Microstructure and creep behaviour of eutectic SnAg and SnAgCu solders

The paper presents creep data, that was gained on specimens of different microstructures. The three specimen types have been flip chip solder joints, pin trough hole solder joints and standard bulk solder specimens. The bulk solder specimen was a dog-bone type specimen (diameter=3 mm, LENGTH=117 mm). The pin trough hole solder joint consisted on a copper wire that was soldered into a hole of a double sided printed circuit board (thickness 1.5 mm). The flip chip solder joint specimen consisted of two silicon chips (4 mm × 4 mm), which were connected by four flip chip joints (one on each corner). SnAg and SnAgCu flip chip bumps (footprint 200 μm × 200 μm, joint height 165–200 μm, centre diameter 90…130 μm) were created by printing solder paste.Constant–load creep tests were carried out on all three specimen types at temperatures between 5 and 70 °C. Creep data was taken for strain rates between 10⁻¹⁰ and 10⁻³ s⁻¹. The specimens were tested in “as cast” condition and after thermal storage.The microstructural properties of the bulk specimens and real solder joints were examined using metallographic sectioning, optical microscopy techniques, and SEM-microprobe analysis. The results of the microstructural analysis were related to the investigated mechanical properties of the solders. Models of SnAg3.5 and SnAg4Cu0.5, that can be used with the ANSYS FEM software package, will be presented.     

Intermetallics Characterization of Lead-Free Solder Joints under Isothermal Aging

Solder interconnect reliability is influenced by environmentally imposed loads, solder material properties, and the intermetallics formed within the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminal plating, board pad plating, and solder materials. These metallurgies react together and form intermetallic compounds (IMCs) that affect the metallurgical bond strength and the reliability of solder joint connections. This study evaluates the composition and extent of intermetallic growth in solder joints of ball grid array components for several printed circuit board pad finishes and solder materials. Intermetallic growth during solid state aging at 100°C and 125°C up to 1000 h for two solder alloys, Sn-3.5Ag and Sn-3.0Ag-0.5Cu, was investigated. For Sn-3.5Ag solder, the electroless nickel immersion gold (ENIG) pad finish was found to result in the lowest IMC thickness compared to immersion tin (ImSn), immersion silver (ImAg), and organic solderability preservative (OSP). Due to the brittle nature of the IMC, a lower IMC thickness is generally preferred for optimal solder joint reliability. A lower IMC thickness may make ENIG a desirable finish for long-life applications. Activation energies of IMC growth in solid-state aging were found to be 0.54 ± 0.1 eV for ENIG, 0.91 ± 0.12 eV for ImSn, and 1.03 ± 0.1 eV for ImAg. Cu₃Sn and Cu₆Sn₅ IMCs were found between the solder and the copper pad on boards with the ImSn and ImAg pad finishes. Ternary (Cu,Ni)₆Sn₅ intermetallics were found for the ENIG pad finish on the board side. On the component side, a ternary IMC layer composed of Ni-Cu-Sn was found. Along with intermetallics, microvoids were observed at the interface between the copper pad and solder, which presents some concern if devices are subject to shock and vibration loading.     

Process windows for low-temperature Au wire bonding

The process windows are presented for low-temperature Au wire bonding on Au/Ni/Cu bond pads of varying Au-layer thicknesses metallized on an organic FR-4 printed circuit board (PCB). Three different plating techniques were used to deposit the Au layers: electrolytic plating, immersion plating, and immersion plating followed by electrolytic plating. Wide ranges of wire bond force, bond power, and bond-pad temperature were used to identify the combination of these processing parameters that can produce good wire bonds, allowing the construction of process windows. The criterion for successful bonds is no peel off for all 20 wires tested. The wire pull strengths and wire deformation ratios are measured to evaluate the bond quality after a successful wire bond. Elemental and surface characterization techniques were used to evaluate the bond-pad surfaces and are correlated to wire bondability and wire pull strength. Based on the process windows along with the pull strength data, the bond-pad metallization and bonding conditions can be further optimized for improved wire bondability and product yields. The wire bondability of the electrolytic bond pad increased with Au-layer thickness. The bond pad with an Au-layer thickness of 0.7 μm displayed the highest bondability for all bonding conditions used. The bondability of immersion bond pads was comparable to electrolytic bond pads with a similar Au thickness. Although a high temperature was beneficial to wire bondability with a wide process window, it did not improve the bond quality as measured by wire pull strength.     

Major factors to the solder joint strength of ENIG layer in FC BGA package

Since electroless nickel and immersion gold (ENIG) process was implemented as the surface finish of printed circuit board (PCB) substrate, there have been lots of reports on the brittle fracture between the Ni–P (phosphorous) layer and solder which results in the poor solder joint strength performance. Galvanic corrosion during immersion Au plating process and P-content in Ni–P layer were considered as major factors in the solder joint strength of ENIG layer in this investigation. The attempt to reduce the galvanic corrosion attack in Ni–P layer was made by changing immersion Au plating process to partial electroless Au plating process. Reducing the galvanic corrosion attack was proved to be effective to improve the solder joint strength of ENIG layer. Evaluation of the solder joint performances in variation with the thickness of the Ni layer leads to the conclusion that the thicker Ni layer has the better solder joint strength performances. The result also showed that higher P-content in Ni layer is more favorable to the solder joint strength.     

Numerical and experimental analysis of the Sn3.5Ag0.75Cu solder joint reliability under thermal cycling

The Sn3.5Ag0.75Cu (SAC) solder joint reliability under thermal cycling was investigated by experiment and finite element method (FEM) analysis. SAC solder balls were reflowed on three Au metallization thicknesses, which are 0.1, 0.9, and 4.0 μm, respectively, by laser soldering. Little Cu–Ni–Au–Sn intermetallic compound (IMC) was formed at the interface of solder joints with 0.1 μm Au metallization even after 1000 thermal cycles. The morphology of AuSn₄ IMC with a small amount of Ni and Cu changed gradually from needle- to chunky-type for the solder joints with 0.9 μm Au metallization during thermal cycling. For solder joints with 4 μm Au metallization, the interfacial morphology between AuSn₄ and solder bulk became smoother, and AuSn₄ grew at the expense of AuSn and AuSn₂. The cracks mainly occurred through solder near the interface of solder/IMC on the component side for solder joints with 0.1 μm Au metallization after thermal shock, and the failure was characterized by intergranular cracking. The cracks of solder joints with 0.9 μm Au metallization were also observed at the same location, but the crack was not so significant. Only micro-cracks were found on the AuSn₄ IMC surface for solder joints with 4.0 μm Au metallization. The responses of stress and strain were investigated with nonlinear FEM, and the results correlated well with the experimental results.     

Reliability evaluation of BOAC and normal pad stacked-chip packaging using low-K wafers

This work evaluates the wire bondability and the reliability tests for the stacked-chip TFBGA wire bond packaging with the Sn–4.0Ag–0.5Cu lead-free solder ball. The bonding-over-active-circuit (BOAC) pad is the top test chip and the normal pad is the bottom test chip and is combined in the stacked-chip packaging. Both test chips are 90 nm low-K dielectric with five copper layers and one layer aluminum pad and a background ranging from 775 μm to 150 μm. According to the simulation results, the maximum normal stress of low-K layer for the BOAC pad is higher than that of the normal pad by 146.4%. However, the maximum shear stress of Cu metal layer for the BOAC pad is lower than that of the normal pad by 64.2%. To compare the bonding pad strength for the BOAC and normal pad low-K wafers, this work uses the simplified two-layer model to extract the effective mechanical properties of the two bonding pad structures. The effective average Young’s modulus of the normal pad and the BOAC pad are 86 GPa and 69 GPa, respectively. The test results indicate that the effective Young’s modulus of the normal pad exceeds that of the BOAC pad by 17 GPa. The wire bondability test of the ball shear and the wire pull test results are superior to the specification by 80% and 83.75%, respectively. All stacked-chip TFBGA packaging samples underwent reliability tests, including HAST, TCT, and HTST. All the wire bondability and reliability tests passed the specification for the BOAC pad and the normal pad low-K structures. Accordingly, this work shows that the proposed stacked-chip TFBGA packaging passes the wire bondability and the reliability tests. The proposed packaging improves the electrical performance, enhances the utility of the active chip area and saves chip area through the use of low-K and BOAC chips. Furthermore, the results show that the equivalent stiffness of the bonding pad structure can be used as the bondability and reliability test index for the chip.     

Solder joint reliability of TFBGA assemblies with fresh and reworked solder balls

In this article, the solder joint reliability of thin and fine-pitch BGA (TFBGA) with fresh and reworked solder balls is investigated. Both package and board level reliability tests are conducted to compare the solder joint performance of test vehicle with fresh and reworked solder balls. For package level reliability test, ball shear test is performed to evaluate the joint strength of fresh and reworked solder balls. The results show that solder balls with rework process exhibit higher shear strength than the ones without any rework process. The results also exhibit that the different intermetallic compound (IMC) formation at solder joints of fresh and reworked solder balls is the key to degradation of shear strength. For board level reliability tests, temperature cycling and bending cyclic tests are both applied to investigate the fatigue life of solder joint with fresh and reworked solder balls. It is observed that package with reworked solder ball has better fatigue life than the one with fresh solder ball after temperature cyclic test. As for bending cyclic test, in addition to test on as-assembled packages, reworked and fresh samples are subjected to heat treatment at 150 °C for 100 h prior to the bending cyclic test. The purpose is to let Au–Ni–Sn IMC resettle at solder joints of fresh solder ball and examine the influence of Au–Ni–Sn IMC on the fatigue life of solder joints (Au embrittlement effect). The final results confirm that reworked solder balls have better reliability performance than fresh one since Au embrittlement dose exist at fresh solder ball.     

Creep of thermally aged SnAgCu-solder joints

The creep behaviour of Sn96.5Ag3.5- and Sn95.5Ag3.8Cu0.7-solder was studied specifically for its dependence on technological and environmental factors. The technological factors considered were typical cooling rates and pad metallizations for solder joints in electronic packaging. The environmental factors included microstructural changes as a result of thermal aging of solder joints. Creep experiments were conducted on three types of specimens—flip–chip joints, PCB solder joints and bulk specimens. flip–chip specimens were altered through the selection of various under bump metallizations (Cu vs. NiAu), cooling rates (40 K/min vs. 120 K/min), and thermal storage (24 h, 168 h, and 1176 h at 125 °C). PCB solder joints were studied by using a copper pin soldered into a thru-hole connection on a printed circuit board having a NiAu metallization. Bulk specimens contained the pure alloys. The creep behaviour of the SnAg and SnAgCu solders varied in dependence of specimen type, pad metallization and aging condition. Constitutive models for SnAg and SnAgCu solders as they depend on the reviewed factors are provided.     

Board level reliability of PBGA using flex substrate

Assurance of board level reliability is necessary and required for adopting any new packages into products. This paper presents board level reliability test results of a flex substrate BGA under thermal and bend cyclic tests. It is well known that solder joint reliability is affected by many factors, such as the size of chip, joint stand-off height, pad design, test board surface finish, substrate gold plating thickness and the utilization of underfill material, etc. However, most of the works have been conducted are BGA on rigid substrates. In this work, thermal cyclic test is performed to re-examine these factors using package housed on a flex substrate. Bending test with two deflections is also performed to investigate solder joint fatigue life and failure modes under mechanically repetitive loading.Two-parameter Weibull model is used to analyze joint fatigue life. Failure analysis is conducted and discussed for each case. Under temperature cycling test, chip size, polyimide thickness and underfill material utilization were found to have significant impacts on joint fatigue life, especially the effect of applying underfill material to the joint. Epoxy thickness was found to have little effect on the joint fatigue life for this case.The effects of test board surface finish and substrate gold plating thickness on the joint fatigue life were found coupled. The term “substrate” here refers to the chip carrier, while the “board” here refers to motherboard, which is the board to assemble test vehicles on. The gold thickness here all refers to the electrolytic gold plating on the substrate. Using organic solderability preservative boards, substrate gold plating thickness affects joint fatigue life slightly, but with Au–Ni test boards, the effect is tremendous. The difference is due to different intermetallic compounds (IMC) formed. In other words, different IMC systems are formed due to different combination of test board surface finish and substrate gold plating thickness. As a result, different IMC induces different failure modes. The joint fatigue life under cyclic bend test with different deflections is also probed and shown. The corresponding failure modes are also discussed.     

Impact of board and component metallizations on microstructure and reliability of lead-free solder joints

Aging and accelerated thermal cycling (ATC) have been performed on 2512 chip resistors assembled with Sn3.8Ag0.7Cu (wt.%) solder. The boards were finished with immersion Ag (IAg), electroless nickel/immersion gold (ENIG), and hot air solder leveling Sn–Pb eutectic solder (HASL), and the components’ terminations were finished with 100% Sn and Sn8.0Pb (wt.%). The boards were reflowed with an average cooling rate of 1.6 °C/s. It was found that the microstructure and reliability of the solder joints depended on the board surface finish. The boards containing small amounts of Pb (from board/component terminations) were the most reliable. Solder joints to copper showed a significantly higher number of cycles to first failure than the joints on nickel. Better reliability of the Sn3.8Ag0.7Cu/Cu joints was attributed to an increased copper content in the bulk due to substrate dissolution.     

板厚影响通孔再流焊点抗热疲劳性能的试验研究

针对不同板厚的通孔再流焊点进行了热冲击的可靠性测试，以非破坏性和破坏性的试验方式，对比分析了板厚对通孔再流焊点的抗热疲劳能力的影响。结果表明，热膨胀系数(CTE)失配是焊点产生裂纹的主要原因，使得板厚严重影响着焊点的抗热疲劳性能：厚板焊点断裂程度重于薄板焊点，其循环后的强度下降也快于后者，但二者的电性能变化差异不大。     

Board level solder joint reliability analysis of a fine pitch Cu post type wafer level package (WLP)

Wafer level packaging (WLP) has many advantages, such as ease of fabrication and reduced fabrication cost. However, solder joint reliability of traditional WLPs is the weakest point of the technology. In this paper, a 0.4 mm pitch Cu post type WLP has been developed for mobile computing application. The Cu post type WLP has 440 I/Os and 12 × 12 mm die size. The initial design WLP has been fabricated and subjected to a thermal cycling (TC) testing. The failure life of the original WLP under TC was 296 cycles. This paper also presents a nonlinear finite element analysis of the board level solder joint reliability and methods for enhancement of the WLP. A viscoplastic constitutive relation is adopted for the solder joints to account for its time and temperature dependence in TC. The fatigue life of the solder joint is estimated by the modified Coffin–Manson equation. The two coefficients in the modified Coffin–Manson equation are also determined. A series of parametric studies are performed by changing the passivation (PI) thickness, redistribution layer (RDL) thickness, polymer height (Cu post height accordingly varies), die thickness, PCB thickness, and PCB CTE. The results obtained from the modeling are useful to formulate design guidelines for board level reliability enhancement of the WLP.     

Reliability and RF performance of BGA solder joints with plastic-core solder balls in LTCC/PWB assemblies

In this paper board-level reliability of low-temperature co-fired ceramic (LTCC) modules with thermo-mechanically enhanced ball-grid-array (BGA) solder joint structure mounted on a printed wiring board (PWB) was experimentally investigated by thermal cycling tests in the 0–100 °C and −40 to 125 °C temperature ranges. The enhanced joint structure comprised solder mask defined (SMD) AgPt pad metallization, eutectic solder and plastic-core solder balls (PCSB). Similar daisy-chained LTCC modules with non-collapsible 90Pb10Sn solder spheres were used for a reference test set. The reliability of the joint structures was analyzed by resistance measurements, X-ray microscopy, scanning acoustic microscopy (SAM) and SEM/EDS investigation. In addition, a full-wave electromagnetic analysis was performed to study effects of the plastic-core material on the RF performance of the LTCC/BGA package transition up to millimeter-wave frequencies. Thermal cycling results of the modules with PCSBs demonstrated excellent fatigue performance over that of the reference. In the harsher cycling test, Weibull’s shape factor β values of 7.9 and 4.8, and characteristic lifetime θ values of 1378 and 783 were attained for the modules with PCSBs and 90Pb10Sn solder spheres, respectively. The primary failure mode in all test assemblies was fatigue cracking in eutectic solder on the ceramic side.     

Thermosonic gold wire bonding to laminate substrates with palladiumsurface finishes

As the laminate substrate industry moves from hot air solder level (HASL) finishes, alternate plating finishes are being proposed such as immersion gold/electroless nickel, electroless palladium, and electroless silver. This paper presents results of an evaluation of the thermosonic gold ball wire bondability of electroless palladium. Two palladium thicknesses, with and without a nickel underlayer, were evaluated from two vendors. In each case, a thin gold passivation layer was deposited by immersion plating over the palladium. The initial evaluation criteria included bondability (number of missed bonds and flame off errors), wire pull strength, standard deviation, bond failure mode, and visual inspection. The bonding window was determined by independently varying force (four levels), power (four levels), and time (two levels). The stage temperature was maintained at 150°C, compatible with BT laminate material. Different preconditioning environments such as a solder reflow cycle, high temperature storage (125°C) and humidity storage (85%RH/85°C) on initial bondability were also considered. Rutherford backscattering and Auger analysis were used to examine the surface finishes. The stability of the bonds was investigated by high temperature storage (125°C) with periodic electrical resistance and pull strength testing     

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.     

Oxidation of copper pads and its influence on the quality of Au/Cu bonds during thermosonic wire bonding process

To understand the copper oxide effect on the bondability of gold wire onto a copper pad, thermosonic gold wire bonding to a copper pad was conducted at 90–200 °C under an air atmosphere. The bondability and bonding strength of the Au/Cu bonds were investigated. The bondability and bonding strength were far below the minimum requirements stated in industrial codes. At elevated bonding temperature of 200 °C, the bondability and bonding strength deteriorated mainly due to hydroxide and copper oxide formation on the copper pad. Oxide formation occurred if no appropriate oxide preventive schemes were applied. At lower bonding temperature, 90 °C, poor bondability and low bonding strength were mainly attributed to insufficient thermal energy for atomic inter-diffusion between the gold ball and copper pad.Copper pad oxidation was investigated using an electron spectroscopy for chemical analysis (ESCA) and thermogravimetric analysis (TGA). An activation energy of 35 kJ/mol for copper pad oxidation was obtained from TGA. This implies that different mechanisms govern the oxidation of copper pad and bulk copper. Hydroxide and copper oxide were identified based on the shifted binding energy. Cu(OH)₂ forms mainly on the top surface of copper pads and the underlying layer consists mainly of CuO. The hydroxide concentration increased with increasing the heating temperatures. After heating at 200 °C, the hydroxide concentration on the copper pad surface was approximately six times that at 90 °C. Protective measures such as passivation layer deposition or using shielding gas are critical for thermosonic wire bonding on chips with copper interconnects.     

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.     

The influence of solder fillet geometry on the occurrence of coronadischarge during operation between 400 and 900 V in partial vacuum

A number of experiments have been made to study the effect that different solder joint designs have on the voltage needed to initiate the onset of corona discharge under low ambient pressures. Wires were joined to both printed circuit board terminations and to electrical feedthroughs. All were constructed from materials known to be suitable for spacecraft. The main factor that determines whether solder joints will be prone to initiate corona discharges or electrical breakdown in a space environment is the actual minimum distance between terminals. Large, rounded solder fillets are desirable, but those containing protrusions or contours around wire strands have little adverse effect. It is shown that it is not always necessary, as is usually required by workmanship specifications related to high voltage connections, for operators to rework solder joints in order to achieve protrusion-free fillets. It would appear from metallurgical analyses of solder joints that it is the tin-rich phase that is sputtered from the fillet's surface during the phenomenon of corona discharge     

The influence of Sn–Cu–Ni(Au) and Sn–Au intermetallic compounds on the solder joint reliability of flip chips on low temperature co-fired ceramic substrates

The formation of intermetallic compounds in the solder joint of a flip chip or chip scale package depends on the under bump metallurgy (UBM), the substrate top surface metallisation, the solder alloy and the application conditions. To evaluate the influence of intermetallic compounds on the solder joint reliability, a detailed study on the influence of the UBM, the gold finish thickness of the substrate top surface metallisation, the solder alloy and the aging conditions has been conducted. Flip chips bumped with different solder alloys were reflow-mounted on low temperature co-fired ceramic substrates. The flip chip package was then aged at high temperature and a bump shear test followed to examine the shear strength of the solder joint at certain aging intervals. It was found that the type of UBM has a great impact on the solder joint reliability. With Ni(P)/Au as the UBM, well-documented gold embrittlement was observed when the gold concentration in the eutectic SnPb solder was about 3 wt%. When Al/Ni(V)/Cu was used as the UBM, the solder joint reliability was substantially improved. Copper dissolution from the UBM into the solder gives different intermetallic formations compared to Ni(P)/Au as UBM. The addition of a small amount of copper in the solder alloy changed the mechanical property of the intermetallic compound, which is attributed to the formation of Sn–Cu–Ni(Au) intermetallic compounds. This could be used in solving the problem of the AuSn₄ embrittlement. The formation and the influence of this Sn–Cu–Ni(Au) intermetallic phase are discussed. The gold concentration in the solder joint plays a role in the formation of intermetallic compounds and consequently the solder joint reliability, especially for the Sn–Ag–Cu soldered flip chip package.     

Single chip bumping and reliability for flip chip processes

Processes of bump deposition based on mechanical procedures together with their reliability data are summarized in this paper. The stud bumping of gold, palladium, and solder is described and also a novel bumping approach for fine pitch solder deposition down to 100 μm pitches using thermosonic bonding on a modified wedge–wedge bonding machine. This wedge bumping doesn’t require a wire flame-off process step. Because of this, no active atmosphere is necessary. The minimum pad diameter which can be bumped using the solder wedge bumping is 50 μm, up to now. This bumping process is highly reproducible and therefore well-suited for different flip chip soldering applications. Palladium stud bumps provide a solderable under bump metallization. Results from aging of lead/tin solder bumps on palladium are shown. The growth of intermetallics and its impact on the mechanical reliability are investigated.