NOVEL STRESS-BASED CRITERION FOR THE DESIGN OF ADHESIVE-BONDED SPACE ELECTRONICS Lassaad Ben Fekih (1) , Olivier Verlinden (1) , Christophe De Fruytier (2) , Georges Kouroussis (1) (1) Université de Mons, Place du Parc 20, 7000 Mons, Belgium, +32(0)65/37 42 16, lassaad.benfekih@umons.ac.be (2) Thales Alenia Space, Rue Chapelle Beaussart 101, 6032, Mont-sur-Marchienne, Belgium, +32(0)71/44 27 89, christophe.defruytier@thalesaleniaspace.com ABSTRACT The design of space electronic boards relies usually on empirical warp-based criteria having as main asset the coverage of a wide range of electronic components. Such versatility may yield over-design and lack of precision which can be at the expense of the payload weight. In this work, a practical design rule has been developed for electronic components being adhesive-bonded to boards subjected to harsh accelerations of the launch. Numerical and analytical developments within worst-case considerations resulted into an analytical expression of the stress intensity factor at one free corner of the adhesive-board interface. This location represents the most critical site for fracture initiation in a bonded electronic assembly. The proposed design criterion stipulates that the stress intensity factor in the adhesive should not exceed a certain critical value. Such threshold has been identified for a selection of common used aerospace adhesives (CV2946, EC-2216 and TRA-BOND 8.2) via peel testing of custom bonded assemblies. 1. INTRODUCTION Space printed board assemblies (PBA) undergo severe thermal and mechanical loads. In this context, structural adhesives are commonly used in order to increase the service life of electronic components especially heavy and large flat ones [1]. Among others, this consolidation technique contributes to mitigate stress concentrations in solder joints. Presently, PBA design criteria are on the one hand empirical [2] and on the other hand global being based on maximum warp [3], strain [4, 5] and curvature of the printed circuit board (PCB) [6]. A basic requirement in the design for space equipment obliges to comply with the linear elastic range of materials [2]. This work is concerned with the elaboration of a criterion of design for bonded electronic assemblies based upon the stress field in the adhesive layer within the framework of linear elasticity. As known, bonded electronics include interfaces between materials. These locations constitute prominent sites for fracture initiation due to the presence of free corners and free edges yielding singular stress increasing asymptotically to the infinite with decreasing radial distance emanating from the free end. In this situation, the computation of stress at free ends using standard finite elements turns to be questionable. Hence, it is decided to resort to linear fracture mechanics theory. According to this approach, the far-field geometry and the applied loading can be encompassed into a so-called stress intensity factor (SIF) [7]. This permits to derive a fracture design criterion stating that failure occurs when the SIF of the adhesive free corner attains a certain critical value. The investigations related to this approach [7-13] are few in number compared to ones dealing with classical yield criteria such as von Mises. Akisanya et al. [9] investigated a combination of theoretical and non-linear finite element analyses to evidence the validity of such fracture initiation criterion for bonded joints. Zhou et al. [10] identified the most likely corners for crack initiation in plastic integrated circuit package by the identification of SIF at corners using a boundary element method. Lu et al. [11] studied the singular stress fields at corners in flip-chip packages and concluded that lengths of the component and the PCB have slight influence on the singular stress field contrary to the PCB-to-component thickness ratio. By studying single-lap joint Goglio et al. [12] confirmed the great importance of the component contact angle in reducing the SIF. They, also, reported the dependence of such variable to the adhesive thickness. Xu et al. [13]