New observations on intermetallic compound formation in gold ball bonds: general growth patterns and identification of two forms of Au4Al

Relatively little information is available on the growth patterns and metallurgy of Au–Al intermetallics in fine-pitch (FP) and ultra-fine pitch (UFP) ball bonding. This paper presents a study of the growth pattern and chemistry of intermetallic compounds formed between a 25 μm 4 N gold wire and aluminium pad metallization after isothermal ageing in air at 175 °C. The data show the intermetallics grow vertically and laterally under the ball and totally consume the Al in the bond pad at <20 h. Then, a third layer of intermetallic grows between Au₄Al and Au₅Al₂. Measurements and observations made with EDX and optical microscopy lead to the conclusion that the new compound is a different form of Au₄Al, most probably a low-temperature version of the α-Au₄Al intermetallic structure. Electrical resistance during intermetallic growth was not measured in this study but wire chemistry and bonding conditions are found to affect the thickness of the intermetallic compounds, which suggests that the resistance of ball bonds during moulding and operation can change.     

High temperature reliability of aluminium wire-bonds to thin film, thick film and low temperature co-fired ceramic (LTCC) substrate metallization

Reliable interconnects are essential for microelectronic systems intended for long life times in harsh environment applications. Intermetallic growth accelerates as the temperature increases, and the material system must be carefully selected to avoid mechanically and/or electrically weak connections. The dominating chip metallization is aluminium, and aluminium wire-bonding is therefore recommended to obtain a mono-metallic system at chip level. A suitable substrate metallization compatible with aluminium wire-bonds at high temperatures (HT) should therefore be found.Test substrates with low temperature co-fired ceramic (LTCC) silver conductors plated with nickel/gold, gold and aluminium thin film, gold thick film, and silver thick film plated with copper/nickel/gold have been manufactured. Wedge/wedge aluminium wire-bonding were performed with 25 μm aluminium wire on the substrates before they were subjected to long term ageing at temperatures up to 250 °C for 6-12 months. Bond-pull strength and electrical resistance were measured during ageing on selected components.The present work shows that long term reliable aluminium wire-bonds for 250 °C operation is feasible both with thin film, thick film and LTCC substrate technology. For the screen-printed conductors, a plating system with nickel is necessary. Aluminium wire bonded to gold thin film displays reliable long term high temperature performance for gold thicknesses up to ∼1 μm.     

Low temperature delamination of plastic encapsulated microcircuits

Plastic encapsulated microcircuits (PEMs) are increasingly being used in applications requiring operation at temperatures lower than the manufacturer’s recommended minimum temperature, which is 0°C for commercial grade components and −40°C for industrial and automotive grade components. To characterize the susceptibility of PEMs to delamination at these extreme low temperatures, packages with different geometries, encapsulated in both biphenyl and novolac molding compounds, were subjected to up to 500 thermal cycles with minimum temperatures in the range −40 to −65°C in both the moisture saturated and baked conditions. Scanning acoustic microscopy revealed there was a negligible increase in delamination at the die-to-encapsulant interface after thermal cycling for the 84 lead PQFPs encapsulated in novolac and for both 84 lead PQFPs and 14 lead PDIPs encapsulated in biphenyl molding compound. Only the 14 lead novolac PDIPs exhibited increased delamination. Moisture exposure had a significant effect on the creation of additional delamination.     

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.     

Environmentally sustainable composite resistors with low temperature coefficient of resistance

Temperature coefficient of resistance (TCR) of thick film resistors are based on fired conducting grains and glass composites. Many analog sensor and control circuits require low (<100 ppm/°C) TCR value. To prepare resistors with low TCR value, knowledge of processing conditions and conduction mechanism parameters are of particular importance because TCR is finalised during firing and cannot be trimmed in the latter stage to a target value as resistors can be. This paper reports the preparation and properties such as microstructural and electrical in particular to sheet resistance, TCR (hot and cold) of eco-friendly composite resistor paste compositions. Our resistor compositions showed the sheet resistance in the range of 1.18–1.38 KΩ/□ and the hot and cold TCR of the compositions reduced substantially from 360 to 100 ppm/°C and 175 to 60 ppm/°C with the addition of TCR modifier.     

Degradation behavior of 600 V–200 A IGBT modules under power cycling and high temperature environment conditions

One challenge for automotive hybrid traction application is the use of high power IGBT modules that can withstand high ambient temperatures, from 90 °C to 120 °C, for reliability purpose. The paper presents ageing tests of 600 V–200 A IGBT modules subjected to power cycling with 60 °C junction temperature swings at 90 °C ambient temperature. Failure modes are described and obtained results on the module characteristics are detailed. Especially, physical degradations are described not only at the package level, like solder attach delaminations, but also at the chip level, with a shift on electrical characteristics such as threshold voltage. Finally, numerical investigations are performed in order to assess the thermal and thermo-mechanical constraints on silicon dies during power cycling and also to estimate the effect of ambient temperature on the mechanical stresses.     

Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection

The effects of different bonding parameters, such as temperature, pressure, curing time, bonding temperature ramp and post-processing, on the electrical performance and the adhesive strengths of anisotropic conductive film (ACF) interconnection are investigated. The test results show that the contact resistances change slightly, but the adhesive strengths increase with the bonding temperature increased. The curing time has great influence on the adhesive strength of ACF joints. The contact resistance and adhesive strength both are improved with the bonding pressure increased, but the adhesive strengths decrease if the bonding pressure is over 0.25 MPa. The optimum temperature, pressure, and curing time ranges for ACF bonding are concluded to be at 180–200 °C, 0.15–0.2 MPa, and 18–25 s, respectively. The effects of different Teflon thickness and post-processing on the contact resistance and adhesive strength of anisotropic conductive film (ACF) joints are studied. It is shown that the contact resistance and the adhesive strength both become deteriorated with the Teflon thickness increased. The tests of different post-processing conditions show that the specimens kept in 120 °C chamber for 30 min present the best performance of the ACF joints. The thermal cycling (−40 to 125 °C) and the high temperature/humidity (85 °C, 85% RH) aging test are conducted to evaluate the reliability of the specimens with different bonding parameters. It is shown that the high temperature/humidity is the worst condition to the ACF interconnection.     

Characterisation and modelling of parasitic effects and failure mechanisms in AlGaN/GaN HEMTs

This work is a study of the degradations of AlGaN/GaN HEMTs induced by 2000 h of ageing tests. The methodology is based on cross-characterisation analysis.The life tests (HTO 150 °C, HTO 175 °C and HTRB 175 °C and Idq 90 °C) have mainly induced a decrease of the saturation drain current, occurring during the first 50 h, followed by a stabilisation. There is a shift of the pinch-off voltage in the range of 0.1–0.2 V while the Schottky contact is rather stable after ageing. The evolution of the electrical characteristics after ageing does not depend on the bias conditions but rather more on the channel temperature. It seems to be neither field nor current driven. Low frequency drain current noise demonstrates that there is no trap creation and the weak evolution of the 1/f noise confirms that there is no degradation in the channel. Moreover, pulsed I–V measurements show a weak evolution of gate lag and drain lag rates after ageing. The same degradation mode is demonstrated for all life tests with rather high activation energy of 1.6 eV. The weak evolution of electrical characteristics observed during the life tests cannot be obviously explained by a single physical mechanism and results from a combination of trap-related effects before stabilisation.     

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.     

Optical dispersion analysis within the IR range of thermally grown and TEOS deposited SiO2 films

SiO₂ thin films, with thickness ranging between approximately 13 and 95 nm, have been thermally grown at 950°C in dry oxygen and chemically vapor deposited at low pressures (0.3 Torr) by decomposition of tetraethylorthosilicate (TEOS) at 710°C, on Si (100) substrates. Dispersion analysis was performed on Fourier transform infrared (FTIR) transmission spectra of these films within the range 900–1400 cm⁻¹. It was found that the spectra were best described within this range, by four Lorentz oscillators located near 1060, 1089, 1165 and 1220 cm⁻¹ almost independent of film thickness. The polarization of the oscillators (proportional to their strength) was found to increase slightly, and their widths to decrease, with film thickness. From the study of the FTIR spectra obtained at room temperature, it was suggested that at this temperature, a considerable number of Si–O–Si angles in these SiO₂ films are distributed in a way expected at higher temperatures and that the distribution of the Si–O–Si angles depends on the thermal history of the film and the method of growth.     

Lifetime of power electronics interconnections in accelerated test conditions: High temperature storage and thermal cycling

We investigate the effect of three testing conditions (thermal shock, Rapid Temperature Change – RTC – and high temperature storage) on the interconnects of a power electronic module. In particular, the mechanical strength of thick aluminium wirebonds is investigated and shows that while it is not affected by storage at 230 °C, it is much more sensitive to thermal cycling. Shock tests are found to be especially severe, despite having a smaller temperature swing than RTC. Regarding the die attach, no noticeable reduction in mechanical strength is found, regardless of the ageing conditions, and despite clear micro-structural evolutions.     

Effect of primary creep and plasticity in the modeling of thermal fatigue of SnPb and SnAgCu solder joints

It has been conventional to simplify the thermo-mechanical modeling of solder joints by omitting the primary (transient) contributions to total creep deformation, assuming that secondary (steady-state) creep strain is dominant and primary creep is negligible. The error associated with this assumption has been difficult to assess because it depends on the properties of the solder joint and the temperature–time profile. This paper examines the relative contributions of plasticity, primary and secondary creep in Sn40Pb and Sn3.8Ag0.7Cu solders using the analysis of a trilayer solder joint structure with finite elements and a newly developed finite difference technique. The influences of temperature amplitude and ramp rate have been quantified. It was found that for the thermal profiles considered, the role of plasticity was negligible for trilayer assemblies with SnPb and SnAgCu solder interlayers. Furthermore, when primary creep was included for SnAgCu, the temperature-dependent yield strength was not exceeded and no plastic strains resulted. Neglect of primary creep can result in errors in the predicted stress and strain of the solder joint. Damage metrics based on the stabilized stress vs. strain hysteresis loop, for symmetric 5 min upper/lower dwell periods, differ widely when primary creep is considered compared to the secondary-only creep model. Creep strain energy density differences between the secondary-only and primary plus secondary creep models for SnPb were 32% (95 °C/min–Δ165 °C thermal profile), 32% (95 °C/min–Δ100 °C) and 35% (14 °C/min–Δ100 °C); similarly for SnAgCu, the differences were 29% (95 °C/min–Δ165 °C), 46% (95 °C/min–Δ100 °C) and 58% (14 °C/min–Δ100 °C). Accumulated creep strain differences between the secondary-only and primary plus secondary creep models for SnPb were 21% (95 °C/min–Δ165 °C), 25% (95 °C/min–Δ100 °C) and 25% (14 °C/min–Δ100 °C); similarly for SnAgCu the differences were 82% (14 °C/min–Δ100 °C), 89% (95 °C/min–Δ100 °C) and 100% (95 °C/min–Δ165 °C). In turn, these discrepancies can lead to errors in the estimation of the solder thermal fatigue life due to the changing proportion of primary creep strain to total inelastic strain under different thermal profiles, particularly for SnAgCu.     

Effect of microwave preheating on the bonding performance of flip chip on flex joint

A microwave (MW) preheating mechanism of anisotropic conductive adhesive film (ACF) has been introduced in order to reduce the bonding temperature for flip chip technology. Thermal curing of epoxy shows a very sluggish and non-uniform curing kinetics at the beginning of the curing reaction, but the rate increases with time and hence requires higher temperature. On the other hand MW radiation has the advantage of uniform heating rate during the cycle. In view of this, MW preheating (for 2/3 s) of ACF prior to final bonding has been applied to examine the electrical and mechanical performance of the bond. Low MW power has been used (80 and 240 W) to study the effect of the MW preheating. It has been found that 170 °C can be used for flip chip bonding instead of 180 °C (standard temperature for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (0.015–0.025 Ω) is low in these samples where the standard resistance is 0.017 Ω (bonded at 180 °C without prior MW preheating). The shear forces at breakage were satisfactory (152–176 N) for the samples bonded at 170 °C with MW preheating, which is very close and even higher than the standard sample (173.3 N). For MW preheating time of 2 s, final bonding at 160 °C can also be used because of its low contact resistance (0.022–0.032 Ω), but the bond strength (137.3–145 N) is somewhat inferior to the standard one.     

High-temperature reliability of Flip Chip assemblies

Flip Chip technology has been widely accepted within microelectronics as a technology for maximum miniaturization. Typical applications today are mobile products such as cellular phones or GPS devices. For both widening Flip Chip technology’s application range and for addressing the automotive electronics’ volume market, developing assemblies capable of withstanding high temperatures is crucial. A typical scenario for integrating electronics into a car is a control unit within the engine compartment, where ambient temperatures are around 150 °C, package junction temperatures may range from 175 °C to 200 °C and peak temperatures may exceed these values.If Flip Chip technology is used under harsh environment conditions, it is clear that especially the polymeric materials, i.e., underfiller, solder mask or the organic substrate base material, are challenged. Generally, the developmental goal for encapsulants compatible with high-temperature applications are materials with high T_g and low degradation even at temperatures >200 °C.According to these demands, a test group of advanced underfill encapsulants has been used for assembling Flip Chip devices. These test vehicles were built using lead-free and lead-containing solders such as SnAgCu and eutectic PbSn and standard FR4 substrates, for evaluating the reliability potential of state-of-the-art underfillers. Material analysis is performed for studying both material degradation as well as temperature-dependent thermo-mechanical and adhesive properties. For assessing reliability, temperature cycling is performed with different maximum test temperatures ranging from 150 °C to 175 °C. The device status is intermediately analyzed by using electrical measurement for detecting bond integrity and acoustomicroscopy for determining the occurrence and growth of delaminations. Extensive failure analysis is added to investigate device failure mechanisms, especially related to the respective test temperature.In summary, an empirical status of the high-temperature potential of state-of-the-art underfillers and material combinations is attained and an outlook on future demands and developments is provided.     

High temperature reliability and interfacial reaction of eutectic Sn–0.7Cu/Ni solder joints during isothermal aging

The interfacial reactions and growth kinetics of intermetallic compound (IMC) layers formed between Sn–0.7Cu (wt.%) solder and Au/Ni/Cu substrate were investigated at aging temperatures of 185 and 200 °C for aging times of up to 60 days. After reflow, the IMC formed at the interface was (Cu, Ni)₆Sn₅. After aging at 185 °C for 3 days and at 200 °C for 1 day, two IMCs of (Cu, Ni)₆Sn₅ and (Ni, Cu)₃Sn₄ were observed. The growth of the (Ni, Cu)₃Sn₄ IMC consumed the (Cu, Ni)₆Sn₅ IMC at an aging temperature of 200 °C due to the restriction of supply of Cu atoms from the solder to interface. After aging at 200 °C for 60 days, the Ni layer of the substrate was completely consumed in many parts of the sample, at which point a Cu₃Sn IMC was formed. In the ball shear test, the shear strength decreased with increasing aging temperature and time. Until the aging at 185 °C for 15 days and at 200 °C for 3 days, fractures occurred in the bulk solder. After prolonged aging treatment, fractures partially occurred at the (Cu, Ni)₆Sn₅ + Au/solder interface for aging at 185 °C and at the (Ni, Cu)₃Sn₄/Ni interface for aging at 200 °C, respectively. Consequently, thick IMC layer and thermal loading history significantly affected the integrity of the Sn–0.7Cu/Ni BGA joints.     

Anisotropic conductive film flip chip joining using thin chips

In this study, flip chip interconnections were made on very flexible polyethylene naphthalate substrates using anisotropic conductive film. Two kinds of chips were used: chips of normal thickness and thin chips. The thin chips were very thin, only 50 μm thick. Due to the thinness of the chips they were flexible and the entire joint was bendable. The reliability properties of the interconnections established with these two different kinds of chips were compared. In addition, the effect of bending of the chip and joint area on the joint reliability was studied. Furthermore, part of the substrates was dried before bonding and the effect of that on the joint performance was investigated.The pitch of the test vehicles was 250 μm and the chips had 25 μm high gold bumps. For resistance analysis there were two four-point measuring positions in each test vehicle. For finding the optimal bonding conditions for the test vehicles, the bonding was done using two different bonding pressures, of which the better one was chosen for the final tests.Furthermore, the test vehicles were subjected to thermal cycling tests between −40 and +125 °C (half-an-hour cycle) and to a humidity test (85%/85 °C). Part of the test vehicles were bent during the tests. Finally, the structures of the joints were studied using scanning electron microscopy.     

Oxynitride gate dielectric prepared by thermal oxidation of low-pressure chemical vapor deposition silicon-rich silicon nitride

Thin oxynitride gate dielectric films were prepared by thermal oxidation of low-pressure chemical vapor deposition silicon-rich silicon nitride at temperature ranging 850–1050 °C. X-ray photoelectron spectroscopy results indicate that the conversion of the as-deposited silicon nitride into oxynitride with different composition or oxide is feasible and the process is governed by the oxidation temperature. For sample oxidized at 1050 °C for 1 h, the nitride film was converted into silicon dioxide with 7 at.% of nitrogen at the interface and the leakage current density can be reduced by several orders of magnitude. By measuring the leakage current, the barrier height (extracted from Fowler–Nordheim plot) at the dielectric/Si interface is found to be in the range of 1.14–3.08 eV for the investigated samples.     

Ball grid array reliability assessment for aerospace applications

Reliability of ball grid arrays (BGAs) was evaluated with special emphasis on space applications. This work was performed as part of a consortium led by the Jet Propulsion Laboratory (JPL) to help build the infrastructure necessary for implementing this technology. Nearly 200 test vehicles, each with four package types, were assembled and tested using an experiment design. The most critical variables incorporated in this experiment were package type, board material, surface finish, solder volume, and environmental condition. The packages used for this experiment were commercially available packages with over 250 I/Os including both plastic and ceramic BGA packages.The test vehicles were subjected to thermal and dynamic environments representative of aerospace applications. Two different thermal cycling conditions were used, the JPL cycle ranged from −30°C to 100°C and the Boeing cycle ranged from −55°C to 125°C. The test vehicles were monitored continuously to detect electrical failure and their failure mechanisms were characterized. They were removed periodically for optical inspection, scanning electron microscopy (SEM) evaluation, and cross-sectioning for crack propagation mapping. Data collected from both facilities were analyzed and fitted to distributions using the Weibull distribution and Coffin–Manson relationships for failure projection. This paper will describe experiment results as well as those analyses.     

Growth of tin whiskers for lead-free plated leadframe packages in high humid environments and during thermal cycling

Growth behavior of tin whiskers from pure tin and tin–bismuth plated leadframe (LF) packages for elevated temperature and high humidity storages and during thermal cycling was observed. In the storage at 60 °C/93% relative humidity (RH) and 85 °C/85%RH the galvanic corrosion occurred at the outer lead toes and shoulders where the base LF material is exposed, forming tin oxide layers of SnO₂. The corroded layers spread inside the film and formed whiskers around the corroded islands. Many whiskers were observed to grow from grain boundaries for the Fe–42Ni alloy (alloy42) LF packages. It was confirmed that the corrosion tends to occur on the side surfaces of outer leads adjacent to the mold flash. The contribution of ionic contaminants in epoxy mold compound (EMC) to the corrosion was not identified. During thermal cycling between −65 °C and +150 °C whiskers grew out of as-deposited grains for pure tin-plated alloy42 LF packages and they grew linearly with an increase of number of cycle. Growth mechanisms of the whiskers from grain boundaries and as-deposited grains were discussed from the deformation mechanism map for tin and mathematical calculation with a steady-state diffusion model.