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.     

Pb-free high temperature solders for power device packaging

Reliabilities of joints for power semiconductor devices using a Bi-based high temperature solder has been studied. The Bi-based solder whose melting point is 270 °C were prepared by mixing of the CuAlMn particles and molten Bi to overcome the brittleness of Bi. Then, joined samples using the solder were fabricated and thermal cycling tests were examined. After almost 2000 test cycles of −40/200 °C test, neither intermetallic compounds nor cracks were observed for CTE (Coefficient of Thermal Expansion) matched sample with Cu interface. On the other hand, certain amount of intermetallic compound such as Bi₃Ni was found for a sample with Ni interface. In addition, higher reliability of this solder than Sn-Cu solder was obtained after −40/250 °C test. Furthermore, an example power module structure using double high temperature solder layers was proposed.     

Thermal fatigue endurance of collapsible 95.5Sn4Ag0.5Cu spheres in LTCC/PWB assemblies

The behavior of thermomechanically loaded collapsible 95.5Sn4Ag0.5Cu spheres in LTCC/PWB assemblies with high (LTCC/FR-4; ΔCTE 10 ppm/°C) and low (LTCC/Arlon; ΔCTE < 10 ppm/°C) global thermal mismatches was studied by exposing the assemblies into two thermal cycling tests. The characteristic lifetimes of the LTCC/FR-4 assemblies, tested over the temperature ranges of 0–100 °C and −40 to 125 °C, were 1475 and 524 cycles, respectively, whereas the corresponding values of the LTCC/Arlon assemblies were 5424 and 1575 cycles. According to the typical requirements for the industrial lifetime duration of solder joints, the former values are inadequate, whereas the latter are at an acceptable level in a few cases. Furthermore, the global thermal mismatch affected the thermal fatigue behavior of the 95.5Sn4Ag0.5Cu spheres in the temperature range of −40 to 125 °C.     

CCGA packages for space applications

Commercial-off-the-shelf (COTS) area array packaging technologies in high reliability versions are now being considered for applications, including use in a number of NASA electronic systems being utilized for both the Space Shuttle and Mars Rover missions. Indeed, recently a ceramic package version specifically tailored for high reliability applications was used to provide the processing power required for the Spirit and Opportunity Mars Rovers built by NASA-JPL. Both Rovers successfully completed their 3-months mission requirements and continued exploring the Martian surface for many more moths, providing amazing new information on previous environmental conditions of Mars and strong evidence that water exists on Mars.Understanding process, reliability, and quality assurance (QA) indicators for reliability are important for low risk insertion of these newly available packages in high reliability applications. In a previous investigation, thermal cycle test results for a non-functional daisy-chained peripheral ceramic column grid array (CCGA) and its plastic ball grid array (PBGA) version, both having 560 I/Os, were gathered and are presented here. Test results included environmental data for three different thermal cycle regimes (−55/125 °C, −55/100 °C, and −50/75 °C). Detailed information on these—especially failure type for assemblies with high and low solder volumes—are presented. The thermal cycle test procedure followed those recommended by IPC-9701 for tin–lead solder joint assemblies. Its revision A covers guideline thermal cycle requirements for Pb-free solder joints. Key points on this specification are also discussed.In a recent investigation a fully populated CCGA with 717 I/Os was considered for assembly reliability evaluation. The functional package is a field-programmable gate array that has much higher processing power than its previous version. This new package is smaller in dimension, has no interposer, and has a thinner column wrapped with copper for reliability improvement. This paper will also present thermal cycle test results for assemblies of this and its plastic package version with 728 I/Os, both of which were exposed to four different cycle regimes. Two of these cycle profiles are specified by IPC-9701A for tin–lead, namely, −55 to 100 °C and −55 to 125 °C. One is a cycle profile specified by Mil-Std-883, namely, −65/150 °C, generally used for ceramic hybrid packages screening and qualification. The last cycle is in the range of −120 to 85 °C, a representative of electronic systems directly exposed to the Martian environment without use in a thermal control enclosure. Per IPC-9701A, test vehicles were built using daisy chain packages and were continuously monitored and/or manually checked for opens at intervals. The effects of many process and assembly variables—including corner staking commonly used for improving resistance to mechanical loading such as drop and vibration loads—were also considered as part of the test matrix. Optical photomicrographs were taken at various thermal cycle intervals to document damage progress and behavior. Representative samples of these are presented along with cross-sectional photomicrographs at higher magnification taken by scanning electron microscopy (SEM) to determine crack propagation and failure analyses for packages.     

Growth behavior of Cu/Al intermetallic compounds and cracks in copper ball bonds during isothermal aging

Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. There are many potential benefits for use of copper in these applications, including better electrical and mechanical properties, and lower cost. Usually, wires are bonded to aluminum contact pads. However, the growth of Cu/Al intermetallic compounds (IMC) at the wire/pad interfaces is poorly understood, and if excessive would increase the contact resistance and degrade the bond reliability.To study the Cu/Al IMC growth in Cu ball bonds, high temperature aging at 250 °C for up to 196 h has been used to accelerate the aging process of the bonds. The Cu/Al IMCs growth behavior was then recorded and the IMC formation rate of 6.2 ± 1.7 × 10⁻¹⁴ cm²/s was obtained. In addition to the conventional yz-plane cross-section perpendicular to the bonding interface, a xy-plane cross-section parallel through the interfacial layers is reported. Three IMC layers were distinguished at the Cu/Al interfaces by their different colors under optical microscopy on the xy-plane cross-sections of ball bonds. The results of micro-XRD analysis confirmed that Cu₉Al₄, and CuAl₂ were the main IMC products, while a third phase is found which possibly is CuAl. During the aging process, IMC film growth starts from the periphery of the bond and propagates inward towards the centre area. Subsequently, with increased aging time, cavities are observed to develop between the IMC layer and the Cu ball surface, also starting at the bond periphery. The cavitation eventually links up and progresses toward the centre area leading to a nearly complete fracture between the ball and the intermetallic layer, as observed after 81 h.     

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.     

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.     

Investigation of Au/Ti/Al ohmic contact to N-type 4H–SiC

In this study, investigation on Au/Ti/Al ohmic contact to n-type 4H–SiC and its thermal stability are reported. Specific contact resistances (SCRs) in the range of 10⁻⁴–10⁻⁶ Ω cm², and the best SCR as low as 2.8 × 10⁻⁶ Ω cm² has been generally achieved after rapid thermal annealing in Ar for 5 min at 800 °C and above. About 1–2 order(s) of magnitude improvement in SCR as compared to those Al/Ti series ohmic systems in n-SiC reported in literature is obtained. XRD analysis shows that the low resistance contact would be attributed to the formation of titanium silicides (TiSi₂ and TiSi) and Ti₃SiC₂ at the metal/n-SiC interface after thermal annealing. The Au/Ti/Al ohmic contact is thermally stable during thermal aging treatment in Ar at temperature in the 100–500 °C range for 20 h.     

Temperature profile effects in accelerated thermal cycling of SnPb and Pb-free solder joints

Accelerated thermal cycling (ATC) has been widely used in the microelectronics industry for reliability assessment. ATC testing decreases life cycle test time by one or more of the following means: increasing the heating and cooling rate, decreasing the hold time, or increasing the range of the applied temperature. The relative effect of each of these cycle parameters and the failure mechanisms they induce has been the subject of many studies; however uncertainty remains, particularly regarding the role of the heating and cooling rate. In this research, three conditions with two ramp rates (14 °C/min and 95 °C/min) and two temperature ranges (ΔT = 0–100 °C and −40 to 125 °C) were applied to resistor 2512 and PBGA 256 test vehicles assembled with SnPb and Pb-free solders. The test results showed that the higher ramp rate reduced the testing time while retaining the same failure modes, and that the damage per cycle increased with the temperature difference. For the resistors, the Pb-free solder joints lasted longer than the SnPb joints at the smaller ΔT, but were inferior at the larger ΔT. In contrast, the Pb-free solder joints in the PBGA test vehicles lasted longer than the SnPb solder under both conditions.     

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.     

Fatigue analysis of miniaturized lead-free solder contacts based on a novel test concept

The paper presents the method of generating lifetime-prediction-laws on special prepared very stiff specimen. The combination of thin- and thick-film technology allows building up test samples on ceramic very similar to electronic packages including the measurement issues. Influences of pad surface metallurgy, microstructure of solder, ineutectic solder alloys and assembly process parameter are regarded now. The investigation objects provide monitoring of electrical and mechanical damage process of SnAgCu solder bump. Different thermo-mechanical loads will be applied in temperature ranges of 0 to +80 °C, −40 to +125 °C and −50 to +150 °C, where the temperature gradient and cycle frequency also vary. A Variation of four different chip sizes allows the determination of fatigue laws for each temperature profile, to be able to compare in between them. The results of these tests will give universal lifetime-prediction laws for SnAgCu base solder joints. Main goals are to find coefficients for lifetime prediction models such as Coffin–Manson- or Norris–Landzberg-relation, which are transferable in between different electronic packages.     

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.     

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.     

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.     

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.     

Reliability of 80 μm pitch flip chip attachment on flex

The interest toward flip chip technology has increased rapidly during last decade. Compared to the traditional packages and assembly technologies flip chip has several benefits, like less parasitics, the small package size and the weight. These properties emphasize especially when flip chip component is mounted direct to the flexible printed board. In this paper flip chip components with Kelvin four point probe and daisy chain test structure were bonded to the polyimide flex with two different types of anisotropically conductive adhesive films and one anisotropically conductive adhesive paste. The reliability of small pitch flip chip on flex interconnections (pitch 80 μm) was tested in 85°C/85% RH environmental test and −40↔+125°C thermal shock test. According to the results it is possible to achieve reliable and stable ohmic contact, even in small pitch flip chip on flex applications.     

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.     

Effect of annealing on the properties of low-k nanoporous SiO2 films prepared by sol–gel method with catalyst HF

Using hydrofluoric acid (HF) as catalyst, nanoporous SiO₂ thin film was synthesized by sol–gel method. By scanning electron microscopy, Fourier transform infrared spectra, thermo gravimetric and differential thermal analysis, ellipsometry, capacitance–voltage and current–voltage measurements, the effects of annealing on film properties were discussed in detail. The introduction of HF results in the less polarizability, the preferable microstructures and the improved thermal stability of the nanoporous silica films. After thermal annealing at 450 °C, the crack-free films with strong hydrophobicity, ultra-low dielectric constant of 1.65, porosity of 78%, and leakage current density of 1.3 × 10⁻⁸ A cm⁻² were obtained.     

New encapsulation development for fine pitch IC devices

The continuous reduction of chip size driven by the market demand has a significant impact on circuit design and assembly process of IC packages. Shrinking chip size and increasing I/O counts require finer bond pad pitch and bond pad size for circuitry layout. As a result, serious wire deflection during transfer molding process could make adjacent wires short, and this issue becomes more critical as a smaller wire diameter has to be applied for the finer pitch wire bonded IC devices.This paper presents a new encapsulation process development for 50 μm fine pitch plastic ball grid array package. Since reduced wire diameter decreases the bending strength of bonded wires significantly, wire deflection during molding process becomes quite serious and critical. Experiments on conventional transfer molding were conducted to evaluate wire span threshold with 23.0 μm diameter gold wire. The results show that the wire span threshold is about 4.1 mm, which is much shorter than the wire span threshold of over 5.0 mm for wire with 25.4 μm diameter. Finite element analysis shows there is a significant difference in the wire deflection between 23.0 μm gold wire and 25.4 μm gold wire diameter under the same action of mold flow. A novel encapsulation method is introduced using non-sweep solution. The wire span could be extended to over 5.0 mm with wire sweep less than 1%. Reliability tests conducted showed that all the units passed 1000 temperature cycles (−55 to 125 °C) with JEDEC moisture sensitivity level 2a (60 °C/60% relative humidity for 120 h) and 3 times reflow (peak temperature at 220–225 °C). It is believed that this solution could efficiently overcome the risk of wire short issues and improve the yield of ultra fine pitch wire bonds in high-volume production.     

Solder joint reliability evaluation of chip scale package using a modified Coffin–Manson equation

Thermal cycle tests were performed for chip scale package (CSP) solder joints with Sn–37mass%Pb under several thermal cycle conditions. Under the conventional thermal cycle conditions, which heat up to approximately 100 °C, microstructure coarsening occurred and solder joints were fractured. The thermal fatigue lives followed the modified Coffin–Manson equation. The exponential factors m and n, and the activation energy Q in that equation were evaluated as 0.33, −1.9 and 15.5 kJ/mol, respectively. When the maximum temperature is room temperature and the temperature range is very narrow, the solder joint fracture occurred without microstructure coarsening, and the thermal fatigue life does not follow the modified Coffin–Manson equation.