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Natural convection on inclined QFN32 electronic package generating constant volumetric heat flux
International Communications in Heat and Mass Transfer 66 (2015) 133–139
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Natural convection on inclined QFN32 electronic package generating constant volumetric heat flux A. Baïri University of Paris, LTIE-GTE EA 4415, 50, rue de Sèvres, F-92410 Ville d'Avray, France
a r t i c l e
i n f o
Available online 29 May 2015 Keywords: Natural convection Electronics Thermoregulation QFN32 CFD Enclosure Packaging
a b s t r a c t Qualification and quantification of the natural convective phenomena are examined in the case of a Quad Flat Non-lead (QFN32). This active electronic package is inclined with respect to the horizontal plane by an angle varying between 0° and 90° corresponding to the horizontal and vertical position respectively. It generates during its operation a constant volumetric heat flux leading to Rayleigh numbers varying in the range 1.31x107 ‐ 1.01x108. The walls of the large air-filled cubic cavity containing this device are maintained isothermal. The temperature and velocity fields are presented for different combinations of the Rayleigh number and inclination angle. The convective heat transfer concerning the whole component exchange surface is determined for all the treated configurations. Correlations of Nusselt–Rayleigh type are proposed. They allow optimizing the thermal design of electronic assemblies used in various engineering domains. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The Quad Flat Non-lead (QFN) is integrated in most modern electronic devices. Its electrical performance allows using it in assemblies involving high frequencies and power densities. Its weight and volume are reduced compared to other conventional devices, allowing its use in various industrial sectors. Digital cameras equipped with this package are increasingly smaller and lighter while becoming more efficient. The assemblies equipped with QFN are also favored in the area of surveillance and security. Their small size and dimensions allow installing them in reduced volumes of industrial assemblies without having to drastically modify the existing arrangements. They are currently generalized in various industrial sectors such as automotive and aerospace. Several studies are carried out to develop this package in most areas of technology. The thermal aspects play an important role in this development. Several QFN packages are commercially available. They are more or less voluminous, varying from (0.8 × 1.5 × 2.0) mm 3 to (1.0 × 9.0 × 9.0) mm3 . Some details and characteristics can be found in the technical note [1] which also contains Standard and Land Pattern Calculator Tool concerning these Integrated Circuits (IC). More information about these products and thermal test methods can be found in [2–4]. Heat sink modules and finned devices are often used in cooling electronics systems, as proposed by Huang et al. [5]. The objective of their numerical study based on the Levenberg–Marquardt Method (LMM) is to reduce the temperature in the end array and optimize the heat sink shape. The proposed design algorithm is applied to 3 different
E-mail addresses: [email protected], [email protected].
http://dx.doi.org/10.1016/j.icheatmasstransfer.2015.05.016 0735-1933/© 2015 Elsevier Ltd. All rights reserved.
heat sinks. Some temperature measurements by means of a thermal camera complete and validate the numerical approach. The thickness optimization of a heat sink base applied to electronics cooling has been considered by Li and Shi [6]. The hot source representing the electronic device is centered on the base of the finned area. In this 3D numerical approach done by means of the finite volume method, the heat transfer coefficient is imposed. Three values have been considered: 50, 15 × 102 and 22 × 103 Wm−2 K−1 corresponding to some applications for air natural convection, air forced convection and liquid forced convection respectively. Temperature measurements are performed to validate the calculated results. The thermal contact resistance between the heat source and the basis of the finned surface plays an important role in this type of problem. This parameter has been addressed in several studies according to the contact type, including those of Laraqi et al. [7], Vintrou et al. [8], Baïri and Laraqi [9] and Yang et al. [10]. These studies often rely on the Biot number to compare conductive and convective thermal resistances involved in the considered problem. Values of the heat transfer coefficient considered in [6] have obviously different effects on the thermal state of the electronic device. Although the values taken into account are representative of some situations, it is important to know more accurately the real values concerning the treated configuration, as it is the case in the studies of Basak et al. [11], Sathiyamoorthy et al. [12] and Abhinav et al. [13]. Using heat sink modules also requires exact knowledge of the fins' thermal conductivity, often metallic, as presented in the review of Reif-Acherman [14]. When high power levels are generated, natural convective heat exchange is sometimes insufficient when air is the convective fluid, as it is the case in some electronic applications. Other heat transfer fluids are then used such as electrostatic fluid accelerators, phase change material or nanofluids. Several studies are devoted to these fluids. The
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A. Baïri / International Communications in Heat and Mass Transfer 66 (2015) 133–139
Nomenclature thermal diffusivity of the air (m2s−1) specific heat at constant pressure (J kg−1 K−1) dimensionless unit vector opposite to the gravity direction g gravity acceleration (m s−2) local convective heat transfer coefficient of the ith elehi ment (Wm−2 K−1) L characteristic length (m) m number of surface elements of the QFN package (−) local Nusselt number of the ith element (−) Nui Nuα average Nusselt number (−) p pressure (Pa) p* dimensionless pressure (−) Pr Prandtl Number (−) heat flux of the ith element (Wm−2) qi Rayleigh number (−) RaL,φ external area of the considered area (m2) Sh area of the ith element of the QFN package (m2) Si temperature of the cavity's walls and initial temperaTc ture of the whole system (K) local temperature of the ith element of the QFN package Ti (K) T* dimensionless temperature (−) dimensionless average surfacic temperature (−) Th⁎ dimensionless average surfacic temperature for an Th,α⁎ angle α (−) ! u velocity vector ! dimensionless velocity vector u V* dimensionless velocity (−) (x, y, z) Cartesian coordinates (m) (x*, y*, z*) dimensionless coordinates (−) a Cp ! eg
Greek symbols αZ inclination angle of the QFN package with respect to the horizontal (°) β air volumetric expansion coefficient (K−1) ! dimensionless nabla operator (−) ∇ dimensionless Laplacian operator (−) ∇*2 φ volumetric heat flux (Wm−3) λ air thermal conductivity (Wm−1 K−1) thermal conductivity of the QFN32 package compoλp nents (Wm−1 K−1) μ air dynamic viscosity (Pa s) ρ air density (kg m−3)
recent review of Öztop et al. [15] contains interesting elements of comparison between natural convective exchange with and without nanofluids. The warpage phenomenon occurs during the manufacturing process of the QFN packages which involves thermosetting polymers for encapsulation. This decreases its reliability and causes contractions and expansions that may produce dysfunctions and even destroy the package if the thermal phenomena are not controlled. The numerical survey of Yang et al. [16] considering a cure-dependent viscoelastic constitutive model allows examination of the material's characteristics all along the curing process of the thermosetting polymer. The measurements confirm the numerical approach's findings that the material coefficient of thermal expansion (CTE) greatly influences the warpage phenomenon and thus the performance of the QFN package. Thus the correct choice of the encapsulation material is very important for the package manufacturing process and its optimal operation. Thermal phenomena should be controlled at some industrial production stages. Conventional methods for cutting individual elements
could involve local heating and cooling which can lead to cracking or even breaking the packages. This is mostly due to the thermal characteristics of the involved materials, mainly their corresponding CTE. Li et al. [17] and Tsai et al. [18] propose optimized laser cutting processes. By examining the experimental results obtained on devices equipped with QFN32 packages, Bahi et al. [19] show the influence of thermal phenomena on their resistance and integrity. Tests show that the thermal storage duration does not affect the delamination and does not degrade the thermal properties of the resin if it is completely polymerized. The authors propose optimized sequential tests for the qualification package, in which the temperature plays an important role. Chen et al. [20] propose a wideband equivalent circuit model of QFN packages for radio frequency (RF) applications which are widely used in domestic devices such as cellular phones. The numerical study shows that the best structure of the QFN32 package for RF applications consists of double bonding wires with a lower dielectric-constant molding compound and larger die-pad. The temperature distribution has a strong influence on the performance of QFN used in the field of instrumental techniques and power conversion. The experimental study of Feld et al. [21] deals with a DC-DC converter using a QFN32 package. The results show that the magnetic-shield cover lowers the operating temperature of the inductor of about 20 °C below the temperature observed during normal operation. This technique is interesting as it increases the converters' efficiency by controlling the heat transfer phenomena occurring within the assembly. Monier-Vinard et al. [22] propose a prediction model for critical component areas with a minimized depreciation. The reduction process is based on a generic fitting technique to achieve a Dynamic Compact Thermal Model. The rupture phenomena are sometimes due to excessive temperatures in the junctions and contacts between the components and the PCB board on which they are welded. This is highlighted by Radivojevic et al. [23] in a study applied to RF amplifiers characterized by high power generation. The authors show that exceeding the critical temperature junctions greatly reduces the assembly's reliability, causes premature fatigue in components and leads either to failures or destruction of all or part of the assembly. Determining the electronic assembly's performance requires considering all the parameters that can influence the overall natural convective heat transfer coefficient. The main parameters are the geometry of the enclosure, the support of the heating element and the package itself, their dimensions, their position relative to the gravity field, their thermal characteristics, the involved temperatures, the generated power, the fluid characteristics and the thermal boundary conditions. This is the objective of the present work dealing with thermoregulation of a QFN32 package positioned in the center of a plate (PCB) contained in a cubic cavity inclined with respect to the horizontal plane by an angle varying between 0° and 90° corresponding to the horizontal and vertical position respectively, with a step of 15°. The solution obtained numerically by means of the finite volume method allows determining the thermal and dynamic phenomena. Nusselt–Rayleigh type correlations are proposed, valid for Rayleigh Numbers in the range of 1.31x107 ‐ 1.01x108. They allow thermal optimization of electronic assemblies used in various engineering domains, and increase their reliability. 2. Treated configurations. Governing equations. Numerical solution The considered QFN32 package is schematically presented in Fig. 1(a). The characteristic length L = 5 mm (external length) and the Cartesian reference (x, y, z) are defined in Fig. 1(b) and (c), while dimensions of the assembly are detailed in Fig. 1(d) and (e). The Die (1) constituting the hot element measures (2.8x2.8x0.28)mm3 according to (x, y, z) directions respectively. Its surface layer (2) of 20 μm thickness is active. The Die is fixed to the lead pad (or die pad) (3) located at the bottom by means of a thin layer of paste (4), which ensures a stable electrical ground. The 32 leads (5) connect the package to the electronic assembly, and the whole device is encapsulated by
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Fig. 1. (a) Schematic of the treated QFN32 package. (b) The characteristic length L. (c) The Cartesian reference (x, y, z). (d,e) Dimensions details (in mm) of the assembly.
means of a molding compound (6). The QFN package can therefore be modeled as a (5.0x5.0x0.9) mm3 parallelepiped, represented in Fig. 2(a). It is welded in the center of a (100x100x1.5) mm3 printed circuit board (PCB), installed in a (200x200x200) mm3 cubic cavity. The whole system is inclined with respect to the horizontal plane by an angle varying between 0° (horizontal, Fig. 2(b)) and 90° (vertical, 2(c)) by steps of 15° as represented in Fig. 2(d). These positions correspond to the real situations realized in the considered industrial electronic assembly. With the adopted configuration and dimensions,
the natural convective flow is assumed to be free and unobstructed. Despite the small dimensions, the surface phenomena are taken into consideration all around the device which is decomposed into three distinct surfaces schematized in Fig. 2(a): (i) the top denoted as (T); (ii) the lateral surfaces denoted as (S); and (iii) the remaining area denoted as (P) consisting of the rear face, the four lateral surfaces of the PCB board, and the front of this board. In this steady state work, the device's materials are assumed to be thermally homogeneous. Their thermal conductivities λp listed in Table 1 are considered as isotropic.
Fig. 2. (a) Details of the QFN package welded on the PCB; (b)–(c) The cavity containing the device in horizontal and vertical position respectively. (d) The 7 treated inclination angles. All dimensions are given in mm.
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A. Baïri / International Communications in Heat and Mass Transfer 66 (2015) 133–139 Table 1 Thermal conductivity λp of the QFN32 package components. λp (Wm−1 K−1) Die (1) + (2) Diepad (3) Paste (4) Leads (5) Molding compound (6)
120 260 2.1 260 0.66
The thermal conductivity of the PCB's resin is equal to 20, 20 and 0.25 Wm−1 K−1 for (x, y, z) directions respectively and considered as temperature independent in the range concerned by this study. The walls of the cavity are maintained isothermal at temperature Tc = 20°C. The active part of the Die generates a power varying between 0 (device off) and 0.8 W. The resulting volumetric heat flux φ assumed to be constant and homogeneous could reach 5.1 GWm−3. The heat exchange surface of the assembly with the environment denoted as Sh is discretized in this numerical survey into m elements whose heat exchange area and temperature are denoted by Si and Ti (i = 1, m) respectively. The dimensionless system for the laminar steady state flows considered in this problem is Continuity equation: ! ! ∇ u ¼0
ð1Þ
Momentum equation: ! ! ! ! ! ! u ∇ u ¼ − ∇ p þ Pr∇2 u þ RaL;φ PrT e g
be incompressible, the Boussinesq approximation is applied and values of the other thermophysical properties of the air are systematically evaluated at the average temperature of each control volume. Given that the present study is focused on the convective phenomena, radiation is not considered. This condition is easily realized by imposing a global infrared emissivity equal to zero at all the walls. The numerical code used to solve the system of Eqs. (1)–(3) is the commercial software Ansys-Fluent [24] based on the control volume method and using the SIMPLE algorithm. The main parameters are exported into an interface developed in the research group in order to calculate the local convective heat transfer around the QFN package. The mesh consists of quadrilateral wall faces and hexagonal cells, as detailed in Fig. 3. It is refined all around the package to determine with precision the thermal gradients distribution over the entire outer surface of the device. The same refinement is done at the cavity's boundaries in order to realize the heat balance and check the convergence calculations. Although the flow strongly depends on the considered combination (RaL,ϕ, α), the mesh of the domain has been kept constant. It corresponds to the highest values of these parameters, involving the most important convective exchanges. This operation does not optimize the computing time for a given case (RaL,ϕ, α), however the total time required to complete the full study is reduced by avoiding to handle one different mesh for each treated cases. The average time saved with this operation is about 22%. The convergence criteria in the numerical process are set to 10‐ 5 for velocity components and 10‐ 6 for energy. Determining the temperature gradient (∂Ti/∂n)wall at the surface of every element (i = 1, m) allows calculation of the local heat flux qi = − λ(∂Ti/∂n)wall and then the distribution of the local convective heat transfer coefficient all around the package with
ð2Þ hi ¼
Energy equation: ! ! for the air : u ∇ T ¼ ∇2 T for the QFN package0 s active part : ∇2 T þ 1 ¼ 0
ð3Þ
! In these equations, the nabla operator ∇ , the Laplacian ∇*2 and the ! unit vector opposite to the gravity direction e g are dimensionless. The ! dimensionless velocity vector u , pressure p*, the Prandtl number Pr and cartesian coordinate x*, y*, z* are defined by ! μC p x; y; z u L L2 p ;x ;y ;z ¼ ; Pr ¼ ;p ¼ u ¼ λ a L ρ a2
!
ð4Þ
where μ, ρ, Cp and a = λ/ρCp are respectively the air's dynamic viscosity, density, specific heat at constant pressure and thermal diffusivity. Given the considered thermal boundary, the dimensionless temperature T* and the Rayleigh number RaL,ϕ based on the characteristic length L are calculated with T ¼
T−T c gβL5 ρ ; RaL;ϕ ¼ ϕ μaλ T h −T c
ð5Þ
being T h the average package's surface temperature. In a survey considering effects of vent locations in an enclosure containing a heat source, Abhinav et al. [13] define the dimensionless temperature with T* = (T − T∞)/(Tmax − T∞) where T∞ is the air ambient temperature while Tmax is the maximum temperature in the fluid domain. The definition adopted here for RaL,ϕ is representative of the present study. The entire domain including the air and the cavity is initially isothermal at the lowest temperature Tc under standard atmospheric pressure. The same thermal condition is applied to the QFN package initially considered as passive (off). The imposed thermal boundary conditions are: a constant volumetric heat flux ϕ in the Die's active part, as well as a no-slip condition on all the internal walls. The air is assumed to
qi : ðT i −T c Þ
ð6Þ
The local Nusselt Number based on the characteristic length L is deduced with Nui ¼
hi L λ
ð7Þ
whose integration over the total considered surface Sh allows determination of the average Nusselt number Nuα for a specific angle α Nuα ¼
1 ∬ Nui dSi Sh Sh
! :
ð8Þ
wall
This parameter constitutes the main parameter in this survey dealing with natural convection. The numerical solution is considered as mesh-independent when the variation in successive Nuα values is less than 3% after refining the mesh. Such condition is attained in this work with 478,607 nodes, 1,450,063 quadrilateral wall faces and 450,465 hexagonal cells. 3. Results Calculations are performed for several combinations of the inclination angle α = 0 − 90° step 15° and the Rayleigh number RaL,ϕ varying between 1.31x107 and 1.01 x 108. The dimensionless temperature T* and velocity V* fields around the device are presented in Fig. 4 at RaL,ϕ = 1.01 x 108 for some configurations in the (x, y) and (y, z) planes. Heat exchanges are in agreement with the temperature and velocity distributions. The presented example concerning the maximum generated power (0.8 W) confirms that the maximum heat flux exchanged device-environment occurs in the immediate vicinity of the active part, towards its top face. Examination of the overall results for all inclinations shows that the average heat flux concerning the area (S) is about 33% higher than that of (T). The average temperature of the
A. Baïri / International Communications in Heat and Mass Transfer 66 (2015) 133–139
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Fig. 3. Some details of the adopted mesh around the device.
upper face of the PCB board (outside the device area) is between 0.5 and 1.2 °C above Tc. The local low heat flux confirms that the mean convective heat transfer on the free part of this upper side of the PCB board is small compared to that of (T) and (S). The convective heat transfer in the upper part of the plate is therefore concentrated on and around the active device. Furthermore, the average convective heat flux corresponding to the back face and the four lateral surfaces of the PCB board are low compared to those of (T) and (S). The maximum heat flux concerning this back board face is concentrated around the opposite surface with respect to the active device.
The difference between the average temperature of the back face of the plate and Tc is of about 0.9 °C. Given the previous comments, the system is simplified, decomposed into only two parts: (i) the “front face” denoted as (F), which groups (T), (S) and the free upper part of the PCB board (P); (ii) the “back face” denoted as (B) which groups the remaining part of (P), i.e. its rear face and four lateral surfaces. To determine the effective natural convective power exchanged between the assembly and the environment, it is necessary to take into account the exchange areas concerning all the assembly elements, and associate them with their corresponding heat flux.
Fig. 4. Temperature and velocity fields around the device in (x, y) and (y, z) planes for RaL,φ = 1.01 x 108 and α = 0°, 45° and 90°.
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A. Baïri / International Communications in Heat and Mass Transfer 66 (2015) 133–139
Being the (T) and (S) areas very low compared to that of the free surface of the upper face of the PCB board, the average convective heat flux exchanged by the front face (F) is close to that of the free surface. It is the same for the area (B). Evolution of the average Nusselt number Nuα versus RaL,ϕ is clearly linear of Nuα ¼ kðα Þ þ nðα ÞRaL;ϕ type for all the elements of the assembly. It is presented as an example in Fig. 5(a) for the (T) area and the 7 inclination angles α = 0 − 90° step 15°. The same trend concerns the overall assembly. Evolution of the coefficients k(α) and n(α) is linear and polynomial of the second order respectively, as represented in Fig. 5(b). The results of this work lead finally to the correlations Nuα ¼ kðα Þ þ nðα ÞRaL;ϕ Fig. 6. Th,α⁎ versus RaL,φ for α = 0 − 90° step 15°.
withf
0:0167α þ 4:1815 : ðTÞ area þ 2:8302 : Overall assembly ( 0:00931α ‐0:0008α 2 þ 0:1183α þ 2:9101 x10−9 : ðTÞ area nðα Þ ¼ −9 ð0:00119α þ 1:197Þx10 : Overall assembly
based on the maximum temperature difference corresponding to the horizontal position (α = 0)
kðα Þ ¼
valid for
ð9Þ
0≤α ≤90 1:31x107 ≤RaL;ϕ ≤1:01x108
being the coefficients of determination associated to k(α) and n(α) equal to 0.9991 and 0.9985 respectively. The average surface temperature of the package's front face increases with RaL,ϕ and decreases with α. Its dimensionless temperature
T h;α
T h −T c α ¼ T h −T c α¼0
ð10Þ
is presented in Fig. 6. The value corresponding to the minimum generated power (0.1 W; RaL,ϕ minimum) is roughly the same for all angles in relative terms: the deviation between the values corresponding to both extreme inclination angles (horizontal and vertical positions) is about 2.1%. For the maximum RaL,ϕ, this deviation is twice as high, reaching 4.5%.
Fig. 5. (a) Nuα versus RaL,φ for α = 0 − 90° step 15°. (b) Evolution of k(α) and n(α) for Nuα ¼ kðα Þ þ nðα ÞRaL;φ correlations for the (T) area.
A. Baïri / International Communications in Heat and Mass Transfer 66 (2015) 133–139
4. Conclusion The natural convective phenomena are examined for a Quad Flat Non-lead (QFN32) inclined with respect to the horizontal plane by an angle varying between 0° (horizontal) and 90° (vertical position). This active electronic device generating a constant volumetric heat flux during its operation is contained in an air-filled cubical cavity whose walls are maintained isothermal. The temperature and velocity fields are presented and completed with an analysis of the thermal state of the assembly. The natural convective heat transfer is quantified by means of Nusselt–Rayleigh type valid for Rayleigh numbers in the range of 1.31x107 ‐ 1.01x108. They allow optimizing the thermal design and increase the reliability of these electronic assemblies, which are used in various engineering domains. Acknowledgments The author and all his colleagues of the Research Laboratory (Laboratoire Thermique, Interfaces, Environnement, LTIE EA 4415) and the Thermal and Energy Engineering Department (Département Génie Thermique et Energie, GTE) of Paris X University (IUT Ville d'Avray) express their deep emotion to the family of their colleague Professor Laurent Proslier. References [1] Trinamic application note 005, Rev. 1.01, http://www.trinamic.com 2013. [2] Integrated Circuits Thermal Test Method Environmental Conditions—Natural Convection (Still Air), Jedec Solid State Technology Association, 2008. (JESD51-2A). [3] Guidelines for Reporting and Using Electronic Package Thermal Information, Jedec Solid State Technology Association, 2005. (JESD51-12). [4] Semiconductor and IC Package Thermal Metrics, Application Report, Texas Instruments, 2012. (SPRA953B–July). [5] C.H. Huang, J.R. Lu, H. Ay, A three-dimensional heat sink module design problem with experimental verification, Int. J. Heat Mass Transf. 54 (2011) 1482–1492. [6] J. Li, Z.S. Shi, 3D numerical optimization of a heat sink base for electronics cooling, Int. Commun. Heat Mass Transfer 39 (2012) 204–208. [7] N. Laraqi, A. Baïri, Theory of thermal resistance between solids with randomly sized and located contacts, Int. J. Heat Mass Transf. 45 (20) (2002) 4175–4180. [8] S. Vintrou, N. Laraqi, A. Baïri, Calculation and analysis of thermal impedance of microelectronic structures from analytical models, Solid State Electron. 67 (1) (2012) 45–52.
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Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Natural convection on inclined QFN32 electronic package generating constant volumetric heat flux A. Baïri University of Paris, LTIE-GTE EA 4415, 50, rue de Sèvres, F-92410 Ville d'Avray, France
a r t i c l e
i n f o
Available online 29 May 2015 Keywords: Natural convection Electronics Thermoregulation QFN32 CFD Enclosure Packaging
a b s t r a c t Qualification and quantification of the natural convective phenomena are examined in the case of a Quad Flat Non-lead (QFN32). This active electronic package is inclined with respect to the horizontal plane by an angle varying between 0° and 90° corresponding to the horizontal and vertical position respectively. It generates during its operation a constant volumetric heat flux leading to Rayleigh numbers varying in the range 1.31x107 ‐ 1.01x108. The walls of the large air-filled cubic cavity containing this device are maintained isothermal. The temperature and velocity fields are presented for different combinations of the Rayleigh number and inclination angle. The convective heat transfer concerning the whole component exchange surface is determined for all the treated configurations. Correlations of Nusselt–Rayleigh type are proposed. They allow optimizing the thermal design of electronic assemblies used in various engineering domains. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The Quad Flat Non-lead (QFN) is integrated in most modern electronic devices. Its electrical performance allows using it in assemblies involving high frequencies and power densities. Its weight and volume are reduced compared to other conventional devices, allowing its use in various industrial sectors. Digital cameras equipped with this package are increasingly smaller and lighter while becoming more efficient. The assemblies equipped with QFN are also favored in the area of surveillance and security. Their small size and dimensions allow installing them in reduced volumes of industrial assemblies without having to drastically modify the existing arrangements. They are currently generalized in various industrial sectors such as automotive and aerospace. Several studies are carried out to develop this package in most areas of technology. The thermal aspects play an important role in this development. Several QFN packages are commercially available. They are more or less voluminous, varying from (0.8 × 1.5 × 2.0) mm 3 to (1.0 × 9.0 × 9.0) mm3 . Some details and characteristics can be found in the technical note [1] which also contains Standard and Land Pattern Calculator Tool concerning these Integrated Circuits (IC). More information about these products and thermal test methods can be found in [2–4]. Heat sink modules and finned devices are often used in cooling electronics systems, as proposed by Huang et al. [5]. The objective of their numerical study based on the Levenberg–Marquardt Method (LMM) is to reduce the temperature in the end array and optimize the heat sink shape. The proposed design algorithm is applied to 3 different
E-mail addresses: [email protected], [email protected].
http://dx.doi.org/10.1016/j.icheatmasstransfer.2015.05.016 0735-1933/© 2015 Elsevier Ltd. All rights reserved.
heat sinks. Some temperature measurements by means of a thermal camera complete and validate the numerical approach. The thickness optimization of a heat sink base applied to electronics cooling has been considered by Li and Shi [6]. The hot source representing the electronic device is centered on the base of the finned area. In this 3D numerical approach done by means of the finite volume method, the heat transfer coefficient is imposed. Three values have been considered: 50, 15 × 102 and 22 × 103 Wm−2 K−1 corresponding to some applications for air natural convection, air forced convection and liquid forced convection respectively. Temperature measurements are performed to validate the calculated results. The thermal contact resistance between the heat source and the basis of the finned surface plays an important role in this type of problem. This parameter has been addressed in several studies according to the contact type, including those of Laraqi et al. [7], Vintrou et al. [8], Baïri and Laraqi [9] and Yang et al. [10]. These studies often rely on the Biot number to compare conductive and convective thermal resistances involved in the considered problem. Values of the heat transfer coefficient considered in [6] have obviously different effects on the thermal state of the electronic device. Although the values taken into account are representative of some situations, it is important to know more accurately the real values concerning the treated configuration, as it is the case in the studies of Basak et al. [11], Sathiyamoorthy et al. [12] and Abhinav et al. [13]. Using heat sink modules also requires exact knowledge of the fins' thermal conductivity, often metallic, as presented in the review of Reif-Acherman [14]. When high power levels are generated, natural convective heat exchange is sometimes insufficient when air is the convective fluid, as it is the case in some electronic applications. Other heat transfer fluids are then used such as electrostatic fluid accelerators, phase change material or nanofluids. Several studies are devoted to these fluids. The
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Nomenclature thermal diffusivity of the air (m2s−1) specific heat at constant pressure (J kg−1 K−1) dimensionless unit vector opposite to the gravity direction g gravity acceleration (m s−2) local convective heat transfer coefficient of the ith elehi ment (Wm−2 K−1) L characteristic length (m) m number of surface elements of the QFN package (−) local Nusselt number of the ith element (−) Nui Nuα average Nusselt number (−) p pressure (Pa) p* dimensionless pressure (−) Pr Prandtl Number (−) heat flux of the ith element (Wm−2) qi Rayleigh number (−) RaL,φ external area of the considered area (m2) Sh area of the ith element of the QFN package (m2) Si temperature of the cavity's walls and initial temperaTc ture of the whole system (K) local temperature of the ith element of the QFN package Ti (K) T* dimensionless temperature (−) dimensionless average surfacic temperature (−) Th⁎ dimensionless average surfacic temperature for an Th,α⁎ angle α (−) ! u velocity vector ! dimensionless velocity vector u V* dimensionless velocity (−) (x, y, z) Cartesian coordinates (m) (x*, y*, z*) dimensionless coordinates (−) a Cp ! eg
Greek symbols αZ inclination angle of the QFN package with respect to the horizontal (°) β air volumetric expansion coefficient (K−1) ! dimensionless nabla operator (−) ∇ dimensionless Laplacian operator (−) ∇*2 φ volumetric heat flux (Wm−3) λ air thermal conductivity (Wm−1 K−1) thermal conductivity of the QFN32 package compoλp nents (Wm−1 K−1) μ air dynamic viscosity (Pa s) ρ air density (kg m−3)
recent review of Öztop et al. [15] contains interesting elements of comparison between natural convective exchange with and without nanofluids. The warpage phenomenon occurs during the manufacturing process of the QFN packages which involves thermosetting polymers for encapsulation. This decreases its reliability and causes contractions and expansions that may produce dysfunctions and even destroy the package if the thermal phenomena are not controlled. The numerical survey of Yang et al. [16] considering a cure-dependent viscoelastic constitutive model allows examination of the material's characteristics all along the curing process of the thermosetting polymer. The measurements confirm the numerical approach's findings that the material coefficient of thermal expansion (CTE) greatly influences the warpage phenomenon and thus the performance of the QFN package. Thus the correct choice of the encapsulation material is very important for the package manufacturing process and its optimal operation. Thermal phenomena should be controlled at some industrial production stages. Conventional methods for cutting individual elements
could involve local heating and cooling which can lead to cracking or even breaking the packages. This is mostly due to the thermal characteristics of the involved materials, mainly their corresponding CTE. Li et al. [17] and Tsai et al. [18] propose optimized laser cutting processes. By examining the experimental results obtained on devices equipped with QFN32 packages, Bahi et al. [19] show the influence of thermal phenomena on their resistance and integrity. Tests show that the thermal storage duration does not affect the delamination and does not degrade the thermal properties of the resin if it is completely polymerized. The authors propose optimized sequential tests for the qualification package, in which the temperature plays an important role. Chen et al. [20] propose a wideband equivalent circuit model of QFN packages for radio frequency (RF) applications which are widely used in domestic devices such as cellular phones. The numerical study shows that the best structure of the QFN32 package for RF applications consists of double bonding wires with a lower dielectric-constant molding compound and larger die-pad. The temperature distribution has a strong influence on the performance of QFN used in the field of instrumental techniques and power conversion. The experimental study of Feld et al. [21] deals with a DC-DC converter using a QFN32 package. The results show that the magnetic-shield cover lowers the operating temperature of the inductor of about 20 °C below the temperature observed during normal operation. This technique is interesting as it increases the converters' efficiency by controlling the heat transfer phenomena occurring within the assembly. Monier-Vinard et al. [22] propose a prediction model for critical component areas with a minimized depreciation. The reduction process is based on a generic fitting technique to achieve a Dynamic Compact Thermal Model. The rupture phenomena are sometimes due to excessive temperatures in the junctions and contacts between the components and the PCB board on which they are welded. This is highlighted by Radivojevic et al. [23] in a study applied to RF amplifiers characterized by high power generation. The authors show that exceeding the critical temperature junctions greatly reduces the assembly's reliability, causes premature fatigue in components and leads either to failures or destruction of all or part of the assembly. Determining the electronic assembly's performance requires considering all the parameters that can influence the overall natural convective heat transfer coefficient. The main parameters are the geometry of the enclosure, the support of the heating element and the package itself, their dimensions, their position relative to the gravity field, their thermal characteristics, the involved temperatures, the generated power, the fluid characteristics and the thermal boundary conditions. This is the objective of the present work dealing with thermoregulation of a QFN32 package positioned in the center of a plate (PCB) contained in a cubic cavity inclined with respect to the horizontal plane by an angle varying between 0° and 90° corresponding to the horizontal and vertical position respectively, with a step of 15°. The solution obtained numerically by means of the finite volume method allows determining the thermal and dynamic phenomena. Nusselt–Rayleigh type correlations are proposed, valid for Rayleigh Numbers in the range of 1.31x107 ‐ 1.01x108. They allow thermal optimization of electronic assemblies used in various engineering domains, and increase their reliability. 2. Treated configurations. Governing equations. Numerical solution The considered QFN32 package is schematically presented in Fig. 1(a). The characteristic length L = 5 mm (external length) and the Cartesian reference (x, y, z) are defined in Fig. 1(b) and (c), while dimensions of the assembly are detailed in Fig. 1(d) and (e). The Die (1) constituting the hot element measures (2.8x2.8x0.28)mm3 according to (x, y, z) directions respectively. Its surface layer (2) of 20 μm thickness is active. The Die is fixed to the lead pad (or die pad) (3) located at the bottom by means of a thin layer of paste (4), which ensures a stable electrical ground. The 32 leads (5) connect the package to the electronic assembly, and the whole device is encapsulated by
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Fig. 1. (a) Schematic of the treated QFN32 package. (b) The characteristic length L. (c) The Cartesian reference (x, y, z). (d,e) Dimensions details (in mm) of the assembly.
means of a molding compound (6). The QFN package can therefore be modeled as a (5.0x5.0x0.9) mm3 parallelepiped, represented in Fig. 2(a). It is welded in the center of a (100x100x1.5) mm3 printed circuit board (PCB), installed in a (200x200x200) mm3 cubic cavity. The whole system is inclined with respect to the horizontal plane by an angle varying between 0° (horizontal, Fig. 2(b)) and 90° (vertical, 2(c)) by steps of 15° as represented in Fig. 2(d). These positions correspond to the real situations realized in the considered industrial electronic assembly. With the adopted configuration and dimensions,
the natural convective flow is assumed to be free and unobstructed. Despite the small dimensions, the surface phenomena are taken into consideration all around the device which is decomposed into three distinct surfaces schematized in Fig. 2(a): (i) the top denoted as (T); (ii) the lateral surfaces denoted as (S); and (iii) the remaining area denoted as (P) consisting of the rear face, the four lateral surfaces of the PCB board, and the front of this board. In this steady state work, the device's materials are assumed to be thermally homogeneous. Their thermal conductivities λp listed in Table 1 are considered as isotropic.
Fig. 2. (a) Details of the QFN package welded on the PCB; (b)–(c) The cavity containing the device in horizontal and vertical position respectively. (d) The 7 treated inclination angles. All dimensions are given in mm.
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A. Baïri / International Communications in Heat and Mass Transfer 66 (2015) 133–139 Table 1 Thermal conductivity λp of the QFN32 package components. λp (Wm−1 K−1) Die (1) + (2) Diepad (3) Paste (4) Leads (5) Molding compound (6)
120 260 2.1 260 0.66
The thermal conductivity of the PCB's resin is equal to 20, 20 and 0.25 Wm−1 K−1 for (x, y, z) directions respectively and considered as temperature independent in the range concerned by this study. The walls of the cavity are maintained isothermal at temperature Tc = 20°C. The active part of the Die generates a power varying between 0 (device off) and 0.8 W. The resulting volumetric heat flux φ assumed to be constant and homogeneous could reach 5.1 GWm−3. The heat exchange surface of the assembly with the environment denoted as Sh is discretized in this numerical survey into m elements whose heat exchange area and temperature are denoted by Si and Ti (i = 1, m) respectively. The dimensionless system for the laminar steady state flows considered in this problem is Continuity equation: ! ! ∇ u ¼0
ð1Þ
Momentum equation: ! ! ! ! ! ! u ∇ u ¼ − ∇ p þ Pr∇2 u þ RaL;φ PrT e g
be incompressible, the Boussinesq approximation is applied and values of the other thermophysical properties of the air are systematically evaluated at the average temperature of each control volume. Given that the present study is focused on the convective phenomena, radiation is not considered. This condition is easily realized by imposing a global infrared emissivity equal to zero at all the walls. The numerical code used to solve the system of Eqs. (1)–(3) is the commercial software Ansys-Fluent [24] based on the control volume method and using the SIMPLE algorithm. The main parameters are exported into an interface developed in the research group in order to calculate the local convective heat transfer around the QFN package. The mesh consists of quadrilateral wall faces and hexagonal cells, as detailed in Fig. 3. It is refined all around the package to determine with precision the thermal gradients distribution over the entire outer surface of the device. The same refinement is done at the cavity's boundaries in order to realize the heat balance and check the convergence calculations. Although the flow strongly depends on the considered combination (RaL,ϕ, α), the mesh of the domain has been kept constant. It corresponds to the highest values of these parameters, involving the most important convective exchanges. This operation does not optimize the computing time for a given case (RaL,ϕ, α), however the total time required to complete the full study is reduced by avoiding to handle one different mesh for each treated cases. The average time saved with this operation is about 22%. The convergence criteria in the numerical process are set to 10‐ 5 for velocity components and 10‐ 6 for energy. Determining the temperature gradient (∂Ti/∂n)wall at the surface of every element (i = 1, m) allows calculation of the local heat flux qi = − λ(∂Ti/∂n)wall and then the distribution of the local convective heat transfer coefficient all around the package with
ð2Þ hi ¼
Energy equation: ! ! for the air : u ∇ T ¼ ∇2 T for the QFN package0 s active part : ∇2 T þ 1 ¼ 0
ð3Þ
! In these equations, the nabla operator ∇ , the Laplacian ∇*2 and the ! unit vector opposite to the gravity direction e g are dimensionless. The ! dimensionless velocity vector u , pressure p*, the Prandtl number Pr and cartesian coordinate x*, y*, z* are defined by ! μC p x; y; z u L L2 p ;x ;y ;z ¼ ; Pr ¼ ;p ¼ u ¼ λ a L ρ a2
!
ð4Þ
where μ, ρ, Cp and a = λ/ρCp are respectively the air's dynamic viscosity, density, specific heat at constant pressure and thermal diffusivity. Given the considered thermal boundary, the dimensionless temperature T* and the Rayleigh number RaL,ϕ based on the characteristic length L are calculated with T ¼
T−T c gβL5 ρ ; RaL;ϕ ¼ ϕ μaλ T h −T c
ð5Þ
being T h the average package's surface temperature. In a survey considering effects of vent locations in an enclosure containing a heat source, Abhinav et al. [13] define the dimensionless temperature with T* = (T − T∞)/(Tmax − T∞) where T∞ is the air ambient temperature while Tmax is the maximum temperature in the fluid domain. The definition adopted here for RaL,ϕ is representative of the present study. The entire domain including the air and the cavity is initially isothermal at the lowest temperature Tc under standard atmospheric pressure. The same thermal condition is applied to the QFN package initially considered as passive (off). The imposed thermal boundary conditions are: a constant volumetric heat flux ϕ in the Die's active part, as well as a no-slip condition on all the internal walls. The air is assumed to
qi : ðT i −T c Þ
ð6Þ
The local Nusselt Number based on the characteristic length L is deduced with Nui ¼
hi L λ
ð7Þ
whose integration over the total considered surface Sh allows determination of the average Nusselt number Nuα for a specific angle α Nuα ¼
1 ∬ Nui dSi Sh Sh
! :
ð8Þ
wall
This parameter constitutes the main parameter in this survey dealing with natural convection. The numerical solution is considered as mesh-independent when the variation in successive Nuα values is less than 3% after refining the mesh. Such condition is attained in this work with 478,607 nodes, 1,450,063 quadrilateral wall faces and 450,465 hexagonal cells. 3. Results Calculations are performed for several combinations of the inclination angle α = 0 − 90° step 15° and the Rayleigh number RaL,ϕ varying between 1.31x107 and 1.01 x 108. The dimensionless temperature T* and velocity V* fields around the device are presented in Fig. 4 at RaL,ϕ = 1.01 x 108 for some configurations in the (x, y) and (y, z) planes. Heat exchanges are in agreement with the temperature and velocity distributions. The presented example concerning the maximum generated power (0.8 W) confirms that the maximum heat flux exchanged device-environment occurs in the immediate vicinity of the active part, towards its top face. Examination of the overall results for all inclinations shows that the average heat flux concerning the area (S) is about 33% higher than that of (T). The average temperature of the
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Fig. 3. Some details of the adopted mesh around the device.
upper face of the PCB board (outside the device area) is between 0.5 and 1.2 °C above Tc. The local low heat flux confirms that the mean convective heat transfer on the free part of this upper side of the PCB board is small compared to that of (T) and (S). The convective heat transfer in the upper part of the plate is therefore concentrated on and around the active device. Furthermore, the average convective heat flux corresponding to the back face and the four lateral surfaces of the PCB board are low compared to those of (T) and (S). The maximum heat flux concerning this back board face is concentrated around the opposite surface with respect to the active device.
The difference between the average temperature of the back face of the plate and Tc is of about 0.9 °C. Given the previous comments, the system is simplified, decomposed into only two parts: (i) the “front face” denoted as (F), which groups (T), (S) and the free upper part of the PCB board (P); (ii) the “back face” denoted as (B) which groups the remaining part of (P), i.e. its rear face and four lateral surfaces. To determine the effective natural convective power exchanged between the assembly and the environment, it is necessary to take into account the exchange areas concerning all the assembly elements, and associate them with their corresponding heat flux.
Fig. 4. Temperature and velocity fields around the device in (x, y) and (y, z) planes for RaL,φ = 1.01 x 108 and α = 0°, 45° and 90°.
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Being the (T) and (S) areas very low compared to that of the free surface of the upper face of the PCB board, the average convective heat flux exchanged by the front face (F) is close to that of the free surface. It is the same for the area (B). Evolution of the average Nusselt number Nuα versus RaL,ϕ is clearly linear of Nuα ¼ kðα Þ þ nðα ÞRaL;ϕ type for all the elements of the assembly. It is presented as an example in Fig. 5(a) for the (T) area and the 7 inclination angles α = 0 − 90° step 15°. The same trend concerns the overall assembly. Evolution of the coefficients k(α) and n(α) is linear and polynomial of the second order respectively, as represented in Fig. 5(b). The results of this work lead finally to the correlations Nuα ¼ kðα Þ þ nðα ÞRaL;ϕ Fig. 6. Th,α⁎ versus RaL,φ for α = 0 − 90° step 15°.
withf
0:0167α þ 4:1815 : ðTÞ area þ 2:8302 : Overall assembly ( 0:00931α ‐0:0008α 2 þ 0:1183α þ 2:9101 x10−9 : ðTÞ area nðα Þ ¼ −9 ð0:00119α þ 1:197Þx10 : Overall assembly
based on the maximum temperature difference corresponding to the horizontal position (α = 0)
kðα Þ ¼
valid for
ð9Þ
0≤α ≤90 1:31x107 ≤RaL;ϕ ≤1:01x108
being the coefficients of determination associated to k(α) and n(α) equal to 0.9991 and 0.9985 respectively. The average surface temperature of the package's front face increases with RaL,ϕ and decreases with α. Its dimensionless temperature
T h;α
T h −T c α ¼ T h −T c α¼0
ð10Þ
is presented in Fig. 6. The value corresponding to the minimum generated power (0.1 W; RaL,ϕ minimum) is roughly the same for all angles in relative terms: the deviation between the values corresponding to both extreme inclination angles (horizontal and vertical positions) is about 2.1%. For the maximum RaL,ϕ, this deviation is twice as high, reaching 4.5%.
Fig. 5. (a) Nuα versus RaL,φ for α = 0 − 90° step 15°. (b) Evolution of k(α) and n(α) for Nuα ¼ kðα Þ þ nðα ÞRaL;φ correlations for the (T) area.
A. Baïri / International Communications in Heat and Mass Transfer 66 (2015) 133–139
4. Conclusion The natural convective phenomena are examined for a Quad Flat Non-lead (QFN32) inclined with respect to the horizontal plane by an angle varying between 0° (horizontal) and 90° (vertical position). This active electronic device generating a constant volumetric heat flux during its operation is contained in an air-filled cubical cavity whose walls are maintained isothermal. The temperature and velocity fields are presented and completed with an analysis of the thermal state of the assembly. The natural convective heat transfer is quantified by means of Nusselt–Rayleigh type valid for Rayleigh numbers in the range of 1.31x107 ‐ 1.01x108. They allow optimizing the thermal design and increase the reliability of these electronic assemblies, which are used in various engineering domains. Acknowledgments The author and all his colleagues of the Research Laboratory (Laboratoire Thermique, Interfaces, Environnement, LTIE EA 4415) and the Thermal and Energy Engineering Department (Département Génie Thermique et Energie, GTE) of Paris X University (IUT Ville d'Avray) express their deep emotion to the family of their colleague Professor Laurent Proslier. References [1] Trinamic application note 005, Rev. 1.01, http://www.trinamic.com 2013. [2] Integrated Circuits Thermal Test Method Environmental Conditions—Natural Convection (Still Air), Jedec Solid State Technology Association, 2008. (JESD51-2A). [3] Guidelines for Reporting and Using Electronic Package Thermal Information, Jedec Solid State Technology Association, 2005. (JESD51-12). [4] Semiconductor and IC Package Thermal Metrics, Application Report, Texas Instruments, 2012. (SPRA953B–July). [5] C.H. Huang, J.R. Lu, H. Ay, A three-dimensional heat sink module design problem with experimental verification, Int. J. Heat Mass Transf. 54 (2011) 1482–1492. [6] J. Li, Z.S. Shi, 3D numerical optimization of a heat sink base for electronics cooling, Int. Commun. Heat Mass Transfer 39 (2012) 204–208. [7] N. Laraqi, A. Baïri, Theory of thermal resistance between solids with randomly sized and located contacts, Int. J. Heat Mass Transf. 45 (20) (2002) 4175–4180. [8] S. Vintrou, N. Laraqi, A. Baïri, Calculation and analysis of thermal impedance of microelectronic structures from analytical models, Solid State Electron. 67 (1) (2012) 45–52.
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