Thermal stress analysis of leads in Quad Flat Package: a parametric study

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1 Thermal stress analysis of leads in Quad Flat Package: a parametric study D. Zhou Faculty of,, zhouding@siswa.um.edu.my A.S.M.A. Haseeb Faculty of, haseeb@um.edu.my A. Andriyana Faculty of, andri.andriyana@um.edu.my Y.H. Wong Faculty of, yhwong@um.edu.my M.F.M. Sabri Faculty of, faizul@um.edu.my B. Y. Low NXP Semiconductor Sdn Bhd Selangor, Malaysia boon.low@nxp.com X. S. Pang NXP Semiconductor (China) Co. Ltd Tianjin, China xingshou.pang@nxp.com P.L. Eu NXP Semiconductor Sdn Bhd Selangor, Malaysia poh-leng.eu@nxp.com L.C. Tan NXP Semiconductor Singapore Pte Ltd Singapore lc.tan@nxp.com Abstract Due to the cyclic variation of temperature during service, leads in Quad Flat Packages (QFP) undergo high thermal stress which can lead to large deformation and failure. Thus, the prediction of thermal stress and strain distributions is a pre-requisite for reliability analysis of such components. Along this line, the present work focuses on determining the thermal stress and strain distributions in high density QFP using Finite Element Method (FEM). Two types of lead are considered: J-lead and gull-wing lead. For each type of the lead, a parametric study is conducted in order to evaluate the effect of lead geometry on stress and strain distributions. Based on the simulation results, optimum lead design is suggested. Keywords thermal stress; reliability; finite element analysis; quad flat package, J lead, gull wing lead I. INTRODUCTION Electronic products today are getting miniaturized which requires the use of high-density lead frames. The compelling benefits of low cost, high density in assembly, light weight, good operability and high working efficiency have led to the growing application of quad flat package (QFP) assemblies. Thermal mismatch deformations arise due to differences in the coefficient of thermal expansion (CTE) between the assembly materials and high solder joint stresses and strains are generated [1]. The need for high-speed circuitry and the associated advances in integrated circuit technology has led to the design of surface mount components (SMC) with higher pin count and smaller package sizes [2]. Generally, the higher pin count and smaller package size are accomplished by reducing the lead thickness and spacing. This optimal design has led to more attention to the reliability requirements for surface mount assemblies [3]. A surface mount assembly is a composite structure consisting of three major parts: the package, the solder joints and the printed circuit board (PCB). Unlike traditional throughhole technology. surface mount technology (SMT) utilizes smaller SMCs which are soldered directly to the surface of a PCB. Consequently, the solder joints are the only mechanical means of attaching the SMC to the PCB [4, 5]. During service, thermal loading generated by chips will expand the leads in the package so that the stress analysis is imperative to be studied to avoid deformation of QFP [6, 7]. A XXX-X-XXXX-XXXX-X/XX/$XX.00 20XX IEEE

2 more challenging thermal issue is the thermal cyclic stress in a QFP component may experience during its operational life [8, 9]. When chips are running hotter, it increases the difference in temperature (ΔT) between itself and the leads as well as solder material. Leads or solder joint fracture or fatigue failure always occur even through multiple electronic package designs kept trying to fix it [10]. As package size shrinks, the heat generated in die is increasing the mismatch in the coefficients of thermal expansion (CTE) between the leads and solder material since the distance is closer between chip and leads which reduced the heat transfer process [11]. The optimization for the package design with fine pitch leads is expected [12]. Displacement and stress distribution play important roles in the lead frames, thus reliability tests are necessary to assess the performance of the both two types of leads with different designs of geometry subjected to thermal stress [13]. Once the local stress exceeds yield stress, there would be plastic deformation. Furthermore, there will be a lethal consequence for the whole system when any part of the lead reaches to fracture stress. The critical locations on the copper leads are compulsory to be found to avoid hazardous deformation or fracture. The relevant parametric study could also improve the design of the package. Hence, Finite element analysis (FEA) would be one of the best choices to evaluate the case which is widely used in the electronic packaging industry for modelling the physics of failure in components of package or solder joints subjected to thermal or mechanical stress condition [14]. In FEA simulation of thermal stress analysis, the initial temperature or stress-free state temperature is often modelled at room temperature (25 ), or at the underfill encapsulant cure temperature. For example, Pang and Chong [15] used room temperature at 25 as the initial stress-free state temperature, while Michealides and Sitaraman and Pang et al. assumed a stress-free state of 140 at the curing temperature of the underfill epoxy [16,17]. Hybrid high density QFP is always desirable to be designed, but there are yield or fracture possibilities for leads and solder joint with the thermal or mechanical loading applied. Seldom works have been done to find out the optimal design in hybrid high density QFP, and stress analysis by using parametric study is found even few in recent work [18,19]. In this study, there is an innovative QFP package designed by NXP Sdn. Bhd. Hybrid 100-pin QFP has specially constituted with gull-wing and J shaped leads which are spaced on a 0.325mm pitch. These leads are the electrical and mechanical connections of the package to the outside world and are soldered to the surface of a PCB. Thermal and mechanical stresses in a surface mount assembly are determined by an elasto-plastic finite element method. Whole-field stress distributions in the gull-wing, J-lead and solder joint are also reported. Furthermore, the results presented herein are conducted with parametric study in angles of geometry of the lead design, and the optimal geometries are suggested. II. RESEARCH METHODOLOGY Two types of 2mm long copper leads with 0.245mm x 0.2mm cross section were used in this study, and the minimal thickness of solder joint is 0.1mm. The boundary condition of the solder joint is prescribed as fixed constraint, and copper lead is defined also fixed attached on solder material. All stress and displacement applied on the boundary between lead and chip are defined as horizontal direction. Ambient temperature 25 was used in this work and 300 temperature generated from chip which is two times higher than maximal working temperature in most existing QFPs [16, 17] was applied on the closest edge to the chip of all cases as boundary condition for dealing with the stress distribution. Hereby, the newly designed package was depicted in Fig. 1 where the package constituted gull wing and J-lead placed side by side. To find out the optimal designs of the geometry of both two types of lead with mechanical stress applied, refer to US patent No A, a parametric study was done with 4 inclination angles 45, 60, 75, 90 for gull wing, and 4 inclination angles 30 ( ), 45 ( ), 60 ( ), 75 ( ) for J-lead [21]. Fig. 1. QFP package with variable angles for parametric study The materials used in this study are listed in table 1 below refer to literature [22]. In order to have fine meshing for convergence and accurate results, the meshing size for both two types of lead were 0.01mm based on the small size of the models. TABLE I. Materials Copper lead MATERIALS OF LEADS AND SOLDER JOINT IN THIS WORK. Young's modulus (E) 1.17x10 5 MPa SAC x10 4 MPa Poisson's ratio (ν) coefficients of thermal expansion (CTE) Thermal conductivity ppm /k w/mm/k ppm /k w/mm/k In ANSYS software, finite element method is applied, the incremental displacement field is represented by interpolation functions together with incremental generalized displacements at a finite number of nodal points in each element. In matrix form the assumed incremental displacement can be written as (1)

3 where is an incremental displacement column matrix, is a nodal incremental displacement column matrix, and is a shape function matrix. The corresponding incremental strain column matrix is where is a differential operators matrix. The corresponding incremental stress column matrix is where and are incremental column matrix of initial strains and column matrix of initial stresses respectively, and is a material matrix which depends on the current state of stress and hardening of the material. Based on the Prandtl-Reuss theory and the Von Mises yield criterion [18-20], it can be shown that the material matrix in Eq. (3) has the following form Where (2) (3) (4) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (5) is elastic material matrix, is plastic material matrix, E is Young's modulus, is Poisson's ratio, is equivalent (Von Mises) stress, is equivalent plastic strain, is stress component in X direction, is stress component in Y direction, is stress component in Z direction, is shear stress in XY plane, is shear stress in XZ plane, is shear stress in ZX plane, is plastic strain component in X direction, is plastic strain component in Y direction, is plastic strain component in Z direction, is plastic shear strain in XY plane, is plastic shear strain in YZ plane and is plastic shear strain in ZX plane. And (6) (7) (8) III. RESULTS AND DISCUSSIONS A. Thermal stress analysis Before the stress distribution was analysed, it was important to observe the temperature distribution of two types of lead since there were 2 different materials consisted in the QFP. The temperature contours for gull wing and J-lead are shown in Fig. 2 where the heat generated from the chip is 300 with red color in the edge connected with chip, and room temperature 25 is shown around the solder material and part of the furthest end of J-lead. This meant the heat transferred from the chip all the way to the solder material was linear in one material, but the phenomenon was complicated around boundary of two different materials. Due to different CTE in two materials, the temperature transmission and heat

4 distribution around the boundary of lead and solder joint was nonuniform. (a) (a) Fig. 2. (a) Temperature distribution of Gull wing 45 ; (b) temperature distribution of J-lead 75 After the temperature distribution has been checked, the stress distribution by applying Von Mises criteria was also carried out. The stress distribution contour is shown in Fig. 3 where the edge with dashed white line is the original model and the deformed model is coloured. 300 temperature was the higher than normal working status in most QFPs, but the maximum stress on both Gull wing and J-lead are shown only MPa and MPa respectively which are less than yield stress 200MPa. It indicated that 300 would not lead the leads with plastic deformation, but cyclic thermal stress within this temperature would be a potential safety hazard for package. The cooling system would be ideal to be employed [1-4]. The maximum stress distributions on gull wing in red color are at the corner on top left of it and the left side of the boundary between lead and solder joint. Similarly, the maximum stress was also concentrated on the top left corner on J-lead which indicated the critical locations of these two types of lead are both in the corner that close to the heat source. (b) Fig. 3. (a) Stress distribution of Gull wing 45 within thermal loading; (b) stress distribution of J-lead 75 within thermal loading. B. Parametric study for different geometries with mechanical loading applied The yield stress for copper lead is 200MPa, different geometries of these two types of lead are critical to affect the maximum stress on it when mechanical loading is applied on the boundary between lead and chip. Fig. 4 is shown below by applying Von Mises criteria that critical locations for all 4 angles of gull wing were always around the top left corner close to chip where the displacement or stress were applied. But right bottom corners are also shown the maximum stress distribution with red color when the angle changes to 75 and 90. (b) 45

5 Fig. 4. Stress distribution contour of Gull wing when yield stress (200 MPa) reached For J-lead, due to the volume of solder material applied became more while the copper lead was extended, the stress distribution was not shown around solder material part for all 4 angles (Fig. 5). The critical location for J-lead was always around top left corner where it is close to loading boundary Fig. 5. Stress distribution contour of J-lead when yield stress (200 MPa) reached In Fig. 6 and 7, the values of displacement and stress applied on the boundary between chip and leads are shown when yield stress on critical location was reached. The results indicated that minimal angle of gull wing could be applied

6 minimal displacement (0.9441µm) where the angle was designed as 45, but this geometry could be applied with maximum stress MPa on the boundary. However, different geometries affected not significantly on displacement applied on J-lead to reach the yield stress on critical location as the values of displacement applied on the boundary were close. The optimal design of J-lead is suggested with 75 since the maximal stress (4.63MPa) could be applied on it (Fig. 7). Displacement (um) Fig. 6. Displacement applied for two types of lead to reach yield stress Stress (MPa) Max displacement to reach Gull wing yield stress 200MPa Fig. 7. Max stress applied for two types of lead to reach yield stress IV. CONCLUSIONS Inclination(degree) J lead Max stress to reach yield stress Gull wing MPa J lead Evaluations of the stress distributions on QFP package and the optimal geometry of lead are of interest in researches on electronic engineering design. Numerical modeling is always sought in a structural design process to minimize the use of experimentations due to the complexities of experimental setup. Here, the parametric study of a series of different geometries were presented. A few statements may be fairly drawn as follows: The 300 C temperature would not lead the critical location on both gull wing and J-lead reach to yield stress. The increased angle produced the larger displacement and smaller stress that could be applied for Gull-wing, and smaller angle as optimal design of it is suggested. The increased angles could not affect displacement, but larger stress could be applied for J-lead. Thus, the larger angle Inclination(degree) design is suggested without the cost of solder material in consideration. ACKNOWLEDGMENT Support from Malaysia Collaborative Research in, Science & Technology (P12: Hybrid-High Density Lead-frame Package) and Partnership grant (RK A) are gratefully acknowledged. REFERENCES [1] L. Zhang, L. Sun, Y.-h. Guo, and C.-w. He, "Reliability of lead-free solder joints in CSP device under thermal cycling," Journal of Materials Science: Materials in Electronics, vol. 25, pp , [2] V. Vasudevan and X. Fan, "An acceleration model for lead-free (SAC) solder joint reliability under thermal cycling," in Electronic Components and Technology Conference, ECTC th, 2008, pp [3] B. Vandevelde, M. Gonzalez, P. Limaye, P. Ratchev, and E. Beyne, "Hermal cycling reliability of snagcu and snpb solder joints: a comparison for several ic-packages," in Thermal and Mechanical Simulation and Experiments in Microelectronics and Microsystems, EuroSimE Proceedings of the 5th International Conference on, 2004, pp [4] J. H. Pang, D. Chong, and T. Low, "Thermal cycling analysis of flipchip solder joint reliability," IEEE Transactions on components and packaging technologies, vol. 24, pp , [5] A. Syed, "Predicting solder joint reliability for thermal, power, and bend cycle within 25% accuracy," in Electronic Components and Technology Conference, Proceedings., 51st, 2001, pp [6] M. Erinc, P. Schreurs, and M. Geers, "Intergranular thermal fatigue damage evolution in SnAgCu lead-free solder," Mechanics of Materials, vol. 40, pp , [7] M. Shah, "Analysis of parameters influencing stresses in the solder joints of leadless chip capacitors," Journal of Electronic Packaging, vol. 112, pp , [8] J. Lau, L. Powers-Maloney, J. R. Baker, D. Rice, and B. Shaw, "Solder joint reliability of fine pitch surface mount technology assemblies," IEEE Transactions on Components, Hybrids, and Manufacturing Technology, vol. 13, pp , [9] J. H. Lau, D. Rice, and C. Harkins, "Thermal stress analysis of tape automated bonding packages and interconnections," IEEE Transactions on Components, Hybrids, and Manufacturing Technology, vol. 13, pp , [10] N. F. Enke, T. J. Kilinski, S. A. Schroeder, and J. R. Lesniak, "Mechanical behaviors of 60/40 tin-lead solder lap joints," IEEE transactions on components, hybrids, and manufacturing technology, vol. 12, pp , [11] S. Vaynman, "Effect of strain rate on fatigue of low-tin lead-base solder," in Electronic Components Conference, Proceedings., 39th, 1989, pp [12] D. R. Frear, "Thermomechanical fatigue of solder joints: A new comprehensive test method," in Electronic Components Conference, Proceedings., 39th, 1989, pp [13] P. Hall, "Creep and stress relaxation in solder joints of surface-mounted chip carriers," IEEE Transactions on Components, Hybrids, and Manufacturing Technology, vol. 10, pp , [14] H. Charles and G. Clatterbaugh, "Solder Joint Reliability Design Implications from Finite Element Modeling and Experimental Testing," Journal of Electronic Packaging, vol. 112, pp , [15] H. Pang and Y. Chong, "FEA modeling of FCOB assembly solder joint reliability," Proc. MicroMat, pp , [16] S. Michaelides and S. K. Sitaraman, "Effect of material and geometry parameters on the thermo-mechanical reliability of flip-chip assemblies," in Thermal and Thermomechanical Phenomena in Electronic Systems,

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