978-1-61284-736-8/11/$26.00 ©2011 IEEE 333 27th IEEE SEMI-THERM Symposium

Development of an Advanced Thermal Interface Material for High Power Devices

My Nguyen and Jason Brandi Henkel Corporation

14000 Jamboree Rd., Irvine, CA 92606 my.nguyen@henkel.com

Abstract

The cure reactions for conventional flexible epoxy resins are generally slow and have out-gassing and branching site reactions. As a result, these resin types have significant hardening after long-term, high-temperature exposure which makes them unsuitable for thermal interface applications. To address these challenges, a new epoxy resin system has been developed for thermal interface materials (TIMs). The main characteristics of the new resin are:

- High thermal stability - No branching - Hydrophobic - Stable at room temperature

Development and characterization of a new type of TIM will be discussed.

Keywords

Thermal Interface, TIM1, epoxy gel, solder, flip-chip BGA

1. Introduction The power density of semiconductor devices continues to

escalate with increasing transistor counts and reduced device features in every new technology. These conditions make thermal interface materials (TIMs) even more integral for solutions to thermal management.

At the IC package level, thermal interface materials (TIM1) are placed between flip-chip dies and heat spreading lids as shown in Figure 1.

Fig. 1: Flip Chip BGA Package with integrated heat

spreader

The thermal management goal is to maintain the die temperature at or below a fixed value to ensure device performance and reliability. The thermal resistance �jc between the die and the heat spreader can be described by the following equation [1]:

�jc = Tj – Tc / P = Rjc (HF) (1) Where Tj is the junction temperature at the die surface, Tc is temperature at the heat spreader, P is the power, HF (heat flux) is the power dissipated per unit area, and Rjc is the thermal impedance normalized over a unit area, which typically has units of oCcm2/W. The value of Rjc is used to define thermal performance requirements and to select the TIM type [2]. Table 1 is a summary of representative performance for various types of TIMs [3]

Material Type Rjc Application notes

Grease 0.17 Pump-out/Separation Gel 0.19 Require Cure

PCM* 0.21 Higher thermal resistance than grease

PCMA** 0.14 High cost *PCM: Phase Change Material **PCMA: Phase Change Metallic Alloy Improved thermal reliability performance is a significant

issue for TIM1 applications. Large die flip-chip packages are facing even greater reliability challenges.

Traditional thermal greases, gels or PCM have shown serious thermal degradation after undergoing 260oC lead-free reflow conditions, reliability tests such as thermal cycling, moisture conditioning, and long term baking at high temperature. TIM delaminating along the edges of the die/lid interface was most often found to be the cause for reliability failure.

The objective of this paper is to discuss the development of a new type of thermal gel that may address these issues. 2. Thermal Interface Material (TIM) Requirements

TIM must be designed to dissipate the thermo-mechanical stresses from mismatches among different materials in the package. Since the die is curved due to mismatches of thermal expansion coefficients among the die, the substrate and the underfill, the TIM material must have a modulus sufficiently low such that it is stress neutral. It has been shown when a high modulus TIM is used, the die stress is much higher resulting in catastrophic delamination. The ideal modulus values for TIM1 were reported in the range of about 30kPa to 500kPa [2].

Nguyen, Development of an advanced thermal interface … 27th IEEE SEMI-THERM Symposium

A summary of the material component requirements for formulating an advanced thermal gel is given in Table 2.

Component Requirements

Resin Thermally Stable Gel (no hardening) Target Modulus: 30-500 kPa

Filler Particle size <50�m High thermal conductivity

TIM Product Paste Format; No thermal degradation

3. TIM Resin Chemistry Silicone-based gels and greases are found in most

commercially available TIM products [4, 5] because of their relatively high thermal stability compared to epoxies or urethanes. Issues related to silicone migration and out-gassing can be minimized when silicone is exposed to temperatures less than 200oC. Because the next generation of TIM1 must be able to withstand the high temperature of lead-free solder reflow processes up to 260oC, there is a need for an alternate TIM resin system compatible with these conditions.

Conventional epoxy gel containing a typical flexible epoxy/hardener system showed significant hardening when exposed to temperatures above 120oC. The increase in modulus was caused by a combination of slow cure, high out-gassing and branching sites reaction. A novel and proprietary epoxy gel system was synthesized to address the above issues [6].

Figure 2 compares the thermal stability of the two epoxy gels as determined by monitoring the modulus values of the unfilled cured resins after cure: 30oC/60%RH/168hrs + 3X 260oC Reflow (MSL3/260C); thermal cycling; and 150oC bake. The new epoxy gel (Gel A) showed satisfactory thermal stability under various test conditions. In addition, the new epoxy gel also showed significantly less out-gassing when exposed to 260oC. (Figure 3)

Fig. 2: Thermal stability of new epoxy gel A.

Fig. 3: Weight loss characteristics of new epoxy gel A Figure 4 illustrates the cure behavior of gel A as measured

by differential scanning calorimetry (DSC). The resin mixture is relatively stable at temperature less than 150oC.

Fig. 4: Cure characteristics of gel A measured by DSC

4. Epoxy Gel Solder Hybrid Next generation microprocessors require advanced thermal interface materials to provide the lowest possible thermal resistance while improving reliability and ease of use and lowering cost. An example of a potential solution is the polymer solder hybrid film product which incorporates a low melting temperature solder alloy with a polymeric phase change binder [7, 8]. One of the main challenges for this type of material is ensuring thermo-oxidative stability so as to avoid thermal degradation over time. The epoxy gel solder hybrid [6] has been designed as a paste material consisting of the liquid epoxy gel and a mixture of two types of solder powders: tin-bismuth and indium (melting points 140oC, 157oC respectively). Figure 5 follows the DSC traces at the following test conditions:

� Run 1 simulates cure conditions for 30 minutes at 160 oC. The two peaks are associated with melting of Sn-Bi and In respectively.

� Runs 2 and 3 are subsequent runs of the same sample. The peaks associated with Sn-Bi and In have nearly disappeared. They are replaced by a melting

0.01

0.1

1

10

100

Modulus After Cure MSL3/260C 250 cycles TCB 100h @ 150C

Mod

ulus

@25

C,M

Pa

Control

Epoxy Gel A

02468

101214

%Wt. Loss@150C/1h

% Wt. Loss@260C/1h

Resin Type

% w

eigh

t los

s

ControlEpoxy Gel A

Nguyen, Development of an advanced thermal interface … 27th IEEE SEMI-THERM Symposium

peak at approximately 60oC, which indicates the complete formation of the low melt alloy (LMA) of In-Sn-Bi.

Fig. 5: Melting behavior of epoxy gel solder hybrid Figure 6 illustrates a repeat of the same experiment

described in Figure 5 but at a 125oC cure which is well below the melting points of In and Sn-Bi. The presence of the 60oC melting peak in Runs 2 and 3 indicates a partial formation of LMA. It also appears that the epoxy gel chemistry exhibits good fluxing action which would help to reduce thermo-oxidative degradation of the polymer solder hybrid.

Fig. 6: Effect of cure temperature Figure 7 shows the morphology of the epoxy gel solder

hybrid after it was cured at 160 oC. A network of fused solder particles in a polymer matrix was observed.

Fig. 7: Morphology of epoxy gel solder after cure

The main characteristics of the epoxy gel solder hybrid are compared against a filler loaded gel in Table 3.

TIM type Thermal Gel Epoxy Gel Solder Hybrid

Matrix Epoxy Gel Epoxy Gel Filler Conductive

powders Solder Powders

Viscosity 45000 cps 47000 Thermal

Conductivity 3 W.m/ oC 10 W.m/ oC

Solder Phase Change Temp.

NA 60oC

Cure Conditions 125 oC/30 min 125 oC-160 oC

5. Thermal Performance and Reliability TIM performance was evaluated using a thermal test

vehicle (TTV). As shown in figure 8, the TTV consists of a 1cm x 1cm thermal test chip mounted on a package substrate. The temperature of the TTV, Tj, is measured by embedded thermal sensors in the die. The temperature of the copper heat sink, Tc, , is determined by a thermocouple at the center of the heat sink. The power input to the thermal die is read using a power meter. The thermal resistance of TIM can be determined from equation (1).

Fig. 8: Thermal Test Vehicle

Figure 9 shows thermal performance results for the two new TIMs: the thermal gel and the epoxy gel solder hybrid. The epoxy gel solder hybrid gave lower thermal impedance than the thermal gel (0.12 vs. 0.17 oCcm2/W). The thermal impedance values for both TIM materials were relatively stable after subjected to 100 hours at 150oC or 250 thermal cycles -50oC to 125oC (TCB).

Nguyen, Development of an advanced thermal interface … 27th IEEE SEMI-THERM Symposium

Fig. 9: TIM thermal performance comparison The epoxy gel is expected to play an important role in

determining thermal performance reliability. Figure 10 compares the reliability performance of the two epoxy-based solder hybrids. The degradation in thermal performance of the standard epoxy-based TIM after reliability can be attributed to the hardening effect of the resin-based system, which caused interface delamination. Another factor that may influence thermal reliability of the polymer solder hybrid TIM is thermal oxidative degradation of the solder alloy. Since the epoxy gel chemistry exhibits good flux properties as evidenced by the in-situ formation of the low melt alloy during cure, oxidation reduction is enabled and thermal reliability performance is improved.

Fig. 10: Effect of matrix resin on thermal reliability

6. Conclusions

A novel liquid epoxy gel has been developed to target reliability improvements for TIM. The new TIM paste, which incorporates the prescribed resin and solder powders, showed relatively low and stable thermal impedance after reliability tests. The material appears to be a promising new TIM advance for use in high power microprocessors.

Acknowledgments The authors would like to thank the management for the

permission to publish this work.

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