Coupling of emerging inspection techniques and thermomechanical

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1 Minapad 2014, May 21 22th, Grenoble; France Coupling of emerging inspection techniques and thermomechanical modeling Diane Ecoiffier INSIDIX 24 rue du Drac F Seyssins (Grenoble) France , diane.ecoiffier(at)insidix.com Abstract Components and assemblies become a dense labyrinth, complicated to explore, whatever for qualification or failure analyses. Analysis technologies for sure are going ahead for better resolution and to obtain more accurate results. But, it is a thorny issue to be able to analyze quite large samples (non destructively) with very high resolution (µm-scale or less) in only few hours! To succeed, one idea is to combine several techniques, having a first non destructive analysis which will help to understand what is happening and to try and localize something, and then identifying the defect with high resolution destructive techniques. On another hand, it is widespread to use modeling. The accuracy of modeling is crucial to have the more realistic prediction of electronic assemblies lifetime, and nowadays assembly complexity make it harder. Indeed, the increase of parameters (eg. complex geometry or high number of materials) implies an increase of errors occurrence not only due to calculation but also due to unknown manufacturing steps, not well-known material parameters, etc. An emerging approach to decrease model development time and errors is to use real measurements data to implement the models. The advantage of the approach is that unknown parameters like mechanical properties of adhesives or strength of interface can be adjusted by matching experimental and modeling results. This paper will present the two approaches, principle and application examples from full board to die level. Key words: Thermo-mechanical behaviour, Warpage measurements, lock-in thermography, material models, FEM simulation, lifetime prediction. Introduction The goal of this paper is to show new emerging approaches allowing faster analyses and lifetime prediction. 1. Fast localization We propose some examples of successful combination of different techniques, using at a first step LIT (Lock-in Infrared Thermography) for shortcircuit or leakage localization and then X-ray or destructive microscopy (<µm) to identify the defect. This new approach can be applied for failure analysis, local thermal dissipation, materials and interface characterizations With LIT, the sample (whatever die or board) is measured in working conditions. An infrared camera observes thermal variations at top surface, and can detect hottest areas due to lack of heat dissipation or leakage. Going from large field of view (whole board or whole component) to smallest field of view (down to 6µm resolution), one can have a fast and accurate view of critical areas. Then, X-ray analyses are performed directly in the area(s) identified with LIT, and circuitry defects can be detected faster because there were already localized. One example concerns a whole board where the defect was localized in only two hours. On another hand, LIT first localization allows incalculable time saving for further destructive analyses! 1.1. Lock-In Thermography principle To obtain high sensitivity thermal imaging, lock-in conditions have been developed [1]. It means that sensitivity improvement for temperature distribution analysis on the surface of the observed sample has been obtained by synchronization of thermal distribution images with periodic stresses (electrical, mechanical, thermal stresses) fig. 1 and 2. By this way, thermal distribution is relevant to material characteristics homogeneities as well as interfaces thermal resistance variation; thermal resistance defect on electrical interconnection dissipation for example.

2 Periodic electrical load Thermography camera Live acquisition Interchangeable optics Sample (b) Connection interface Figure 1: LIT (Lock-In Thermography) system schematic overview Load Temperature at each pixel Amplitude (c) Figure 3: step by step analysis. LIT (phase-shift) image of one quarter of the board (~70mm²) where a hot spot is detected (white circle). (b) Optical Zoom in: Hot spot is superposed to IR image, to better localize. (c) X-ray image at hot spot location showing a short between 2 tracks (red arrows). Phase shift ϕ Figure 2: LIT (Lock-In Thermography) principle. The camera acquires images (~each ms) during several periods. At each pixel it calculates the mean amplitude of temperature variation and the phase shift with regard to load. The results given by software are two images : 1 amplitude and 1 phase-shift cartography. The advantage of the system used is its flexibility: large field of view (up to 200x250mm²) to local analyses (1.6x2mm²) are possible thanks to panel of infrared optics, the system is portable and easy to adapt to sample and working environment Case studies from board to die level The examples shown below will illustrate how fast the analyses are and the ability to have large visualization and close up to the defect position. An embedded board have over-consumption after few working days. As the board is large (~200x200mm²), it is observed with large filed of view (fig. 3). Then zoom in a hot area detected is done and the hot spot is well localized. Finally, an x- ray analysis at high resolution is performed directly at hot spot location, and a short-circuit is identified. This example well illustrate key advantages of LIT : no need of reference nor preparation, sample in working condition, and most of all the analyses where done in only two hours. The same procedure can be applied to molded components (eg. Fig.4) and dies, and further analyses will consist in microscopy or MEB directly in the area identified by LIT. Figure 4: Hot spot localization on a molded BGA assembled on board. A leakage was detected on the board and as previously from large to small field analyses leads to identify the defective component. 2. Lifetime Prediction Guarantee thanks to measurements Concerning qualification and lifetime evaluation of components and assemblies, FEM simulation tools are used to evaluate the reach mechanical stress level in devices and assemblies. However, the accuracy of modeling is crucial to have the more realistic prediction of electronic assemblies lifetime, and nowadays assembly complexity make it harder. Indeed, the increase of parameters (eg. complex geometry or high number of materials) implies an increase of errors occurrence not only due to calculation but also due to unknown manufacturing steps, not well-known material parameters, etc.

3 An emerging approach to decrease model development time and errors is to use real measurements data to implement the models. The advantage of the approach is that unknown parameters like mechanical properties of adhesives or strength of interface can be adjusted by matching experimental and modeling results. We propose several examples of successful combination of measurements and modeling for adhesive development, devices or thin dies for 3D packages. The measurements are done with TDM (Topography and Deformation Measurements) tool, the system developed by Insidix to characterize warpage variation during reflow or thermal cycle. We will mainly show TDM results and how they were used to adapt and to check thermo-mechanical behavior models Warpage and Dilatation measurements TDM (Topography and Deformation Measurements) system was developed to answer new issues of the electronic industries, and to characterize deformation of boards and components when they see thermo-mechanical loads, as those typical for assembly processes or operating conditions (gluing, reflow, aging, etc) [2]. TDM system combines an optical measurement capability (full field and high resolution) and a thermal system that can reproduce reflow profiles (Jedec, with ramps of +3 C/s and -6 C/C) and aging cycles (from -70 C to +350 C), and also a dedicated software to pilot all parts and thermal regulation (cf. Fig. 5 and 6). Figure 5 : TDM system principle Figure 6: Principle of TDM measurements. Example of reflow profile (b) ex. typography of a BGA balls side (c) Graph Extracted values from topographies measured of warpage VS temperature (batch of 8 samples, max in pink and mean in violet). The results obtained with TDM are the warpage variations of packages and assembly. The data obtained as warpage values or topography data files can be directly implemented into simulation software or programs Bulk material and interface strength examples Dealing with cost and space reduction issues, 3D packages will include thinner and thinner dies. Unfortunately, reducing silicon die thickness induce high flexibility that can prevent from good stacking. It is thus mandatory to check process and deposit layers consequences on die warpage. An ongoing study with CEA-Leti deals with this characterization coupling measurements and also modeling. Models give an orientation to material choice and deposit process. With measurement, they check and adapt the model [3]. Concerning warpage measurements, the work consist in checking the influence of mix combinations die thickness, number of layers and material and deposition process (ex. Fig. 7). The measurements help to adapt, in simulation calculation, some material parameters that were not well-known (or suspect). 250 temperature ( C) time (s) (b)

4 (b) Warpage (µm) Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 1 value on each diagonal (ex. Fig. 8). Then, the values are compared with models (ex. Fig. 9) Température ( C) Figure 7: Ex of topography variation of a thin die with deposit layer. (b) Warpage VS temperature graph for several dies. Another use of this approach concerns adherence and interface integrity lifetime. Adhesives used for bonding, show strong temperature and loading history dependent behavior. Prediction of mount stability and bond failure is therefore a key issue. Viscoelastic material models are used to evaluate stresses and strains in bonded joints. However, representative material data for adhesives is difficult to find. An experimental framework was built up at TNO to determine relevant mechanical properties of adhesives for improving stress and deformation prediction. Several measurements were done to obtain the following mechanical properties as a function of temperature CTE, viscoelastic shear and Young s modulus, elastic temperature dependent bulk modulus. A 1 st measurement campaign was lead on adhesive bars (5 different types of materials, mainly epoxy and silicone based), to determine those properties. The data obtained are set in a FEM model (described in [4]). Then, to validate the model, assemblies are designed with the 5 adhesives using an aluminum base plate (60x60x0.2mm 3 ). 25 C W=-1120µm -60 C W=-4000µm 80 C W=+1030µm 25 C_end W=+380µm Figure 8: Shape of one plate with adhesive at different temperatures of thermal cycle. W gives the warpage value of each surface. For each sample, warpage values are extracted at each temperature the value can be calculated from min/max of the topography or max Figure 9: Warpage z as a function of temperature. The blue line represents the measured warpage, the red line represents calculated warpage obtained with the viscoelastic material model. The graph below illustrates that the model well follow reality. Thus, the viscoelastic models are capable to reliably predict stress relaxation and creep effects in bonded joints as a result from thermal loading history. Warpage variations and CTE measurements using TDM are suitable for implementing and check bonded joint lifetime models. Conclusion The 2 approaches introduced in this paper aim at having fast analyses and high level guarantee of results. The advantage is to be applicable to all levels (from die to full integrated system, from development to failure analyses). New challenges with dies and packages density and complexity increasing, and working under tight deadlines, make it obvious that these emerging approaches are full of future promise. So do not hesitate anymore, test it! On another hand they are presented quite separately, but it is also possible to use LIT thermography measurements, for heat dissipation characterization for instance, to check thermal behavior models. Acknowledgements The work illustrated in this paper were done with European labs, that we thank for their collaboration and results sharing 1- Jan de Vreugd from TNOoptomechatronics, Stieltjesweg 1, 2628 CK, Delft, the Netherlands 2- Jean Charbonnier from CEA-Leti, Grenoble, France References Presented Paper [1] O. Breitenstein, Lock-in IR Thermography for Functional Testing of Electronic Devices, 7th Int. Conf. on Quantitative Infrared

5 Thermography (QIRT 2004), Rhode-St.- Genese, Belgium, , Proceedings pp. B [3] J. Charbonnier et al., Bow management with temperature for thin film chips integration, EMPC 2013, Grenoble France, 9-12 september [4] J. de Vreugd, M.J.A. Voert, J.R. Nijenhuis, J.A.C.M. Pijnenburg, E. Tabak, Improved stress prediction in adhesive bonded optical components, SPIE Amsterdam, Journal Article: [2] Michael Hertl, Diane Weidmann (Ecoiffier), "Innovative Assessment of Thermomechanical Stress Effects in electronics components and assemblies", Edfas Journal, Vol. 13, pp. 4-11, August, 2011.