Fluid–solid coupling thermo-mechanical analysis of high power LED package during thermal shock testing

The virtual design by numerical simulation to model various accelerated reliability testing conditions is adopted to validate and improve the reliability of the high power LED package. In this study, the reliability of the high power LED package during thermal shock testing is investigated by fluid–solid coupling thermo-mechanical modeling by considering nonlinear time and temperature dependent material properties. Through fluid–solid coupling transient thermal transfer analysis, it is found that the maximum thermal gradient exceeds 75 K during the rapid cooling process and 91 K during the rapid heating process of the thermal shock testing which is ignored in the traditional isothermal assumption. The calculation results indicate that the equivalent plastic strain range of the bonding wire within the LED package with consideration of the temperature gradient is much higher than that with the isothermal assumption. The assumption of the isothermal condition is not appropriate which will lead to overestimation of the predicted lifetime. The viscoelastic behaviors of the silicone have significant influences on the lifetime prediction of the bonding wire and silicone with low elastic modulus and coefficient of thermal expansion (CTE) can significantly enhance the reliability of the bonding wire under the thermal shock loading. The results in this study could provide a guideline on design for reliability in the high power LED packaging.     

Reliability of Cu wire bonds in microelectronic packages

In this study the thermo-mechanical response of 25 μm Cu wire bonds in an LQFP-EPad (Low Profile Quad Flat-Exposed Pad) package was investigated by numerical and experimental means. The aim was to develop a methodology for fast evaluation of the packages, with focus on wire bond fatigue, by combining finite element analysis (FEA) and mechanical fatigue testing. The investigations included the following steps: (i) simulation of the warpage induced displacements in the encapsulated LQFP-176-Epad package due to temperature changes, (ii) reproducing the thermally induced stresses in the wire bond loops in an unmolded (non-encapsulated) LQFP package using an accelerated multiaxial mechanical fatigue testing set-up under the displacement amplitudes determined in case (i) and determination of the loading cycles to failure (N_f), (iii) FEA of the experiments performed in (ii) based on the boundary conditions determined in (i) to calculate the states of stress and strain in the wire bonds subjected to multiaxial mechanical cyclic loading. Our investigations confirm that thermal and mechanical cyclic loading results in occurrence of high plastic strains at the heat affected zone (HAZ) above the nail-head, which may lead to fatigue failure of the wire bonds in the packages. The lifetime of wire bonds show a proportional relation between the location and angle of the wire bond to the direction of loading. The calculated accumulated plastic strain in the HAZ was correlated to the experimentally determined N_f values based on the volume weighted averaging (VWA) approached and presented in a lifetime diagram (∆ d - N_f) for reliability assessment of Cu wire bonds. The described accelerated test method could be used as a rapid qualification test for the determination of the lifetimes of wire bonds at different positions on the chip as well as for related improvements of package design.     

Fatigue testing method for fine bond wires in an LQFP package

A mechanical testing setup was developed to study the fatigue response of fine thermo-sonic wire bond connection in low profile quad flat packages (LQFP). The testing set-up was designed to induce pre-defined multi-axial stresses in the wire bond loops of non-encapsulated packages in order to mimic their deformation behavior during the thermo-mechanical loading. Lifetime curves were obtained up to 1.0E7 loading cycles with fatigue failure occurring in the heat affected zone of the ball bond. The experimental fatigue data in combination with extended FEA provided the basis for a Coffin Manson lifetime model. The proposed fatigue testing procedure can be applied as a highly efficient method for evaluation of various wire bonded packages by using a limited number of test samples and simultaneous testing of several wire bonds.     

Thermal transient characteristics of die attach in high power LED PKG

The reliability of packaged electronics strongly depends on the die attach quality because any void or a small delamination may cause instant temperature increase in the die, leading sooner or later to failure in the operation. Die attach materials have a key role in the thermal management of high power LED packages by providing the low thermal resistance between the heat generating LED chips and the heat dissipating heat slug. In this paper, thermal transient characteristics of die attach in high power LED PKG have been studied based on the thermal transient analysis using the evaluation of the structure function of the heat flow path. With high power LED packages fabricated by die attach materials such as Ag paste, solder paste and Au/Sn eutectic bonding, we have demonstrated for characteristics such as cross-section analysis, shear test and visual inspection after shear test of die attach and how to detect die attach failures and to measure thermal resistance values of die attach in high power LED PKG. From the differential structure function of the thermal transient characteristics, we could know the result that die attach quality of Au/Sn eutectic bonding with the thermal resistance of about 3.5 K/W was much better than this of Ag paste and solder paste with the thermal resistance of about 11.5–14.2 K/W and 4.4–4.6 K/W, respectively. From this results, it is possible to fabricate high power LED with a small thermal resistance and a good die attach quality by applying Au/Sn eutectic bonding die attach with a high reliability and a good repeatability.     

Crack mechanism in wire bonding joints

High voltage and high current power modules are key components for traction applications. While the modules are exposed to harsh stress conditions all over their lifetime, high reliability is of decisive importance in this field of application. In power electronic packages wire bonding is used for the electrical interconnection from the chips to the output pins. Wire bond lift-off and solder fatigue are limiting the reliability. In this work we investigate the initiation and growth of cracks in the wire bonds using finite-element analysis.     

Reliability of thick Al wire bonds in IGBT modules for tractionmotor drives

Reliability enhancement of thick Al wire bonds during thermal fatigue test has been investigated from a metallurgical viewpoint. Al wire bonds degrade with the increase of crack length during thermal fatigue tests with high ΔT_j due to the tensile stress generated by the thermal expansion coefficient mismatch between Al wires and Si. It is also found that cracks propagate along the small grain boundaries of Al wires at the bonding interface. It is predicted that the Al wire bonds may not degrade due to thermal fatigue if ΔT_j is controlled below 40 K, i.e., keeping it within the actual temperature fluctuation range in IGBT modules for traction motor drives. The reliability of Al wire bonds can be enhanced by increasing the grain size of the Al wire at the bonding interface. The high temperature bonding is considered to be a good candidate for enhancing the reliability of Al wire bonds     

Lifetime assessment of Bisphenol-A Polycarbonate (BPA-PC) plastic lens,used in LED-based products

In this investigation, the accelerated optical degradation of two different commercial Bisphenol-A Polycarbonate (BPA-PC) grades under elevated temperature stress is studied. The BPA-PC plates are used both in light conversion carriers in LED modules and encapsulants in LED packages. BPA-PC plates are exposed to temperatures in the range of 100–140 °C. Optical properties of the thermally-aged plates were studied using an integrated sphere. The results show that increasing the exposure time leads to degradation of BPA-PC optical properties, i.e. decrease of light transmission and increase in the yellowing index (YI). An exponential luminous decay model and Arrhenius equation are used to predict the lumen depreciation over different time and temperatures. Accelerated thermal stress tests together with the applied reliability model are used to predict the lifetime of plastic lens in LED lamps in real life conditions.     

Effects of Underfill Materials and Thermal Cycling on Mechanical Reliability of Chip Scale Package

The thermomechanical reliability of chip-scale packages (CSPs) with various underfills was evaluated by measure the electrical resistance under thermal shock and four-point bending fatigue tests. The underfill containing cycloaliphatic-type epoxy resin had lower resistance than without cycloaliphatic-type epoxy resin under thermomechanical fatigue test because the cycloaliphatic-type epoxy resin was able to mechanically relax more than the other types. The lifetimes of the CSPs under thermomechanical fatigues were strongly dependent on the properties of the underfill.     

Boundary element analysis of thermal fatigue effects on high power IGBT modules

The technology of high power IGBT modules has been significantly improved these last years against thermal fatigue. The most frequently observed failure modes, due to thermal fatigue, are the solder cracks between the copper base plate and the direct copper bonding (DCB) substrate and bond wire lift-off. Specific simulation tools are needed to carry out reliability researches and to develop device lifetime models. In other respects, accurate temperature and flux distributions are essential when computing thermo-mechanical stresses in order to assess the lifetime of high power modules in real operating conditions. This study presents an analysis method based on the boundary element method (BEM) to investigate thermal behavior of high power semiconductor packages subjected to power cycling loads. The paper describes the boundary integral equation which has been solved using the BEM and applied to the case of a high power IGBT module package (3.3 kV–1.2 kA). A validation of the numerical tool is presented by comparison with experimental measurements. Finally, the paper points out the effect on the thermal stress of the IGBT chips position on the DCB substrate. In particular, a light shifting of the silicon chips may be sufficient to delay significantly the initiation and the propagation of the cracks, allowing a higher device lifetime of the studied module.     

Cyclic bending reliability of wafer-level chip-scale packages

In this paper, both experimental and numerical studies are conducted to investigate board-level reliability of wafer-level chip-scale packages under four-point cyclic bending conditions that combine different deflection amplitudes and excitation frequencies. In addition to the fatigue lives of the test vehicle, locations and modes of fractured solder joints are observed. In the numerical modeling, inertia forces along with rate-dependent material properties of the solder joints are considered in order to capture frequency-dependent characteristics of this particular test methodology. Through the dynamic finite element analysis, plastic strain energy densities accumulated per bending cycle within the critical solder joint are calculated and together with the experimental results, parameters for the Morrow fatigue model are calibrated.     

A fast mechanical test technique for life time estimation of micro-joints

A novel accelerated mechanical testing method for reliability assessment of micro-joints in the electronic devices is presented as an alternative to time consuming thermal and power cycling test procedures. A special experimental set-up in combination with an ultrasonic resonance fatigue testing system and a laser Doppler vibrometer is used to obtain fatigue life curves of micro-joints under shear loading. Using this method fatigue life curves of Al wire bonded micro-joints were obtained up to 10⁹ number of loading cycles and discussed with regard to micro-mechanisms of the bond failure. Failure analysis of the fatigued micro-joints showed that the predominant failure mechanism of power cycling tests, bond wire lift-off, was reproduced by the mechanical testing procedure. Life time of the micro-joints was modelled using a Coffin–Manson type relationship and showed a good correlation to life time curves obtained by power cycling tests. The major advantage of the proposed fast mechanical testing method is the significant reduction of the testing time in comparison with conventional thermal and power cycling tests. Furthermore subsequent examination of the failure surface provides a reliable tool for improvement of the bonding process. The proposed high frequency fatigue testing system can be applied as a rapid qualification and screening tool for various kinds of interconnects in electronic packaging.     

Effect of substrate flexibility on solder joint reliability. Part II: finite element modeling

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue life significantly. The mechanism of substrate flexibility on improving solder joint thermal fatigue was investigated by thermal mechanical analysis (TMA) technique and finite element modeling. The reliability of solder joints in real flip chip assembly with both rigid and compliant substrates was evaluated by accelerated temperature cycling test. Finite element simulations were conducted to study the reliability of solder joints in flip chip on flex assembly (FCOF) and flip chip on rigid board assembly (FCOB) applying Anand model. Based on the finite element analysis results, the fatigue lives of solder joints were obtained by Darveaux’s crack initiation and growth model. The thermal strain/stress in solder joints of flip chip assemblies with different substrates were compared. The results of finite element analysis showed a good agreement with the experimental results. It was found that the thermal fatigue lifetime of FCOF solder joints was much longer than that of FCOB solder joints. The thermal strain/stress in solder joints could be reduced by flex buckling or bending and flex substrates could dissipate energy that otherwise would be absorbed by solder joints. It was concluded that substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during temperature cycling.     

Reliability model for Al wire bonds subjected to heel crack failures

In power electronic packages wire bonding is used for the electrical contact of the chips and for interconnections on the module substrate. Limiting factors for the reliability are solder fatigue and wire bond failures. In this work we investigate the material fatigue of aluminum bonding wires stressed by cyclic lateral bonding area displacement. Bond wire heel crack failures observed by experiments are found to be strongly dependent on the loop geometry. Based on a finite element model that accounts for elastic-plastic material properties, a life-time model for the Al wire (Coffin-Manson representation) is derived from the experiments.     

Solder fatigue failures in a new designed power module under Power Cycling

Today a point has been reached where lifetimes of power modules are limited by the standard packaging technologies, such as wire bonding. To surpass these limits, a new power module was designed using Cu clips as interconnects instead of Al wire bonds. With this new design the structure robustness should be improved and lead to a reliability gain but in counterpart it requires an additional solder layer in order to fix the clip onto the die. This paper studies the failure mechanisms occurring in these two solder layers under power cycling. The behavior of solder layers is precisely analyzed by performing power cycling tests and by taking advantage of Finite Elements simulations. Furthermore an experimental and numerical sensitivity study on test parameters is conducted. Results obtained enable the definition of solder lifetime prediction models.     

Flip chip interconnect systems using copper wire stud bump and leadfree solder

This research focuses on flip chip interconnect systems consisting of wire stud bumps and solder alloy interconnects. Conventional gold (Au) wire stud bumps and new copper (Cu) wire stud bumps were formed on the chip by wire stud bumping. Cu wire studs were bumped by controlling the ramp rate of ultrasonic power to eliminate the occurrence of under-pad chip cracks that tend to occur with high strength bonding wire. Lead free 96Sn3.5Ag0.5Cu (SnAgCu) alloy was used to interconnect the wire studs and printed circuit board. A comparison was made with conventional eutectic 63Sn37Pb (SnPb) alloy and 60In40Pb (InPb) alloy. Test vehicles were assembled with two different direct chip attachment (DCA) processes. When the basic reflow assembly using a conventional pick and place machine and convection reflow was used, 30% of the lead free test vehicles exhibited process defects. Other lead free test vehicles failed quickly in thermal shock testing. Applying the basic reflow assembly process is detrimental for the SnAgCu test vehicles. On the other hand, when compression bonding assembly was performed using a high accuracy flip chip bonder, the lead free test vehicles exhibited no process defects and the thermal shock reliability improved. Cu stud-SnAgCu test vehicles (Cu-SnAgCu) in particular showed longer mean time to failure, 2269 cycles for the B stage process and 3237 cycles for high temperature bonding. The C-SAM and cross section analysis of the Cu stud bump assemblies indicated less delamination in thermal shock testing and significantly less Cu diffusion into the solder compared to Au stud bumped test vehicles. The Cu stud-SnAgCu systems form stable interconnects when assembled using a compression bonding process. Moreover, Cu wire stud bumping offers an acceptable solution for lead free assembly     

Effect of substrate flexibility on solder joint reliability

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue lifetime significantly. The reliability of solder joint in flip chip assembly for both rigid and compliant substrates was evaluated by accelerated temperature cycling test. Experimental results strongly showed that the thermal fatigue lifetime of solder joints in flip chip on flex assembly was much improved over that in flip chip on rigid substrate assembly. Debonding area of solder joints in flip chip on rigid board and flip chip on flex assemblies were investigated, and it was found that flex substrate could slow down solder joint crack propagation rate. The mechanism of substrate flexibility on improving solder joint thermal fatigue was investigated by thermal mechanical analysis (TMA) technique. TMA results showed that flex substrate buckles or bends during temperature cycling and this phenomenon was discussed from the point of view of mechanics of the flip chip assembly during temperature cycling process. It was indicated that the thermal strain and stress in solder joints could be reduced by flex buckling or bending and flex substrates could dissipate energy that otherwise would be absorbed by solder joints. It was concluded that substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during temperature cycling.     

Effect of Size of Lid-Substrate Adhesive on Reliability of Solder Balls in Thermally Enhanced Flip Chip PBGA Packages

Six design cases of lid-substrate adhesive with various combinations of widths and heights were analyzed to investigate how the size of the adhesive affects the reliability of the solder balls of thermally enhanced flip chip plastic ball grid array (FC-PBGA) packages in thermal cycling tests. Analysis results were compared with data on the reliability of conventional FC-PBGA packages. Thermal-mechanical behavior was simulated by the finite element (FE) method and the eutectic solder was assumed to exhibit elastic-viscoplastic behavior. The temperature-dependent nonlinear stress/strain relationship of the adhesive was experimentally determined and used in the FE analysis. Darveaux's model was employed to obtain the predicted fatigue life of the solder ball. Simulation results reveal that the fatigue life of the solder balls in thermally enhanced FC-PBGA packages is much shorter than that in conventional FC-PBGA packages, and the life of solder balls increases with both the width and the height of the adhesive. However, the effect of the width of the adhesive on the reliability of the solder ball is stronger than that of the height. Moreover, increasing either the width or the height reduces the plastic strain in the adhesive at critical locations, indicating that the reliability of the adhesive can be improved by its size. The predicted results of the life of solder balls for some selected studied packages are also compared with experimental data from thermal cycling tests in the paper     

Reliability test and failure analysis of high power LED packages

A new type application specific light emitting diode(LED) package(ASLP) with freeform polycarbonate lens for street lighting is developed,whose manufacturing processes are compatible with a typical LED packaging process.The reliability test methods and failure criterions from different vendors are reviewed and compared.It is found that test methods and failure criterions are quite different.The rapid reliability assessment standards are urgently needed for the LED industry.85℃/85 RH with 700 mA is used to test our LED modules with three other vendors for 1000 h,showing no visible degradation in optical performance for our modules,with two other vendors showing significant degradation.Some failure analysis methods such as C-SAM,Nano X-ray CT and optical microscope are used for LED packages.Some failure mechanisms such as delaminations and cracks are detected in the LED packages after the accelerated reliability testing.The finite element simulation method is helpful for the failure analysis and design of the reliability of the LED packaging.One example is used to show one currently used module in industry is vulnerable and may not easily pass the harsh thermal cycle testing.     

On the difference between thermal cycling and thermal shock testing for board level reliability of soldered interconnections

In this work, the endurance behavior of a ball grid array package is determined at two different temperature cycle conditions: a thermal cycle test and a thermal shock test. The observed failure distributions and failure modes allow deriving a translation factor between the two test conditions. Finite element analyses are carried out to design a predictive simulation model for the fatigue lifetime. Using an isothermal approach, a discrepancy is found between the experimental and simulated results. Incorporating the experimentally measured temperature gradients into a semi-transient model leads to a better match with the observed lifetimes. In particular for the fast thermal shock loading this turns out to be necessary. The results prove that temperature gradients in BGA-packages play an important role in board level reliability testing.     

Effects of solder joint shape and height on thermal fatigue lifetime

Solder joint thermal fatigue failure is a major concern for area array technologies such as flip chip and ball grid array technologies. Solder joint geometry is an important factor influencing thermal fatigue lifetime. In this paper, the effects of solder joint shape and height on thermal fatigue lifetime are studied. Solder joint fatigue lifetime was evaluated using accelerated temperate cycling and adhesion test. Scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), scanning acoustic microscopy (nondestructive evaluation) and optical microscopy were utilized to examine the integrity of the joint and to detect cracks and other defects before and during accelerated fatigue tests. Our accelerated temperature cycling test clearly shows that solder joint fatigue failure process consists of three phases: crack initiation, crack propagation and catastrophic failure. Experimental results indicated that both hourglass shape and great standoff height could improve solder joint fatigue lifetime, with standoff height being the more effective factor. Experimental data suggested that shape is the dominant factor affecting crack initiation time while standoff height is the major factor influencing crack propagation time.