Kaspar M. B. Jansen

Numerical modeling of warpage induced in QFN array molding process

Warpage is a critical issue for QFN array molding process. In this paper, a cure-dependent viscoelastic constitutive model is established to model the cure-induced warpage in array molding process. For the relaxation modulus functions of the packaging polymer, the equilibrium moduli are modeled with a model based on scaling analysis and the relaxation behavior of the transient part is described by the cure-dependent relaxation amplitude and reduced relaxation times which are based on the time-conversion superposition principle. The cure-dependent parameters are characterized by using an integrated approach of dynamical mechanical analysis (DMA) and differential scanning calorimetry (DSC) measurements. Finite element modeling is carried out for three configurations of a carrier package map mould and the warpage induced during the curing process and cooling down is predicted. The results show that warpage induced during the curing process has significant contribution on the total warpage of the map.

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

Abstract 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 100–140xa0°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.

Characterization of the viscoelastic properties of an epoxy molding compound during cure

Abstract In the electronics industry epoxy molding compounds, underfills and adhesives are used for the packaging of electronic components. These materials are applied in liquid form, cured at elevated temperatures and then cooled down to room temperature. During these processing steps residual stresses are built up resulting from both cure and thermal shrinkage. These residual stresses add up to the stresses generated during thermal cycling and mechanical loading and may eventually lead to product failure. The viscoelastic properties of the encapsulation material depend highly on temperature and degree of cure. This paper investigates the increase of elastic modulus and the changes in the viscoelastic behavior of an epoxy molding compound, during the curing process. This is done using the shear setup of a Dynamic Mechanical Analyzer DMA-Q800. The cure dependent viscoelastic behavior is determined during heating scans of an intermittent cure experiment. In such an experiment the material is partially cured and then followed by a heating scan at 2xa0°C/min. During this heating scan continuous frequency sweeps are performed and the shear modulus is extracted. The Time–Temperature superposition principle is applied and the viscoelastic shear mastercurve is extracted. Analyzing the shear modulus, the cure dependent viscoelastic material behavior was modeled using the cure dependent glass transition temperature as a reference and a cure dependent rubbery modulus. It is shown that partial curing would increase the glass transition temperature and rubbery shear modulus. It also shifts the viscoelastic mastercurve to the higher time domain. Taking Tg as the reference temperature for different heating scans, the mastercurves collapse to one graph. In addition, using a Differential Scanning Calorimeter (DSC), the growth of the glass transition temperature, T g DSC , with respect to the conversion level is obtained. These values are coupled to the values of glass transition temperature in DMA apparatus, T g DMA , for calculating the conversion level at each step of curing process in shear mode test.

Prediction of cure induced warpage of micro-electronic products

Prediction of residual stresses in micro-electronic devises is an important issue. Virtual prototyping is used to minimize residual stresses in order to prevent failure or malfunction of electronic products. Already during encapsulation stresses build up due to polymerization induced shrinkage of the molding compound. Differences in coefficient of thermal expansion of the involved materials cause additional stresses during cooling down from molding to ambient temperature. Since industry is availed by reliable prediction methods, detailed material models are required. In electronic packaging, mechanical properties of most of the involved materials have constant mechanical properties. However, the viscoelastic properties of the encapsulation material depends highly on temperature and degree of cure. Reliable predictions of residual stresses require simulation models which take into account the effect of temperature and conversion level. In this paper, properties of molding compound are discussed which are relevant for the prediction of warpage of micro-electronics products. The models for the individual properties are combined to one single model suitable for finite element simulations. The numerical implementation in finite element code is not standard and is done by using user-subroutines. Validation experiments are performed in order to verify the developed material model which is done by measuring and predicting the warpage of a mold map. A Topography and Deformation Measurement (TDM) device is used to measure the deformations at elevated temperatures in a non-intrusive way such that the developed material model could be validated in a broad range of temperature. Finally, simulations are carried out with simplified material models of molding compound. The results of these simulations are compared with results obtained with the cure dependent viscoelastic model and real warpage data. From these comparisons it is concluded that for reliable prediction of warpage, the cure dependent viscoelastic model is has to be used.

Effect of filler concentration of rubbery shear and bulk modulus of molding compounds.

In the electronics industry epoxy molding compounds, underfills and adhesives are used for the packaging of electronic components. These materials are applied in liquid form, cured at elevated temperatures and then cooled down to room temperature. During these processing steps residual stresses are built up resulting from both cure and thermal shrinkage. In order to minimize these stresses inorganic fillers are added. These fillers have several opposing effects on the residual stresses because they decrease the cure shrinkage and thermal contraction but increase the modulus below and above the glass transition temperature. In this paper an extensive study on the cure-dependent rubbery moduli of a series of silica spheres filled epoxy resins is carried out both experimentally and theoretically. Low frequency dynamic mechanical analysis (DMA) was used to measure the rubbery modulus build-up during cure. A model based on scaling analysis was applied to describe the evolution of the rubbery shear modulus. The effect of the filler percentage on the rubbery shear and bulk moduli as well as the coefficients of thermal expansion were measured and compared with models from the theory of particulate-filled composites.

Modeling and characterization of molding compound properties during cure

Abstract During the encapsulation of electronic components stresses are generated due to curing effects and the difference in thermal shrinkage between molding compound and die. These residual stresses add up to the stresses generated during thermal cycling and mechanical loading and may eventually lead to product failure. In this paper we focus on three commercial molding compounds and analyze in detail the increase in elastic modulus and the change in viscoelastic behaviour during cure. This was done with a special shear tool which allows to measure mechanical properties with sufficient accuracy in the liquid as well as in the solid state. The cure dependent viscoelastic material behaviour was modeled using a cure dependent shift factor and rubber modulus. The viscoelastic behaviour of the molding compounds is also shown not to be stable. During postcure the materials slowly continue to crosslink thereby systematically changing their viscoelastic behaviour. The material models presented here therefore only account for the initial curing stage and do not include postcure.

Influence of matrix viscoelastic properties on thermal conductivity of TCA – Numerical approach

Abstract Thermally conductive adhesives (TCA) and electrically conductive adhesives (ECA) are one of the major concerns of the contemporary micro-electronics. They are especially important in application where, e.g. effective heat dissipation is the key factor for reliability issues. Currently there is a lot of ongoing research in order to improve the basic thermal property of adhesives, which is mainly heat conductance. According to the literature data the heat conductance can vary from 0.1 up to 60xa0W/mxa0K. It depends not only on the filler material, its content and configuration but also on thermo-mechanical properties of matrix. On the other hand numerical simulation becomes nowadays an inevitable tool for rapid non-destructive and low-cost experiments. The basic problem of numerical experiments is accuracy. Nevertheless the accuracy can be improved by combining the numerical and traditional experiments. This can be achieved by means of partial validation of numerical results by traditional experiments or by precise and appropriate material properties measurement. In fact, the above approach was applied in current work in order to simulate the influence of curing temperature and time on the thermal conductance of polymers. Thermally conductive adhesives belong to polymer based materials. In order to apply numerical simulation it is required to have an appropriate description of the thermal and mechanical behavior of polymers. Most often polymers are described by cure dependent or independent linear viscoelastic model [3] , [5] . Having this model, which parameters in fact can be measured experimentally, it is possible to simulate the stress and strain field caused by polymer curing and shrinkage phenomena and finally assess the thermal conductance accordingly. Current paper focuses on a problem of numerical simulation of TCA in order to recognize the trend dependence of thermal conductivity due to viscoelastic model of polymers and filler particles contact area.

Comprehensive material characterization of organic packaging materials

In this study, two highly filled molding compounds were used as example to demonstrate the characterization scheme. In addition, two low filled packaging polymers are included for comparison. The characterization scheme consists of the steps sample preparation, measurement of the material data, and modeling the material behavior. The ‘sample preparation’ step included a DSC analysis to understand the cure reaction and to establish the cure kinetics model. In the ‘measurement’ step, two different sets of equipment were applied. The elongation modulus is determined by dynamic mechanical analysis (equipment: DMA ‘Q800’) in a wide range of temperatures and frequencies. The other parameters are measured by pressure-volume-temperature experiments (equipment: PVT ‘Gnomix’). Conducting these characterization tests, the bulk modulus (K), coefficient of thermal expansion (CTE), and the cure shrinkage was determined. The paper describes this comprehensive characterization with the measurement setups and parameter selection. E(T,t), K(T,t), CTE(T), Tg and cure shrinkage are determined to define a complete and consistent material model [JAN07]. Subsequently, the characterization results are presented, discussed and further work to implement the complete material model into FEM simulation tools like ANSYS™ is outlined.

Fast reliability qualification of SiP products

For the purpose of rapidly identifying the functional weak points of SiP products and defining appropriate design rules, a new methodology is proposed to achieve fast reliability qualification. This new methodology is based on the moisture absorption behavior along the critical interface of a SiP carrier and on the most sensitive zone to delamination of the SiP carrier, determined by simulation and experimentally checked. In this paper, a new accelerated preconditioning is proposed and a new non destructive thermal method to monitor the delamination is presented. The effectiveness of this new stress test to accelerate the failure mechanism of the SiP carrier and the ability to detect delamination are evaluated by performing a DOE.

Inhibition of glycosaminoglycan incorporation influences collagen network formation during cartilage matrix production

To understand cartilage degenerative diseases and improve repair procedures, we investigate the influence of glycosaminoglycans (GAGs) on cartilage matrix biochemistry and functionality. Bovine articular chondrocytes were cultured in alginate beads with(out) para-nitrophenyl-beta-d-xyloside (PNPX) to inhibit GAG incorporation into newly formed proteoglycans. As expected, GAG deposition in alginate beads decreased with increasing PNPX concentration. Next to GAGs, collagen deposition and cross-linking also decreased. In the presence of PNPX, GAGs and collagen were deposited further away from the chondrocyte than in the control and increased amounts were found in the culture medium. These changes resulted in decreased functional properties of the construct. We conclude that in our culture system, intact proteoglycans play a role in deposition of collagen and thus the formation of a functional matrix. The effect of less proteoglycans on the collagen network could explain why cartilage repair is ineffective in osteoarthritis and help us with development of new therapies.