NUMERICAL AND EXPERIMENTAL STUDY OF THE EFFECT OF THERMOCOUPLE WIRE ATTACHMENT ON THERMAL CHARACTERIZATION OF A HIGH PERFORMANCE FLIP-

CHIP PACKAGE

Sravan Gondipalli1, Siddharth Bhopte1, Bahgat Sammakia1 & Varaprasad Calmidi2

1State University of New York at Binghamton2Endicott Interconnect Technologies, Inc.

Binghamton, New York, USA, 13902Phone: +1 (607) 777-6021

Fax: +1 (607) 777-4683Email: sgondip2@binghamton.edu

ABSTRACT

Thermal resistance of an electronic package is a measure of the package’s ability to transfer the heat generated by the chip to the printed wiring board (PWB) or to the ambient. For a high performance flip-chip package, the primary heat transfer path for most applications is through a heat sink or cold plate attached to the package lid. Hence, a test method that evaluates the heat transfer path from the chip to the lid is the most relevant one. The thermocouple wire attachment can have a significant effect on the measurement of the surface temperature during the thermal characterization of a high-performance flip chip package. A cold-plate based test method is used for thermal characterization and internal thermal resistance is used as the basis of all comparisons. A detailed numerical study is presented to study the effect of heat conduction losses through the thermocouple wires on the package thermal resistance calculations.

Keywords - Thermal Characterization, Flip chip package, Package thermal resistance, Heat conduction, Thermocouple attachment method, Thermocouple type

NOMENCLATURE

ANSI American National Standard InstituteAWG American wire gaugeu Velocity, m/s ρ Mass density, kg/m3

g Acceleration due to gravity, m/s2

t Time, s p Pressure, PaT Temperature, oC μ Dynamic viscosity, Pa-s C

p Specific heat capacity

k Thermal conductivity, W/m-Kkx, ky Thermal conductivity due to radial heat conduction,

W/m-Kkz Thermal conductivity due to axial heat conduction,

W/m-K

Sr

Radiation source term in energy equation

Q///

Chip volumetric heat generation rate, W/m3-K Tj Chip junction temperature, oC

Tc Copper lid temperature, oC Tb Thermocouple bead temperatureΘ Package internal thermal resistance, oC/W P Power dissipation, WPWB Printed Wiring BoardTIM Thermal Interface Material

INTRODUCTION

Thermocouples are frequently used for temperature measurement because of their simplicity of use, wide temperature ranges, low cost and data acquisition. Small size and low conductivity thermocouple wires are better in order to reduce the error due to the heat losses along the wire. The main causes of errors are heat conduction between the thermocouple wire and the package being measured; interfacial resistance between the thermocouple measuring bead and the component and the displacement of the thermocouple bead away from the surface of the componentthat is being measured. Type K or J will have lower thermal conductivity values compared to Type T and will result in low conduction losses [1, 2]. Borca-Tasciuc, T., et al used a simple analytical model to demonstrate that the temperature measurement in rapid thermal processing can become accurate by using fine diameter thermocouple wires with low emissivity [3].

Tarnopolsky et al [4] conducted a leaf-laboratory experiment to determine conduction error as a function of thermocouple-type, wire diameter, insulation and length of contact between wire and leaf. It was also noted that the T/C heat transfer coefficient due to conduction to a leaf was one order magnitude higher than the heat coefficient due to radiation and convection. And also, it was noted that by stripping off the insulation from the thermocouple wires and switching from type-T wire to a low thermal conductivity wire (e.g. Type-K), the conduction error can be reduced. Park et al. [5] performed a study of surface mounted thermocouples and considered the error introduced by heat conduction along the leads of a thermocouple during rapid transient cooling. Through numerical calculations, they demonstrated that during boiling at the surface, conduction through the thermocouple wires creates a significant depression of the surface temperature at the attachment points, for which measuring the local surface temperature becomes difficult. Thermocouple

978-1-4244-5343-6/10/$26.00 ©2010 IEEE

wire less than AWG (American Wire Gauge) 36 might result in increased thermocouple resistance [6]. Woodbury et al. [7] discussed through a finite element model, the deterministic errors in thermocouple measurements and their impact on inverse heat conduction problem solutions.

The current investigation is an extension of the study performed by Bhopte et al [8], where a detailed experimental and numerical analysis was carried out on the thermal characterization of a flip chip package. In this paper, the study is further extended to analyze the effect of the conduction losses through the thermocouple wires, thermocouple attachment method, thermocouple bead displacement and type of thermocouple on the package thermal resistance.

PACKAGE DESCRIPTION

Figure 1 shows the cross-section of the flip-chip package. A semiconductor chip is soldered to the top side of the laminate composite.

Figure 1: Schematic of high-performance flip chip package [8]. (not to scale)

Layer DimensionThickness (mm) Material k z

W/mKkxyW/mK

Substrate 55 mm 0.46 Composite 20 0.7

Stiffener

55 mm (outer)

24.9 mm (inner)

0.5 Stainless Steel-400

23.4 23.4

Chip 18.9 mm 0.74 Silicon ~100 ~100

TIM 18.9 mm0.0508 –0.0762

Silicon 2 2

Stiffener adhesive

to lid

2 mm wide ring

0.39 Silicone 1 1.2

Underfill 18.9 mm 0.1 Filled epoxy

0.6 0.6

Stiffener adhesive

to substrate

Same as stiffener

0.1 Polyimide 0.17 0.17

Copper lid

53 mm 1 Ni platted

copper380 380

BGA1 mm pitch22 mil ball

sphere

~0.15 collapse

Sn/Pb solder

0.08 3

Table1: Dimensions, thickness and material properties of package layers [8].

A stainless steel stiffener in the shape of a picture frame is attached to the laminate around the chip site. High

conductivity thermal adhesive was used to attach the copper lid to the back side of the chip and also, to couple the stiffeners to the copper lid. Calmidi et al have discussed that direct attachment of the lid and its coupling to the stiffener enhances the thermal performance [9]. On the other side of the laminate ball grid array (BGA) interconnections are provided for the electrical interconnections to the board. The size of the board is 125 mm x 130 mm. There are two 1.4 mil thick power planes inside the board. Dimension, thickness and material properties of package components are summarized in Table 1.

EXPERIMENTAL MEASUREMENTS

Figure 2 shows module mounting arrangement of the test, which is similar to the method described in [10]. The module on the PWB is placed lid down on the cold plate.

Figure 2: Module mounting arrangement for the test [8]. (not to scale)

The following measurements were taken by Bhopte et al. [8]on three similar flip chip packages using the ANSI Type T (copper-constantan) with PTFE (polytetrafluoroethylene) type insulation at 60W & 80W power dissipation. However only the results for 60W power dissipation are discussed in this paper. The AWG size was 36 (nominal wire diameter of 0.13 mm). Thermocouple bead is placed at the center of the copper lid to measure the case temperature. The bead is firmly attached to the lid using conductive silver epoxy. An additional non-conductive epoxy is used to cover the silver epoxy to secure the bead in place [2]. A thin layer of thermal grease is applied between the module lid and cold plate for efficient heat transfer. The cold plate is maintained at a uniform temperature of 20ºC using re-circulating chilled water. A balsa wood insulator is placed behind the PWB to minimize heat losses to the ambient. A dead weight is used to vary the thickness of thermal grease. Measurement with the dead weight was repeated to understand the repeatability of the data.

Figure 3: Thermal resistance for package 3 with 36AWG Type – T thermocouple [8]

Considering all the measurements for packages 1, 2 and 3, minimum and maximum calculated package thermal resistance was 0.084 ºC/W and 0.118 ºC/W respectively (refer to figure 3).

NUMERICAL MODELING

Based on the data given in Table 1, a detailed numerical model of the flip chip package is generated. Commercial computational fluid dynamics (CFD) code IcePak 4.2.6™ is used as a simulation tool. IcePak™ solves complex three-dimensional Navier Stokes equation coupled with the energy equation. Note that even though heat transfer from the package to the cold plate is primarily by conduction, the experimental setup is such that there is convective heat transfer between objects. One, there is heat loss (convection and radiation) from the backside of the PWB and two, there is heat transfer (convection and radiation) from the portion of the PWB around the package and the cold plate. These effects were explicitly included in the numerical model in order to evaluate their relative importance. The governing equations for steady state laminar flow and heat transfer are written as follows [1, 2 and 8]:

0. V (1)

gVpVV 2).( (2)

'''2).( QSTkTVC rp (3)

BASELINE CASE

In this study, baseline cases have 5 mil thermal grease thickness between the lid and the cold plate. The effect of thermocouple wire is not included in the models. The baseline case is solved for 60W power dissipation. Figure 4 shows temperature contours for the baseline case with 60W power dissipation. The grid sensitivity analysis and the numerical model are validated with the numerical model established by Bhopte et al [8]. For the baseline case, temperatures were monitored at the chip center (Tj) and at the lid center (Tc). The package resistance was calculated to be 0.088 oC/W.

(a) (b)

Figure 4: Temperature contours for the flip chip package for 60W power dissipation at (a) chip and stiffener surfaces; (b) copper lid

PARAMTERIC STUDY

To investigate the effects of the thermocouple attachment, the thermocouple bead is mounted at the top surface center of the copper lid to measure the case temperature. This is done using thermally conductive, silver epoxy (1 W/m-K) to provide good electrical and thermal contact between the thermocouple and the surface of the copper lid, followed by an additional clear insulating epoxy (0.15 W/m-K) to secure the placement of the bead. Based on dimension measurements during the experiment study, the non-conductive clear epoxy is modeled as a cube of dimension 2.6 mm sitting above another cube of dimension 1.3 mm (silver epoxy). The cold plate hole that accommodates the epoxy bead is modeled as a cube of dimension 3 mm. Since the epoxy bead is smaller than the cold plate hole, there is a small clearance between the cold plate hole and the epoxy bead. Simulations were carried out with the cold plate hole and the thermocouple slot having the properties of thermal grease (1 W/m-K) filling the clearance. The surface temperature of the packages for all the tests reported in this study was measured using AWG 36 ANSI Type T (copper-constantan) thermocouples with PTFE (polytetrafluoroethylene) type insulation. The following sections will be concentrating on errors in temperature measurement due to heat conduction along thermocouples, the effect of thermocouple attachment, thermocouple bead displacement, and type of the thermocouple, on the package thermal resistance calculations.

Effect of modeling complexity on package thermal resistance calculations:

This section is divided into two parts, in terms of numerical modeling of the thermocouple wire: Thermocouple wire model and the Orthotropic thermocouple wire model.

Thermocouple wire model:

In the thermocouple wire model, the thermocouple wire is modeled (Figure 5) with two wires of copper and constantan as cuboids insulated with PTFE and an outer insulation combining both with a diameter of 0.127 mm.

Figure 5: Temperature contours of the thermocouple (Cu-Con) wires and bead excluding insulation

Orthotropic thermocouple wire model:

In the Orthotropic thermocouple wire model, all the layers in the thermocouple wire are replaced by a single wire of orthotropic material properties (Figure 6), hence reducing the model complexity. Figure 7 shows the heat flow path from the chip to the thermocouple wire by means of axial and radial heat conduction. The thermal conductivities, kx and ky for radial heat conduction were calculated by the resistances, R1, R2 and R3 in series, as designated in Figure 6. Similarly, the thermal conductivity, kz for axial heat conduction was calculated using the same resistances in parallel.

RT = R1+ R2+ R3 (Resistances in series) kx and ky

321

1111

RRRRT

(Resistances in parallel) kz

All the case and the bead temperature measurements reported in this study were monitored at the respective centers of the lid and the bead. Since, the thermocouple bead is in the path of the temperature gradient and is also enclosed with thermal

grease; the bead temperature is anticipated to be lower than the lid temperature.

R_lid and R_bead are two thermal resistance metrics of interest. They are defined as follows:

R_lid = (Tj - Tc)/P

R_bead = (Tj - Tb)/P

Figure 8 demonstrates that the models, Actual T/C wire model and the Orthotropic T/C wire model show no difference in the package thermal resistance (R_lid) for the case temperature monitored at the center of the top-surface of the lid. However, when the case temperature is monitored at the center of the thermocouple bead, the numerical model shows an insignificant 3% variability in the package thermal resistance values (R_bead).

Figure 6: Schematic of thermocouple wire and heat flow path (not to scale).

Figure 7: Temperature contours of the orthotropic thermocouple wire model

Cu ConCu Con

Figure 8: T/C wire model vs. Orthotropic T/C wire model

In Fig. 8, it is to be noted that R_lid is smaller than the R_bead because the thermocouple bead is at a lower temperature than the lid temperature that it is intended to sense.. Figure 9 shows the overview of the package thermal resistance and chip temperature of all the numerical models considered: Baseline package with no thermocouple wire, package with only bead, Actual thermocouple wire and the Orthotropic thermocouple wire model. It is noted that no fixed variation in the package resistance is observed in all the four cases. Therefore, the presence of thermocouple wire shows negligible effect on the package thermal resistance values (R_lid and R_bead).

Figure 9: Overview of package thermal resistance for all the numerical models considered

Effect of Thermocouple wire displacement on the package thermal resistance:

(a)

(b) (c)

Figure 10: Contour plots of thermocouple wire and silver epoxy moving away from the lid; (a) top; (b) center; (c) bottom (Orthotropic T/C wire model)

The thermocouple wire attachment plays a significant role in measuring the case temperature while predicting the package thermal resistance. The displacement of the thermocouple measuring junction is expected to affect the thermal resistance values. In this section, the displacement of the thermocouple wire along with the bead is attached to the copper lid in three different positions. Figure 10(a) represents the thermocouple wire and bead, both displaced from the top surface center of the lid to the top of the clear epoxy cube, while figures 10(b) and 10(c) show that the thermocouple wire with bead at the center of the non-conductive clear epoxy cube and the bottom of the clear epoxy (Orthotropic T/C wire model), respectively.

Figure 11 demonstrates that the Orthotropic T/C wire models-top, center and bottom (Orthotropic T/C wire model) show no difference in the package thermal resistance (R_lid) for the case temperature monitored at the center of the top-surface of the lid. However, again, when the case temperature is monitored at the center of the thermocouple bead, the numerical model shows a significant 30% variability in the package thermal resistance values (R_bead) when the thermocouple wire along with the bead is moved from the

bottom to the center and 50% variability when moved from center to the top.

Figure 11: Effect of thermocouple displacement on the package thermal resistance

Therefore, the thermal resistance values (R_bead) rise as the thermocouple measuring junction moves away from the contact surface. This is due to the thermal impedance between the thermocouple measuring junction and the lid surface, which might lead to the temperature measurements closely associated to the surrounding materials of the thermocouple junction than the surface temperature. From the above analysis, it is clear that the thermocouple measuring junction should make a direct and reliable contact with the top-surface of the case in order to obtain precise package thermal resistance values.

Effect of thermocouple attachment on the package thermal resistance:

In order to test the effect of the thermocouple attachment method, the following numerical study investigates on threedifferent methods used for attaching thermocouples to the packages. One attachment method used an electrically and thermally conductive silver epoxy followed by a small amount of insulating clear epoxy in order to cover the silver epoxy. The thermal conductivity of silver epoxy was estimated to be 1 W/m-K and of the insulating epoxy was 0.15 W/m-K. The other attachment methods considered used only the thermally non-conductive quick drying clear epoxy and silver epoxy alone, to attach the thermocouples on to the packages. It should be noted that the mass of the surrounding material for the thermocouple measuring junction should be a small amount so that the thermocouple does not lag the true surface temperature.

Again, clearly, Fig. 12 demonstrates that the numerical models using thermally conductive silver epoxy, clear epoxy and silver epoxy followed by a small amount of clear epoxy show no difference in the package thermal resistance (R_lid) for the case temperature monitored at the center of the lid.

Figure 12: Comparison of two thermocouple attachment methods: with and without silver epoxy.

However, when the case temperature is monitored at the center of the thermocouple bead, the numerical model shows 4% variability in the package thermal resistance values (R_bead). Therefore, from the above analysis, though not much variation is found but by using silver epoxy and clear epoxy, the numerical model predicts the package thermal resistance values closer to the experimental values.

Variation of the package thermal resistance using different thermocouple types:

Although all the tests reported in this study were measuredusing ANSI Type T thermocouples, this section discusses on how the package thermal resistance varies according to the type of the thermocouple selected like the ANSI Type J and K. Since the Type K and J thermocouple wires have lower thermal conductivity compared to Type T, this will result in lower conduction losses, eventually, leading to less error in surface temperature measurements.

Thermocouples with smaller gauge wires with lower conductivity enhanced the results of the thermal characterization parameters. Figure 13 shows the variation of the package thermal resistance with thermocouple types T, J and K. Type T, Type J and Type K show no variation for the case temperature monitored at the center of the lid. When the case temperature is monitored at the center of the thermocouple bead, Type T and Type J show 16% variation in thermal resistance values whereas Type K shows a 23% variation.

Therefore, from the above analysis, it can be clearly noted that Type K or J can result in more accurate thermal resistance values than Type T and lead closer to the experimental range.

Figure 13: Effect of the package thermal resistance with different thermocouple types: T, J and K

CONCLUSION

The numerical investigation in this paper presented that the inclusion of the thermocouple wire has negligible effect the package thermal resistance. Parametric study was presented to investigate the effect of the thermocouple attachment, displacement and type of the thermocouple used for measuring the surface temperature of the copper lid.

For all the experiments considered in this study, the minimum and maximum calculated package thermal resistance was 0.084 ºC/W and 0.118 ºC/W respectively. For all the numerical models analyzed above, minimum and maximum calculated package thermal resistance was 0.087 ºC/W (R_lid for Thermocouple wire model) and 0.18 ºC/W (R_bead for using clear epoxy to attach thermocouple to the packages) respectively. Hence minimum and maximum values predicted by the numerical models vary from 25 - 50% of the minimum and maximum experimental values. Also if only R_lid is considered the minimum and maximum thermal resistance values are 0.087ºC/W and 0.097 ºC/W. These values agree within 15% of the minimum and maximum experimental values. Additional work is needed to characterize the measurement errors more precisely.

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