Reliability of thermal interface materials: A review

Applied Thermal Engineering 50 (2013) 455e463

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Reliability of thermal interface materials: A review Jens Due b, Anthony J. Robinson a, * a b

Department of Mechanical and Manufacturing Engineering, Parsons Building, Trinity College Dublin, Ireland Siemens Wind Power, Keele University Science Park, United Kingdom

h i g h l i g h t s < This paper reviews the body of work which has been performed on TIM reliability. < Test methodologies for reliability testing are outlined. < Reliability results for the different TIM materials are discussed. < A test procedure is proposed for TIM selection BOLife and EOLife performance.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 January 2012 Accepted 7 June 2012 Available online 28 June 2012

Thermal interface materials (TIMs) are used extensively to improve thermal conduction across two mating parts. They are particularly crucial in electronics thermal management since excessive junctionto-ambient thermal resistances can cause elevated temperatures which can negatively influence device performance and reliability. Of particular interest to electronic package designers is the thermal resistance of the TIM layer at the end of its design life. Estimations of this allow the package to be designed to perform adequately over its entire useful life. To this end, TIM reliability studies have been performed using accelerated stress tests. This paper reviews the body of work which has been performed on TIM reliability. It focuses on the various test methodologies with commentary on the results which have been obtained for the different TIM materials. Based on the information available in the open literature, a test procedure is proposed for TIM selection based on beginning and end of life performance. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Thermal interface material TIM degradation Reliability Accelerated stress test

1. Introduction When two solid surfaces are clamped to one another under moderate loading, micro and macroscopic surface imperfections cause the actual area of contact to be a very small fraction of the apparent contact area [1]. If heat must be transferred across the mating interface, such as in the case of an electronic package and a heat sink, conduction only occurs across the actual area of contact with the remaining area relying on other mechanisms with a substantially higher resistance to heat transfer. The net result is a thermal contact resistance which, when under thermal loading, causes a notable temperature drop to occur across the otherwise thin interface, as depicted in Fig. 1. Reducing the thermal contact resistance between electronic packages and the next level of thermal hardware is crucial to the performance of the overall system. This is because the performance and reliability of electronic devices deteriorate with increasing * Corresponding author. Tel.: þ353 85 156 3366; fax: þ353 1 679 5554. E-mail address: [email protected] (A.J. Robinson). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.06.013

temperature and any additional thermal resistance between the heat source and heat sink will cause the temperature of the source to rise. The most common strategy to reduce thermal contact resistance is illustrated in Fig. 1. Here, a thermal interface material (TIM) of moderate bulk thermal conductivity is sandwiched between the two mating surfaces. The TIM is meant to conform to any surface asperities and displace micro and/or macroscopic air voids, thereby providing a path of improved heat conduction [2]. As depicted in Fig. 1, the thermal resistance across the thermal joint is comprised of three individual resistances in series. The effective thermal resistance is given by,

R ¼ Rconact1 þ Rcond þ Rcontact2

(1)

where Rcontact1 is the contact resistance between the lower surface and the TIM, Rcond ¼ BLT/kTIM is the bulk resistance of the TIM layer and Rcontact2 is the contact resistance between the TIM and the upper surface. Mechanically, TIMs must be pliable so that they can conform to the mating surface asperities whilst creating a small bond-line thickness (BLT) under reasonable assembly pressures; however

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Contact Pressure

TCold

Distance

Heat Sink RContact2 BLT

RCond

TIM

R

RContact1

Electronic Package

ΔTTIM

THot ΔTContact ΔT

Temperature Fig. 1. Schematic representing a real TIM and associated temperature distribution.

the thickness must be large enough to enable the TIM to comply with surface irregularities and non-planarities. Thermally, TIMs should exhibit low contact resistance with reasonably bulk thermal conductivity. There are many different commercially available TIM technologies. They include, though are not limited to, greases, phase change materials, elastomeric pads, putties and epoxies [3,4]. TIMs are often fabricated by dispersing high conductivity particles, such as metals (e.g. silver, copper) or ceramics (e.g. aluminium oxide, zinc oxide or boron nitride) within a lower conductivity organic phase, such as silicone grease. The aim is to create a material with the mechanical properties closer to that of the organic phase though with a higher effective thermal conductivity. Even still, the effective thermal conductivity of the best commercially available TIMs are on the order of 5e10 W/mK, which is considerably lower than the thermal conductivities of typical mating components. As outlined in Refs. [5,6], thermal greases typically offer comparatively high performance and reduced manufacturing cycle times. They have the ability to flow and conform to the interfaces, do not require post-dispense processing and they have higher effective bulk thermal conductivities compared to other categories of TIMs. In addition, the thermal joint made using greases can be reworked facilitating easy repair and upgrade. For these reasons and others, thermal greases are extensively employed in microelectronics and power module cooling applications. Although thermal greases provide comparatively good thermal performance, they do suffer from various failure mechanisms which can severely compromise the functioning of the electronic device in which it had been installed. In the literature it is generally accepted that thermal greases fail primarily due to pump-out or dryout. However, other failure mechanisms, such as those associated with the presence and subsequent evaporating of viscosity lowering solvents [7], and exposure to humidity have been noted [4]. In any case, the degradation in the thermal performance of the TIM depends on the temperature of operation, the time of usage, the mechanical loading and the material properties [8]. Thermal grease pump-out is caused by the TIM being sandwiched between layers of different materials with different coefficients of thermal expansion (CTE) [9]. The CTE mismatch causes warpage and when subject to cyclic thermal loading during service life, alternately squeezes and releases the TIM forcing it to flow out of the thermal joint. Since the squeeze flow is not reversed during the down cycle, the end result is a diminishing quantity of grease over time. Thermal grease dry-out is caused by the separation of the filler from the polymer matrix at elevated temperatures [5,6] and/

or due to thermal cycling [10]. Here, the polymer matrix tends to flow out of the interface preferentially and results in drying-out of the thermal grease. In either case, the eventual presence of air causes voids, delamination and/or cracking of the TIM. These result in an increase in the effective thermal resistance of the interface and ultimately an increase in the junction temperature of the silicon device and possibly failure. Thermal interface materials are generally characterized for the Beginning of Life (BOLife) stage, thus giving an indication of the performance at the time of installation of a new TIM in the electronic device. End of Life (EOLife) performance data, indicating the reliability of a TIM over its service life, is also crucial in characterizing TIM performance, though as critical as it is, is not generally available. The lack of reliability data adversely affects design cycle times, cost and potentially the reliability of the electronic component itself. As noted by Goel et al. [4], estimating the performance degradation of TIMS in use conditions is a big challenge. The objective of this paper is to provide an up to date review of the state of the art regarding TIM reliability testing and characterization. 2. Reliability testing Long-term stability and reliability of a TIM refers to its ability to maintain the BOLife interface thermal resistance over an extended period of time or extensive use condition. Low quality or inappropriately chosen TIM material will degrade with time causing the thermal resistance of the interface and the junction temperature of the electronic device to increase substantially, potentially leading to failure. This being the case, package designers must be facilitated with information regarding the expected failure characteristics of the candidate TIMs in order to make informed decisions in the context of the final environment within which the device will operate. In general, the reliability information is generated by some form of accelerated stress test or a combination of many different stress tests in order to be comprehensive. The stress tests can be roughly sectioned into two categories; those which piggy-back on accepted standards for generic microelectronic component reliability testing, such as JEDEC [11] and those which have been specifically designed for TIM reliability test and measure. The long-term reliability of a TIM depends on the package within which it is installed, the environment it is exposed to and the cyclic nature of its operation. These beings the case, the exact same TIM may be used in two completely different devices and in one case fail prematurely and in the other actually improve in performance. With this in mind, it is difficult to conceive of a ‘silverbullet’ test methodology that can encompass all possible end use scenarios for a particular TIM, considering the vast array of use condition environments. In order to be comprehensive from a materials selection and screening point of view, an array of accelerated stress tests are typically performed in order to closely represent the range of possible conditions under which the TIM may be exposed. The three most common stress test categories for TIMs are the temperature and humidity stress test (THT/HAST), temperature/power cycling stress test (TC/PC) and high temperature storage/bake/soak test (HTS). Other tests include preconditioning, thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). All of the above will be discussed in more detail in subsequent sections. From what appears in the literature, it is evident that TIM reliability testing is not near as mature as that of other components such as solder joints etc. It is also evident that there is not a very clear understanding with regard to the fundamental mechanisms causing TIM failure, as there are very few publications on the basic

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physics of TIM degradation. These being the case, there does not exist a consensus on testing protocol which in part leads to a rather sparse and fragmented collection of TIM reliability data and physical knowledge. Even still, it is possible to outline a generic yet comprehensive test methodology that is, in part, adhered to by most of the past researchers. In this report, this framework will subsequently be used to discuss the individual test methods and results in more detail. Fig. 2 shows the evaluation flow of what can be considered a fairly comprehensive set of reliability tests for screening a TIM, and is similar in structure to that outlined by Chen et al. [8]. It involves the characterization of NT samples subsequent to Ns stress tests, including though not limited to precondition, HAST, TC and HTS. It is important that the pre and post-stressed sample sizes are chosen such that the comparisons between them are statistically significant. For their TIM degradation study, Chen et al. [8] implemented a method outlined in Ref. [12] to select the sample size, N, required for each stress test in order to perform what they called “apple-to-apple” comparisons. For Ns stress tests the total prestress sample size is thus NT ¼ N$Ns. The pre-stressed samples are characterized for a particular performance metric, such as the junction temperature, Tj. thermal resistance, Rth, or some other parameter (in some cases visual and/ or optical). This forms the baseline, in terms of average performance, as well as the standard deviation to which the post-stressed samples can be compared to determine if any changes are statistically significant. In some reliability tests the samples are tested after a predetermined time (or number of cycles) under stress and simple before-and-after comparisons are made [5,8,13]. In other cases, the samples are tested intermittently on order to gauge the rate of degradation (or improvement) over time [4,6,8,9,14e18]. In very few studies, the degradation trends are analysed and statistical models are created to represent the performance over time at use conditions [4,19]. For thermally activated degradation, such as HTS testing, Arrhenius-type relationships have been used [4,19]. Other relationships are suggested to characterize thermo-mechanical stresses, such as temperature/power cycling [4]. Ultimately, the relationships are used to help extrapolate the degradation trend at use condition. 3. Summary of literature As mentioned, there exists a rather incongruent collection of TIM reliability publications in the open literature. Different researchers have tested different TIMs with different test assemblies with an assortment of stress tests with different stress levels and durations. Appendix A is an attempt to collate the relevant studies in order to give a broad sense of the work which has been done to date.

Fig. 2. Long-term reliability evaluation flow diagram.

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Table A1 provides an overview of the types of TIMs which have undergone reliability testing. It is evident that thermal greases are the most studied TIM, which must stem from the fact that it likely the most common TIM in use. This is followed by gels and then phase change materials and adhesives. Table A2 is a stoplight chart meant to assemble the types of reliability tests performed with the corresponding analysis methodology and can be cross-references with Table A1. 3.1. Test vehicles Past researchers have used an array of different thermal joints to determine TIM degradation during reliability stress testing. The choice of the thermal joint largely depends on the method by which the degradation is meant to be quantified. Roughly, they can be divided into thermal test vehicles (TTVs) and non-TTVs. TTVs are dummy devices that thermally and structurally replicate the actual device. Typically, the TTVs have imbedded heating elements and thermal sensors in order to characterize the thermal performance at intervals during the prolonged stress tests in order to gauge degradation and/or develop predictive models. As seen in Table A2, most reliability studies use TTVs since they most closely match the in-situ condition of the TIM. To a lesser extent, fixturebased non-TTV thermal joints are used. This is typically the case when a different characterization technique is deployed to quantify performance changes with time. For example, some studies utilise the laser flash technique for monitoring the TIM thermal resistance over time during accelerated stress testing [5,6,15], for which TTVs are not an option. Other studies rely on visual techniques including IR thermography [10,13], which negate the use of a TTV.

3.2. Stress tests TIMs have been subjected to a variety of stress tests, which are listed in Table A2. In order, the most common stress tests applied to TIMs are temperature cycling (TC), accelerated temperature & humidity stress test (THT & HAST), high temperature storage (HTS) and power cycling (PC). Other tests, such as pre-conditioning, forced mechanical cycling (FMC), differential scanning calorimetry (DSC), and thermo-gravimetric analysis (TGA) have also been used to characterize TIMs.

3.3. Performance metric Reliability tests must in some way gauge the change in performance over time by monitoring a parameter which is indicative of a physical change in the TIM layer. Changes in the performance metric which is being monitored should be related to the degradation/improvement of the TIM. Monitoring the thermal resistance is by far the ideal metric by which to ascertain TIM performance changes under prolonged stress as it ultimately helps determine the junction temperature for a given heat load. This being the case, it is not surprising that most studies choose RTH as the primary performance metric, as indicated in Table A2. Changes in the junction temperature can also be qualitatively indicative of TIM degradation/improvement and has also been used in some studies. Since TIM failure is largely a result of voiding, cracking and delamination, it is also not surprising that many studies employ visual observation, whether through transparent substrates [10,16], acoustic microscopy [5e7,13e15] or by de-attaching the test assembly [9,19] to monitor structural changes in the TIM layers.

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3.4. Sampling method The performance metric which gauges the TIM performance must be compared to a baseline in order to assess the degree of degradation over time. This is achieved by sampling the test assembly under stress and comparing it to the pre-stressed test. In some cases, the stress test is allowed to continue to completion and the post-stressed assembly is then tested. This before & after sampling can be considered a ‘go-no-go’ methodology and, referring to Table A2, is a fairly common technique. More common, however, is the case by which the test assembly is tested at intermittent intervals. This is a preferable technique since it allows one to gauge the rate of degradation over the entire period of stress. It can also be advantageous with regard to developing models to predict degradation over time. 3.5. Predictive models Albeit not common for TIM characterization, one key outcome of stress tests is the development of degradation models which can predict the change of the thermal resistance over time at use condition based on the accelerated stress data. Typically this involves generating two RTH versus t curves, i.e. at two different stress levels, and then developing an empirical relationship which ostensibly extrapolates to use condition. This is an invaluable tool for package designers since, based on the performance of other thermal management hardware in the package, the thermal resistance threshold of the TIM should be known at the design stage. With knowledge of the use condition and environment, the degradation model(s) can then be used to predict the end of life thermal resistance of the TIM and determine if it will stay below the defined threshold during its entire design lifetime. 4. Stress tests for characterizing TIM reliability 4.1. Elevated temperature and humidity stress test (THT/HAST) Elevated temperature and humidity tests induce moisture activated degradation of TIMs. These stress tests employ temperature and humidity under non-condensing conditions to accelerate the penetration of moisture into the thermal joint and ultimately into the TIM. The aim is to simulate the TIM degradation when the package is not active i.e. when the device is in shipping and storage or when it is switched off during use. Based on the open literature, these can be divided into the unbiased “85  C/85% RH” steady state humidity life test (THT) [5,9,15,16,19,20,22] and unbiased “130  C/85%RH” highly accelerated temperature and humidity stress test (HAST) [4,7,8,14,16,19e21]. HAST will typically generate the same failure mechanism as THT but care must be taken at higher temperatures to avoid initiating unrealistic failure modes, such as those which may be associated with a change in the material due to a change of phase, curing etc., which will not occur at end use temperatures. TIM THT tests involved prolonged exposure of the thermal joint to an environment held at 85% relative humidity and 85  C from 1000 h to over 2000 h. Nearly all of the data available shows a reduction in the thermal resistance (w6%e50%), thus an improvement in TIM performance. For silicone-based TIMs, this improvement is thought to be a result of either enhanced wetting at the interface due to the presence of silicone [15], a reduction in the BLT brought about by a reduced TIM viscosity at the elevated temperatures [5] or possibly a combination of the two. In comparison, the PCM TIM tested in Ref. [19] experienced a 40% degradation compared with a 12% improvement observed in Ref. [20] for their PCM TIM for ostensibly the same test condition.

This illustrates the sensitivity of this reliability tests to the TIM composition, test vehicle, test duration etc. For HAST, the standard generally adhered to is JESD22-A118 which is commonly used in environmental testing of non-TIM electronic devices and components [15]. Typically, a HAST for TIMs involves exposure in an environment of 85% relative humidity and between 120  C and 130  C for 200e400 h. In contrast to the THT tests, greases have shown performance degradation with up to a 50% increase in the thermal resistance [21] and 50% voided area [7] have been observed subsequent to HAST. For gels, between 20 and 60% degradation has been noted [8,14] and 50% for PCMs [19]. For TIMs such as silicon greases, the elevated temperature and humidity test causes moisture driven degradation of the greaseinterface layer [5]. The failure mode in this stress test is the absorption of moisture into the TIM with subsequent influence of moisture and temperature on the material itself. As discussed in Ref. [21], conditions of high temperature and moisture cause a deleterious chemical reaction (hydrolysis) in siloxane-based TIMs (such as silicon greases) that cause them to degrade such that the effective thermal resistance increases. When moisture is present, a chemical reaction takes place leading to the formation of silanols. Although the reaction is reversible, the silanol formation causes a loss of hydrophobicity and adhesion property which causes localized delamination on metal surfaces [21]. As will be discussed, the elevated temperature and humidity stress test data can be used with a modified Arrhenius equation to extrapolate to the TIM’s use condition and environment. 4.2. High temperature storage (HTS) or bake test This stress test employs elevated temperature to accelerate the deleterious influence of high temperature on TIM performance. The aim of this accelerated stress test is to simulate the continuous high temperature experienced by the TIM [19]. The standard generally adhered to is JESD22-A103. A bake test involves placing the test assembly into a constant temperature oven for a prolonged period of time. The oven temperature largely depends on the TIM under test since the TIM should exist in the same physical state in both use and accelerated test conditions [4]. The temperature should be determined on the basis of the phase transitions in the material [4] as well as other factors such as the rate of degradation and the reliability issues associated with the other components in the test device [19]. For TIMs, high temperature soak/storage/bake tests tend to be in the range of 95e150  C, generally for hundreds to thousands of hours [4e8,16,18,19,21]. Most studies observe degradation in the TIM resulting in an increase in the thermal resistance. For greases, gels and PCMs both degradation [8,18,21] and improvement [5,6,16] have been observed during HTS. Degradation in thermal performance of greases and gels is due to thermal decomposition of the material. For siloxane-based TIMs it is thought to be a result of hardening of the bulk polymer with subsequent cracking that causes the increased thermal resistance [21] and can be relatively severe (w15e50%) [8,18,21]. Using TGA (to be discussed) Lou et al. [18] showed a tenuous relationship between weight loss and degraded performance. Contradictory results to those above have been obtained by Refs. [5,6,16], where the thermal resistance of greases decreased by up to around 12e15%. The decrease in the thermal resistance was attributed to a reduction in the BLT in combination with improved wetting of the greases due to lower viscosities at elevated temperatures [5,6]. For similar HTS conditions, PCMs show a much more dramatic degradation in performance, generally greater than 200% [4,19]. Bharatham et al. [19] in fact observed that the post-stressed PCM TIMs after elevated temperature tests (HAST & HTS) were hard,

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brittle and powdery. TGA tests showed little material loss and it was concluded that the wetting agent additives in the polymer matrix preferentially evaporated causing the small amount of weight loss with a disproportionately damaging effect on adhesion and thus the thermal contact resistance. They noted that this effect was enhanced at lower clamping pressures highlighting the need for heat sink retention to prevent excessive material loss. As with the HAST test data, the HTS stress test data can also be used with a modified Arrhenius equation to extrapolate to the TIM’s use condition and environment. 4.3. Temperature cycling (TC) Temperature cycling tests are aimed to induce TIM failure/ degradation associated with cyclic changes in temperature during operation. This stress test employs alternating high- and lowtemperature extremes in a thermal chamber to determine the TIMs ability to withstand temperature induced mechanical stresses which can cause permanent changes in the physical and/or chemical characteristics over repeated loading. Accelerated TC tests should adhere to a standard similar to JESD22-A104D or JESD-A105C; standards developed for non-TIM electronic components such as surface-mount solder attachment [6]. There is an admitted absence of a standard method for environmental exposure TIM reliability testing [15]. Typically, a TC test for TIMs involves exposure in an air environment cycling between a low-temperature extreme (55  C) and a high temperature extreme (150  C), with the test assembly being isothermal. In addition, some cases specify the rate of change of temperature as well as dwell times at each extreme. The entire test assembly is exposed to hundreds to thousands of cycles and the performance is typically measured at discrete intervals to gauge the rate of change of performance. For this stress test the main mechanism associated with TIM degradation, in particular greases, is pump-out. As mentioned, pump-out results from a mismatch in the CTE of the two mating surfaces. The CTE mismatch causes warpage which depends on the mating material properties, the operating temperature [4] and the die size of the package [14]. Under cyclic thermal-mechanical loading the result is migration of the TIM from the interface [4]. The available reliability data for accelerated TC tests are inconsistent which leads to ambiguity with regards to the actual physical mechanisms causing changes in performance over time. This must partly be due to the lack of a standardized test protocol. It would also be largely influenced by the large variation in TIM material chemistry and composition which subsequently react differently in this stress environment. Even still, it can generally be said that thermal greases are found to improve in performance during TC stress testing [6,16,20,22]. Gowda et al. [6] exposed several greases to 1000 cycles between 50  C and 150  C and observed thermal resistance reductions in the range of 10.5e25%. In most cases the improved performance was attributed to a net reduction in the BLT as well as enhanced wetting resulting from the lower viscosities at the elevated temperatures, similar to their HTS test. However, improvement was also noted for a grease with no BLT change indicating that the latter mechanism can be significant on its own. This was confirmed by exposing the test TIMs to one cycle of the high temperature extreme for their TC and HTS tests. Improvement between 7% and 12% was observed. Interestingly, visual observation indicated significant voiding at the contact interface suggesting that there is interplay between TIM voiding and BLT/wetting with regarding to their overall influence on the thermal resistance. Islam et al. [7] and Paisner et al. [16] visually observed voiding, delamination and pump-out. Reduced thermal resistance of greases has

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been noted by Refs. [16,20,22] whilst degradation was measured by Ref. [18]. A selection of putties, adhesives, gap fillers and pads were tested by Ref. [15] showing on average a reduction in the thermal resistance. For the silicone-based TIMs it was postulated that silicone was released from the bulk TIM onto the contacting surface which improved wetting. This silicone extraction was noted to be a potential hazard due to contamination of surrounding components [15,19]. It is not clear whether silicone extraction occurs in greases as well. Gel-based TIMs tend to show small changes in thermal resistance with temperature cycling [8,14]. It is thought that the cross linking in the gel mitigates bleed out and helps maintain adhesion to the contacting surface. PCM-based TIMs have also been shown to be very stable [19]. No previous work has been found that aims at predicting longterm temperature cycling performance for the TIMs use condition environment, although it is discussed briefly in Ref. [4]. This is possibly due to the fact that it is not suited to thermally activated failure models, where the modified Arrhenius equation is applicable, and is not a strain cycling phenomenon (i.e. the propagation of cracks) which is suited to traditional fatigue theories such as CoffineManson and continuum damage mechanics type relations. The later has been used to predict failure of adhesives [23], though not TIM adhesives to the best of knowledge. 4.4. Power cycling (PC) Power cycling reliability tests are performed to mimic temperature excursions due to powering on and off of the device. The on/ off powering induces temperature cycling and this test aims to simulate the worst case conditions encountered in typical application. The standard that can be adhered to is JESD22-A105C. Power Cycling tests resemble most closely the actual use condition since it realistically captures the thermo-mechanical loading conditions during temperature/power cycles [4,6,19]. This is largely due to the fact that temperature gradients, both lateral and axial, exist between the base component and the thermal attach, which are not present in isothermal tests such as temperature cycling [4]. As a result, the stress field during the reliability test more closely matches the one experienced in real life. A typical PC test involves the use of a TTV with an internal heater and temperature sensor(s). The TTV must be structurally identical to the actual device. During the test, the power is applied and removed for several thousands of cycles. Tests are either controlled by the maximum and minimum temperatures [9,19] or by simply timing the on/off durations of the heater [4,16]. Generally, the temperatures cycle between the extremes of ambient and 80e110  C. For low modulus TIMs such as greases, PC tests are crucial in evaluating pump-out due to CTE mismatch of the die and thermal attach. Greases tend to show considerable degradation (w20e60%) under PC conditions. As noted in Ref. [9], pump-out will depend on the mechanical squeezing action as well as the temperature and power dissipation. The dependence on the temperature is related to the average bulk viscosity dependence on temperature. The dependence on power is related to the fact that temperature gradients will exist across the TIM such that there could exist large variations in viscosity across the TIM layer; in particular between the hot die and the cold thermal attach [9]. This may cause a difference in TIM movement and influence the pump-out characteristics of the grease. Consistent with the TC test observations, gel and PCM-based TIMs have shown better resistance to power cycling compared with thermal greases.

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4.5. Other applicable tests 4.5.1. Forced mechanical cycling (FMC) Although the power cycling test may be the most accurate technique in assessing the reliability performance of TIMs, it is cost and time intensive, in particular if many TIMs are to be screened during the device design cycle [5,8]. In order to accelerate pumpout, Chiu et al. [9] developed a FMC test technique. The technique involves rigidly fixing one side of the test assembly (say the die) and sandwiching the TIM between it and a thermally regulated chuck. The chuck is attached to a machine capable of controlled cyclic displacement which ostensibly squeezes and releases the TIM mimicking the warpage characteristics of the device. The TIM performance can be monitored by powering the die and simply monitoring the junction temperature or the thermal resistance across the thermal joint. The FMC test has been used to gauge the reliability characteristics of thermal greases by Refs. [5,6,9]. One of the more striking results is provided by Ref. [5] where FMC was the only accelerated stress test that caused degradation in the thermal grease performance, compared with fixture-based THT, HTS and TC tests. As discussed earlier, owing to the decreased viscosity at elevated temperatures, the latter tests induced a BTL reduction which was partially responsible for the reduction in the thermal resistance. For the FMC tests, the BLT changes cyclically about constant average value. This being the case, the loss of material due to pump-out cannot be compensated by a reduced BLT. As a result, pump-out will cause more extreme voiding for the FMC test. 4.5.2. Pre-conditioning During packaging, the component(s) containing TIM may experience conditions which alter their post-dispense characteristics. This pre-conditioning includes, though is not limited to, cure and reflow and will define the end of line (EOL) performance of the TIM. An example of when this would be applicable would be if one or more soldering processes were performed subsequent to the TIM dispense stage. Chen et al. [8] implemented the JEDEC standard JESD22-A113 for evaluating degradation of a gel TIM subsequent to preconditioning. This included multiple reflows, humidity soak and IR reflow. No measured degradation was reported. Wang et al. [14] performed a high temperature cure and multiple reflows to mimic soldering reflow processes and soldering rework. They also did not observe a measurable change in performance which was confirmed by visual inspection. Islam et al. [7] observed that the void percentage coverage was minimal after dispense and progressively increased during the cure and high temperature reflow stages, with the most significant change occurring for the latter. They attributed the appearance of the voids to the evaporation of solvents within the polymer matrix. 4.5.3. Thermo-gravimetric analysis (TGA) The thermo-gravimetric analysis method has been used to assess the stability of TIMs at elevated temperatures [4,18,19]. This test involves subjecting the TIM to one or more heating rate and the weight of material loss is monitored with time. The data can then be used to gain a qualitative indication of weight loss [18,19] or can be used to develop semi-empirical relations to predict reaction kinetics [4]. For example, Ref. [4] tested one thermal grease and one PCM and used the FlynneWalleOzawa method to estimate the reaction kinetics of degradation; ostensibly using the weight lossetime relation to determine the activation energy. The activation energy and weight loss was then extrapolated to use condition to predict the level of material loss over a component lifetime.

Lou et al. [18] performed HTS tests and TGA for an array of silicone and non-silicone thermal greases. They showed that notable increases in the thermal resistance during HTS tests generally corresponded with TIMs that suffered some weight loss during TGA. However, they also showed that in certain TIMs a dramatic loss of material may only be associated with a moderate increase in the thermal resistance. As such, it cannot unequivocally be said that a large/small amount of material loss will cause a like large/small degradation in TIM performance. The tests conducted by Bharatham et al. [19] echo this point, with results that show a very small material loss for a PCM TIM with a correspondingly large thermal resistance increase during HTS testing. As mentioned, it was believed that the minor weight loss corresponded with a volatile agent in the polymer matrix that originally aided adhesion. Although not specifically a TGA, Ref.. [7] monitored weight loss of gel and grease type TIMs subsequent to elevated temperature stress tests. These TIMs have a comparatively high percentage of volatile substances and a linear relationship between void area coverage and weight loss was determined. It is quite clear that although TGA provides the relative comparison of TIM degradation, the accurate prediction of absolute EOLife performance is not possible with this technique [4]. In particular, TGA does not capture failure mechanisms that do not have weight loss associated with them at elevated temperatures. 4.5.4. Differential scanning calorimetry (DSC) Differential scanning calorimetry (DSC) is a thermal analysis technique where a test sample and a reference sample are placed in a chamber, each with a dedicated heater element. The heaters are controlled to maintain the temperature of the test and reference samples to a predetermined value even during a thermal event, such as a change of phase. The amount of energy which must be supplied to or withdrawn from the sample to maintain a zero temperature differential between the sample and the reference is monitored. Changes in the sample that are associated with absorption or evolution of heat cause a change in the differential heat flow which is then recorded as a peak. Significant peaks in the difference are indicative of thermodynamic transitions and/or chemical reactions (curing, change of phase etc.) as a function of temperature. Isothermal DSC refers to the case whereby the samples are held at a constant temperature and the time taken to reach the peak is indicative of transition. An Arrhenius dependence of time to reach the reaction onset or the peak temperature can be used in this scenario. Non-uniform DSC refers to the case in which a rate of temperature increase is selected and the temperature associated with the peak is monitored. DSC can be used to estimate transition characteristics of a material. With regards to TIMs, it is useful since thermodynamic transitions may be accompanied by significant changes in material properties which may impact performance [4]. It is particularly crucial if the behaviour change occurs within the operating temperature range of the component [26]. It can also be useful if performed in combination with other stress tests to help identify/ eliminate failure modes [4] or post stress changes in material properties [21]. 5. Prediction of EOLife performance Accelerated stress tests are performed in order to speed up the ‘aging’ of the material so that the performance at the end of its use life can be predicted during the design cycle of the device. The stress tests are somewhat artificial, in that the actual stress test condition(s) are not typical of use condition; roughly defined by

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a long-term average operating condition based on realistic usage. For example, HTS tests would generally be performed at temperatures which are higher than the realistic operating temperature. This is done to accelerate the degradation so that the reliability test(s) can be performed in a manageable timeframe. This being said, it is quite obvious that the final performance obtained from the stress test cannot be used to predict the TIM’s final EOLife performance. In fact, the use condition information needs to be extrapolated from the data generated from the stress test. This is typically achieved by regression curve fitting based on Arrhenius type of relationships. The form of the Arrhenius relationship depends on the degradation characteristic of the TIM, determined by plotting of, say, the thermal resistance versus time. For thermally activated failure where RTH tends to increase monotonically with time (i.e. continually degrade), such as with HTS tests [19], a linear in time model can be deployed. Here, the Arrhenius relationship is given as,

  Ea RTH ðT; tÞ ¼ RTH ðT; 0Þ þ b exp  $t kT

(2)

where RTH(T,t) is the degraded TIM resistance at time t, RTH(T,0) is the pre-reliability TIM resistance, b is the acceleration coefficient, Ea is the activation energy and k is Boltzmann’s constant [19]. Another commonly encountered degradation trend is one which is asymptotic i.e. where an initial increase in RTH tends to level off until no further degradation takes place with time. To predict this type of failure, an asymptotic in time model may be more appropriate;

  Ea $lnðt þ 1Þ RTH ðT; tÞ ¼ RTH ðT; 0Þ þ b exp  kT

(3)

The above two relations have been used to predict EOLife thermal resistance for TIMs under use condition by Refs. [4,19] for HTS and HAST stress tests. In order to solve for Ea and b, interval data from two tests under different stress conditions must be available i.e. two temperatures for HTS and two temperatures for HAST at 85% RH. A degradation model applied to thermo-mechanical stress tests, such as that of temperature and power cycling, was not found in the open literature. For fatigue induced failures, whether due to purely mechanical or thermally induced mechanical stresses, CoffineManson based models are generally employed to predict the number of cycles to failure. This type of model is not strictly appropriate for TIMs since it is suited for failure which is dominated by crack initiation and growth. The mechanisms of TIM failure are quite different and failure is generally less dramatic i.e. a monotonic increase in RTH compared with a complete fracture of a material or bond. Also, a CoffineManson type of model would not predict the EOLife thermal resistance, which is essential for the design of the device. Goel et al. [4] suggest that for thermomechanical stresses, a Power Law degradation model can be employed, given as;

RTH ðT; tÞ ¼ RTH ðT; 0Þ þ b$Sn $t

(4)

where S represents the stress condition, e.g. change in temperature. The constant b and the exponent n can be determined through fitting of the experimental test data. Once the degradation model(s) are developed from the accelerated stress test data, they can be used to predict the degradation trends at use condition. In order to facilitate this, a long-term average operating condition must be estimated based on a realistic usage scenario. This will result in a RTH versus time curve for the realistic scenario which can be used to predict whether the performance will exceed a predetermined threshold before the end of its use life.

461

6. Conclusions and recommendations for TIM characterization There does not exist a standard protocol for TIM reliability characterization. In fact, most of the reliability tests performed in the past have, for lack of a better alternative, used methods and standards that have been developed for characterizing non-TIM components, which have completely different and generally unrelated failure mechanisms. The most comprehensive TIM characterization methodology was proposed by Goel et al. [4]. Here they suggested a procedure for screening and selecting appropriate TIMs from BOLife to EOLife. Based on Ref. [4] and the above survey, the following can be considered a guideline for screening and selecting TIMs. 6.1. BOLife screening BOL1: Candidate TIMs should pass an initial criterion based on a required thermal conductivity as demanded by the package designers. The ASTM D5470 Tester such as those developed in Refs. [27,28] are suitable for measuring the bulk conductivity of TIM candidates since it is straight forward to implement and can quantify the contact resistance by testing multiple BLTs and extrapolating the contact resistance. This tester can also determine the sensitivity of thermal performance and BLT on pressure loading. This stage will assist in short-listing of potential TIMs on the basis of bulk thermal conductivity and required pressure load [4] to achieve a given BLT. These together will give a non-conservative estimate of the expected BOLife thermal resistance. It will likely be underestimated since it only estimates the thermal resistance associated with the bulk material and not the contact resistances associated with the TIM-interface regions. BOL2: The TIMs which pass BOL1 should then be characterized in a thermal test vehicle (TTV) for BOLife performance in a situation which is closer to in-situ operation i.e. application specific. This stage will result in a refined short-list of candidate TIMs. 6.2. EOLife screening EOL1: TIMs that pass BOLife screening can then be evaluated for EOLife performance to approximate their reliability over the life of the device. Since it is structurally identical to the real device, accelerated stress tests should be performed with a TTV. Preferably the sample size should be large enough that the results are statistically significant. A possible sequence of tests, listed in order of time intensiveness so as to progressively refine the short-list between stress tests, may be as follows; (i) Differential scanning calorimetry to establish if the physical state of a TIM is similar during both accelerated test and realistic use conditions. (ii) Elevated temperature and humidity testing to gauge the influence of humid environments on TIM reliability. (iii) High temperature storage testing to ascertain the effect of elevated temperature on TIM long-term performance. (iv) Power cycle testing to determine if the TIM(s) are susceptible to thermal cycling induced failure, such as pump-out. EOL2: For (ii)e(iv), degradation models should be developed to predict the degradation trends from the accelerated stress data. By determining long-term average operating conditions, the models can be used to extrapolate to use condition. With the threshold thermal resistance for adequate performance being pre-defined, the long-term average condition, required life span and the degradation models can be used to form the final short-list of TIM(s) that will perform reliably during the device lifetime.

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Appendix A Table A1 TIM(s) tested. Grease Wang et al. [14] Chen et al. [8] Bharatham et al. [19] Chiu et al. [9] Goel et al. [4] Tonapi et al. [5] Khuu and Bar-Cohen [15] Gupta et al. [13] Gowda et al. [6] Islam et al. [7] Paisner et al. [16] Mnebe and Ferger [10] Lira and Dal [21] Sinha [25] Vas-Varnai et al. [17] Luo et al. [18] Gowda et al. [22] Bjorneklett et al. [23] Ramaswamy et al. [20] He [24] Out of 20 Percentage

Gel

PCM

Putty

Adhesive

Gap Pad/Filler

X

X

X

Epoxy

Other

Unknown

X X X X X X

X

X

X

X X X X X X

X

X

X X X X

X

X

1 5

1 5

X X

X X 6 30

12 60

3 15

1 5

3 15

1 5

2 10

Table A2 Test vehicles, stress tests, performance metric, sampling method and predictive models.

Researcher(s) Wang et al. [14] Chen et al. [8] Bharatham et al. [19] Chiu et al. [9] Goel et al. [4] Tonapi et al. [5] Khuu et al. [15] Gupta et al. [13] Gowda et al. [6] Islam et al. [7] Paisner et al. [16] Mnebe & Ferger [20] Liza & Dal [22] Sinha [26] Vas-Varnai et al. [17] Luo et al. [18] Gowda et al. [23] Bjorneklett et al. [24] Ramaswamy et al [21] He [25] Out of 20 Percentage

TTV

PreC

HAST

TC

PC

THT

FMC

HTS

DSC

TGA

Tj

Rth

Weight

Visual

Before/ After

Interval Samples

Model

12 60

3 15

8 40

13 65

7 35

7 35

3 15

9 45

3 15

4 20

5 25

13 65

3 15

11 55

8 40

15 75

3 15

References [1] M.M. Yovanovich, E.E. Marotta, Thermal spreading and contact resistances, in: A. Bejan, A.D. Kraus (Eds.), Heat Transfer Handbook, Wiley, Hoboken, New Jersey, 2003, pp. 261e395. [2] R. Prasher, Thermal interface materials: historical perspective, status, and future directions, Proceedings of the IEEE 94 (8) (2006). [3] J. Liu, B. Michel, M. Rencz, C. Tantolin, C. Sarno, R. Miessner, K.-V. Schuett, X. Tang, S. Demoustier, A. Ziaei, Recent progress of thermal interface material researchdan overview, in: Proceedings of the 14th Workshop on Thermal Issues in ICs and Systems (THERMINIC), Rome, Italy, September 24e26, 2008. [4] N. Goel, T.K. Anoop, A. Bhattacharya, J.A. Cervantes, R.K. Mongia, S.V. Machiroutu, L. Hau-Lan, Y-C. Huang, K.-C. Fan, B.-L. Denq, C.-H. Liu, C.-H. Lin, C.-W. Tien, J.-H. Pan, Technical review of characterization methods for thermal interface materials (TIM), in: Thermal and Thermomechanical Phenomena in Electronic Systems, ITHERM’08, Orlando, FL, May 28e31, 2008. [5] S. Tonapi, K. Nagarkar, D. Esler, A. Gowda, Reliability testing of thermal greases, Electronics Cooling Magazine (November 1st, 2007).

[6] A. Gowda, D. Esler, S.N. Paisner, S. Tonapi, Reliability testing of silicone-based thermal greases, in: Proc. 21st IEEE SEMI-THERM Symp., 2005, p. 64. [7] N. Islam, S.W. Lee, M. Jimarez, J.Y. Lee, J. Galloway, TIM degradation in flip chip packages, in: Thermal and Thermomechanical Phenomena in Electronic Systems, 2008. ITHERM’08, Orlando, FL, May 28e31, 2008. [8] C.I. Chen, C.Y. Ni, H.Y. Pan, C.M. Chang, D.S. Liu, Practical evaluation for longterm stability of TIM, Experimental Techniques 33 (1) (2009) 28e32. [9] C.-P. Chiu, B. Chandran, M. Mello, K. Kelly, An accelerated reliability test method to predict thermal grease pump-out in flip-chip applications, in: Electronic Components and Technology Conference, Orlando, Florida, May 29eJune 1, 2001. [10] I.M. Nnebe, C. Feger, Drainage-induced dry-out of thermal greases, IEEE Transactions on Advanced Packaging 31 (3) (2008) 512e518. [11] http://www.jedec.org/. [12] D.C. Montgomery, Design and Analysis of Experiments, sixth ed.; John Wiley & Sons, New York, NY, 2007. [13] A. Gupta, Y. Liu, N. Zamorac, T. Paddock, Thermal imaging for detecting thermal interface issues in assembly and reliability stressing, in: Thermal and

J. Due, A.J. Robinson / Applied Thermal Engineering 50 (2013) 455e463

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

Thermomechanical Phenomena in Electronics Systems, 2006. ITHERM ’06, San Diego, CA, May 30eJune 2, 2006. T.H. Wang, H.-Yu Chen, C.-C. Lee, Y.-S. Lai, High-power-used thermal gel degradation evaluation on board-level HFCBGA subjected to reliability tests, international microsystems, in: Packaging, Assembly and Circuits Technology Conference, IMPACT’09, Taipei, Taiwan, October 21e23, 2009. V. Khuu, A. Bar-Cohen, Effects of temperature cycling and elevated temperature/humidity on the thermal performance of thermal interface materials, IEEE Transactions on Device and Materials Reliability 9 (3) (2009) 379e391. S.N. Paisner, M. Touzelbaev, R.-A. Gamal, Y. Yizhang, New developments for a no-pump-out high-performance thermal grease, in: Thermal and Thermomechanical Phenomena in Electronic Systems, ITHERM’10, Las Vegas, NV, June 2e5, 2010. A. Vass-Varnai, Z. Sarkany, M. Rencz, Method for in-situ reliability testing of tim samples, in: Thermal Investigations of ICs and Systems THERMINIC, Leuven, Belgium, October 7e9, 2009. X. Luo, Y. Xu, D.D.L. Chung, Thermal stability of thermal interface pastes, evaluated by thermal contact conductance measurement, Journal of Electronic Packaging 123 (2001) 309e311. L. Bharatham, W.S. Fong, J. Torresola, C.C. Koang, Qualification of phase change thermal interface material for wave solder heat sink on FCBGA package, in: Electronics Packaging Technology Conference, Shenzhen, China, December 7e9, 2005. C. Ramaswamy, S. Shinde, F. Pompeo, W. Sahlinski, S. Bradley, Phase change materials as a viable thermal interface material for high-power electronic applications, in: Intersociety Conference on Thermal Phenomena, ITHERM’04, Las Vegas, NV, June 1e4, 2004. S. Lira, B. Dal, Degradation mechanisms of siloxane-based thermal interface materials under reliability stress conditions, in: 42nd Annual International Reliability Physics Symposium, Phoenix, AZ, April 26e29, 2004. A. Gowda, A. Zhong, D. Esle, J. David, S. Tonapi, K. Srihari, F. Schattenmann, Design of a high reliability and low thermal resistance interface material for microelectronics, in: Electronics Packaging Technology Conference, Singapore, December 10e12, 2003.

463

[23] A. Bjomeklet, T. Tuhus, L. Halboh, H. Kristiansen, Thermal resistance, thermomechanical stress and thermal cycling endurance of silicon chips bonded with adhesives, in: 9th IEEE SEMI-THERM Symposium, Austin, TX, February 2e4, 1993. [24] Y. He, DSC and DMTA studies of a thermal interface material for packaging high speed microprocessors, Thermochimica Acta 392e393 (2002) 13e21. [25] A. Sinha, Evaluation of bare die chip reliability due to underfill materials during mechanical actuation and thermal cycling, in: 2005 ASME International Mechanical Engineering Congress and Exposition, IMECE’05, Orlando, FL, November 5e11, 2005. [26] A. Gowda, D. Esler, S. Tonapi, Voids in thermal interface material layers and their effect on thermal performance, in: 2004 Electronics Packaging Technology Conference, Singapore, December 8e10, 2004. [27] P. Teertstra, I. Savija, M.M. Yovanovich, Design, assembly and commissioning of a test apparatus for characterizing thermal interface materials, in: Proc. IEEE Inter-Society Conference on Thermal Phenomena, (ITHERM), San Diego, CA, May 30eJune 1, 2002. [28] R. Kempers, P. Kolodner, A. Lyons, A.J. Robinson, A high-precision apparatus for the characterization of thermal interface materials, Review of Scientific Instruments 80 (2009) 095111.

Nomenclature Symbol, description, unit BLT: bond-line thickness, m b: acceleration coefficient, K/W s Ea: activation energy, J/kg kTIM: thermal conductivity, W/m2 K k: Boltzmann’s constant, J/kg K R: thermal resistance, K/W T: temperature, K t: time, s