigration fatigue reliability and morphological patterns of Sn–37Pb and Sn–3Ag–1.5Cu/Sn–3Ag–0.5Cu composite solder bumps in a flip–chip package assembly with Ti/Ni(V)/Cu UBM. The flip–chip test vehicle was subjected to test conditions of five combinations of applied electric currents and ambient temperatures, namely, 0.4 A/150 °C, 0.5 A/150 °C, 0.6 A/125 °C, 0.6 A/135 °C, and 0.6 A/150 °C. The electrothermal coupling analysis was employed to investigate the current crowding effect and maximum temperature in the solder bump in order to correlate with the experimental electromigration reliability using the Black’s equation as a reliability model. From available electromigration reliability models, we also present a comparison between fatigue lives of Sn–37Pb solder bumps with Ti/Ni(V)/Cu and those with Al/Ni(V)/Cu UBM under different current stressing conditions.     

Effects of solder alloy constitutive relationships on impact force responses of package-level solder joints under ball impact test

The ball impact test was developed as a package-level measure for the board-level drop reliability of solder joints in the sense that it leads to fracturing of solder joints around intermetallics, similar to that from a board-level drop test. We investigated numerically the effects of constitutive relationships of solder alloy on transient structural responses of a single package-level solder joint subjected to ball impact testing. This study focused on the characteristics of the ascending part of the impact force profile. According to the piecewise linear stress-strain curve obtained for the Sn-4Ag-0.5Cu solder alloy, parametric studies were performed by varying either segmental moduli or characteristic stresses of the curve at fixed ratios, with regard to the lack of available rate-dependent material properties of solder alloys.     

Studies on solder bump electromigration in Cu/Sn–3Ag–0.5Cu/Cu system

This paper aims to understand the solder bump electromigration phenomenon in the Cu/Sn–3Ag–0.5Cu/Cu system. A temperature of 453 K with a current density of 10 kA/cm² was applied. A void nucleated at the highest current density point at the cathode. As the void grew along the cathode side, a solder depletion occurred on the opposite side of the electron entry point, resulting in an open failure. A unique purposely-designed 3D model simulation methodology provides a good understanding of the void nucleation and growth behavior. The temperature of the solder joint during the electromigration test was measured successfully by the resistance change in the junction line between the two joints.     

Effects of process conditions on reliability, microstructure evolution and failure modes of SnAgCu solder joints

In this study, microstructure evolution at intermetallic interfaces in SnAgCu solder joints of an area array component was investigated at various stages of a thermal cycling test. Failure modes of solder joints were analyzed to determine the effects of process conditions on crack propagation. Lead-free printed-circuit-board (PCB) assemblies were carried out using different foot print designs on PCBs, solder paste deposition volume and reflow profiles. Lead-free SnAgCu plastic-ball-grid-array (PBGA) components were assembled onto PCBs using SnAgCu solder paste. The assembled boards were subjected to the thermal cycling test (−40 °C/+125 °C), and crack initiation and crack propagation during the test were studied. Microstructure analysis and measurements of interface intermetallic growth were conducted using samples after 0, 1000, 2000 and 3000 thermal cycles. Failures were not found before 5700 thermal cycles and the characteristic lives of all solder joints produced using different process and design parameters were more than 7200 thermal cycles, indicating robust solder joints produced with a wide process window. In addition, the intermetallic interfaces were found to have Sn–Ni–Cu. The solder joints consisted of two Ag–Sn compounds exhibiting unique structures of Sn-rich and Ag-rich compounds. A crystalline star-shaped structure of Sn–Ni–Cu–P was also observed in a solder joint. The intermetallic thicknesses were less than 3 μm. The intermetallics growth was about 10% after 3000 thermal cycles. However, these compounds did not affect the reliability of the solder joints. Furthermore, findings in this study were compared with those in previous studies, and the comparison proved the validity of this study.     

Properties of low temperature Sn–Ag–Bi–In solder systems

The metallurgical and mechanical properties of Sn–3.5 wt%Ag–0.5 wt%Bi–xwt%In (x = 0–16) alloys and of their joints during 85 °C/85% relative humidity (RH) exposure and heat cycle test (−40–125 °C) were evaluated by microstructure observation, high temperature X-ray diffraction analysis, shear and peeling tests. The exposure of Sn–Ag–Bi–In joints to 85 °C/85%RH for up to 1000 h promotes In–O formation along the free surfaces of the solder fillets. The 85°C/85%RH exposure, however, does not influence the joint strength for 1000 h. Comparing with Sn–Zn–Bi solders, Sn–Ag–Bi–In solders are much stable against moisture, i.e. even at 85 °C/85%RH. Sn–Ag–Bi–In alloys with middle In content show severe deformation under a heat cycles between −40 °C and 125 °C after 2500 cycles, due to the phase transformation from β-Sn to β-Sn + γ-InSn₄ or γ-InSn₄ at 125 °C. Even though such deformation, high joint strength can be maintained for 1000 heat cycles.     

Characteristics of Sn8Zn3Bi solder joints and crack resistance with various PCB and lead coatings

Reliability of QFP (quad flat package) solder joints after thermal shock was investigated for PCB’s and connecting leads plated with several different alloy coatings before soldering. Sn–8 wt%Zn–3 wt%Bi (hereafter, Sn–8Zn–3Bi) was selected as a solder, and FR-4 PCB’s finished with Cu/Sn, Cu/OSP and Cu/Ni/Au were used as substrates. The leads of the QFP were Cu plated with Sn–10 wt%Pb, or Sn, or Sn–3 wt%Bi. The QFP chips were mounted on the substrates using a Sn–8Zn–3Bi solder paste and reflowed in air atmosphere. The pull strength and microstructure for the soldered leads of QFP were evaluated before and after thermal shock testing. The leads plated with Sn or Sn–3Bi showed approximately 40–50% higher pull strength than the reference value of a Sn–37%Pb solder joint for all PCB-finishes. However, in the case of leads coated with Sn–10Pb, the pull strength of the leads soldered to a Sn-finished PCB was 21% lower than the reference value. In microstructure analysis of the joints with Sn–10Pb-plated leads, cracks were found along the bonded interface for Sn-finished PCB. The cracks were believed to start from the low melting temperature phase, 49.38 wt% Pb–32.58 wt%Sn–18.03 wt%Bi, found around the crack, and then propagated through Cu–Zn intermetallic compound. Meanwhile, even when using Sn–10Pb-plated leads, the PCB’s finished with Cu/Ni/Au coating had about 30% higher strength than the reference value, and cracks were hardly found on the soldered joint. Thus, even with Sn–10Pb-plated leads the Cu/Ni/Au-finished PCB’s were evaluated to be as reliable as the reference joint.     

Effect of Ag micro-particles content on the mechanical strength of the interface formed between Sn–Zn binary solder and Au/Ni/Cu bond pads

In this study, addition of Ag micro-particles with a content in the range between 0 and 4 wt.% to a Sn–Zn eutectic solder, were examined in order to understand the effect of Ag additions on the microstructural and mechanical properties as well as the thermal behavior of the composite solder formed. The shear strengths and the interfacial reactions of Sn–Zn micro-composite eutectic solders with Au/Ni/Cu ball grid array (BGA) pad metallizations were systematically investigated. Three distinct intermetallic compound (IMC) layers were formed at the solder interface of the Au/electrolytic Ni/Cu bond pads with the Sn–Zn composite alloys. The more Ag particles that were added to the Sn–Zn solder, the more Ag–Zn compound formed to thicken the uppermost IMC layer. The dissolved Ag–Zn IMCs formed in the bulk solder redeposited over the initially formed interfacial Au–Zn IMC layer, which prevented the whole IMC layer lifting-off from the pad surface. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces.     

The performance and fracture mechanism of solder joints under mechanical reliability test

The drop resistance and fracture behavior of Sn–37Pb, Sn–3.0Ag–0.5Cu (SAC305), Sn–1.0Ag–0.5Cu (SAC105), and Sn–8.5Zn–0.5Ag–0.01Al–0.1 Ga (SnZn-5e) solder ball joints under the board-level drop test (BLDT) and the ball impact test (BIT) were studied. The results show that the drop reliabilities in terms of the characteristic life ratio from the Weibul plot are SnZn-5e : Sn–37Pb:SAC105:SAC305 = 3.1:2.9:2.1:1. It was observed that failure of Sn–37Pb occurred at the eutectic tin–lead phase whereas it took place at the brittle interface between the (Cu,Ni)₆Sn₅ inter-metallic compound and Ni layer in SAC305. The failure of SAC105 was found to be located within the solder matrix as well as at the interface of the inter-metallic compound. The failure of SnZn-5e depends on the morphology of the interfacial inter-metallic compound. The failure modes of Sn–37Pb and SAC305 after the BIT were similar to those after the BLDT. The maximum impact force (F_max) and the initial fracture energy (E) from the BIT can be used to evaluate the drop reliability of solder joints.     

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.     

Influence of trace alloying elements on the ball impact test reliability of SnAgCu solder joints

In this work, we present ball impact test (BIT) responses and fracture modes obtained at an impact velocity of 0.8 m/s on SAC (Sn–Ag–Cu) package-level solder joints with a trace amount of Mn or RE (rare earth) additions, which were bonded with substrates of OSP Cu and electroplated Ni/Au surface finishes respectively. With respect to the as-mounted conditions, the Ni/Au joints possessed better impact fracture resistance than those with Cu substrate. Subsequent to aging at 150 °C for 800 h, multi-layered intermetallic compounds emerged at the interface of the Ni/Au joints and gave rise to degradation of the BIT properties. This can be prevented by RE doping, which is able to inhibit the growth of interfacial IMCs during aging. As for aged Cu joints, the Mn-doped samples showed the best performance in impact force and toughness. This was related to the hardened Sn matrix, and most importantly, a greater Cu₃Sn/Cu₆Sn₅ thickness ratio at the interface. Compared to Cu₆Sn₅, Cu₃Sn with a similar hardness but greater elastic modulus possessed better plastic ability, which was beneficial to the reliability of solder joints suffering high strain rate deformation if no excess Kirkendall voids formed.     

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.     

Drop impact reliability testing for lead-free and lead-based soldered IC packages

Board-level drop impact testing is a useful way to characterize the drop durability of the different soldered assemblies onto the printed circuit board (PCB). The characterization process is critical to the lead-free (Pb-free) solders that are replacing lead-based (Pb-based) solders. In this study, drop impact solder joint reliability for plastic ball grid array (PBGA), very-thin quad flat no-lead (VQFN) and plastic quad flat pack (PQFP) packages was investigated for Pb-based (62Sn–36Pb–2Ag) and Pb-free (Sn–4Ag–0.5Cu) soldered assemblies onto different PCB surface finishes of OSP (organic solderability preservative) and ENIG (electroless nickel immersion gold). The Pb-free solder joints on ENIG finish revealed weaker drop reliability performance than the OSP finish. The formation of the brittle intermetallic compound (IMC) Cu–Ni–Sn has led to detrimental interfacial fracture of the PBGA solder joints. For both Pb-based and Pb-free solders onto OSP coated copper pad, the formation of Cu₆Sn₅ IMC resulted in different failure sites and modes. The failures migrated to the PCB copper traces and resin layers instead. The VQFN package is the most resistant to drop impact failures due to its small size and weight. The compliant leads of the PQFP are more resistant to drop failures compared to the PBGA solder joints.     

Experimental characterization and mechanical behavior analysis of intermetallic compounds of Sn–3.5Ag lead-free solder bump with Ti/Cu/Ni UBM on copper chip

Due to today’s trend towards ‘green’ products, the environmentally conscious manufacturers are moving toward lead-free schemes for electronic devices and components. Nowadays the bumping process has become a branch of the infrastructure of flip chip bonding technology. However, the formation of excessively brittle intermetallic compound (IMC) between under bump metallurgy (UBM)/solder bump interface influences the strength of solder bumps within flip chips, and may create a package reliability issue. Based on the above reason, this study investigated the mechanical behavior of lead-free solder bumps affected by the solder/UBM IMC formation in the duration of isothermal aging. To attain the objective, the test vehicles of Sn–Ag (lead-free) and Sn–Pb solder bump systems designed in different solder volumes as well as UBM diameters were used to experimentally characterize their mechanical behavior. It is worth to mention that, to study the IMC growth mechanism and the mechanical behavior of a electroplated solder bump on a Ti/Cu/Ni UBM layer fabricated on a copper chip, the test vehicles are composed of, from bottom to top, a copper metal pad on silicon substrate, a Ti/Cu/Ni UBM layer and electroplated solder bumps. By way of metallurgical microscope and scanning-electron-microscope (SEM) observation, the interfacial microstructure of test vehicles was measured and analyzed. In addition, a bump shear test was utilized to determine the strength of solder bumps. Different shear displacement rates were selected to study the time-dependent failure mechanism of the solder bumps. The results indicated that after isothermal aging treatment at 150 °C for over 1000 h, the Sn–Ag solder revealed a better maintenance of bump strength than that of the Sn–Pb solder, and the Sn–Pb solder showed a higher IMC growth rate than that of Sn–Ag solder. In addition, it was concluded that the test vehicles of copper chip with the selected Ti/Cu/Ni UBMs showed good bump strength in both the Sn–Ag and Sn–Pb systems as the IMC grows. Furthermore, the study of shear displacement rate effect on the solder bump strength indicates that the analysis of bump strength versus thermal aging time should be identified as a qualitative analysis for solder bump strength determination rather than a quantitative one. In terms of the solder bump volume and the UBM size effects, neither the Sn–Ag nor the Sn–Pb solders showed any significant effect on the IMC growth rate.     

Growth mechanism of intermetallic compounds and damping properties of Sn–Ag–Cu-1 wt% nano-ZrO2 composite solders

Nano-sized, nonreacting, noncoarsening ZrO₂ ceramic particles reinforced Sn–Ag–Cu composite solders were prepared by mechanically dispersing nano-particles into Sn–Ag–Cu solder and investigated their microstructure, kinetic analysis and mechanical properties i.e., shear strength, hardness and high temperature/mechanical damping characteristics. From microstructures evaluation, it was clear that composite solders containing ZrO₂ ceramic nano-particles significantly impact on the formation of intermetallic compounds (IMCs) at their interfaces as well as refined microstructure in the solder ball regions. The growth behavior of IMCs layer at the interfaces in composite solders was lower than that of plain Sn–Ag–Cu solders. Moreover, after long time aging, some microcracks were clearly observed at the interface due to the formation of excessive IMC layer and softening nature of plain Sn–Ag–Cu solder joints.     

Correlation between localized strain and damage in shear-loaded Pb-free solders

A digital image correlation (DIC) algorithm was employed to measure microscopic strain-field evolution in shear-loaded model solder interconnections made out of a number of Sn-based alloys. Four different solder alloys studied were Sn–36Pb–2Ag, Sn–3.8Ag–0.7Cu (SAC), Sn–3.3Ag–3.82Bi, and Sn–8Zn–3Bi. The measured strain fields were correlated with damage observed at the scale of the sample, and at microscopic length scales.Local strain differs significantly from applied global strain and has been shown to depend on the geometry of the samples as well as the microstructure (on a grain level) of the solder.Strain fields in all solder interconnections were found to localize near but not at the solder–substrate interface and along grain boundaries in the solders. The eventual failure path as observed on the scale of the sample (parallel to the two solder–substrate interfaces with a cross-over from one interface to the other somewhere in the connection) showed a good correlation with measured strain fields in all interconnections.In contrast to the similarity on a macroscopic scale, on a microscopic scale the failure mechanisms were observed to be material specific.