Investigation on Thermal Stress Induced Defects in Silicone Encapsulation

Thermal stress induced delaminations and cracks are some of the main failure modes in silicone encapsulated electronic devices. Experimental and finite element analysis were conducted in the present work to determine the main factors that affect thermal stress and their relations with internal defects in silicone encapsulant. Silicone elastomers with three different moduli and adhesive strengths were utilized as encapsulants in cavities with three different sizes. The encapsulation failure risks were evaluated by the ratio between thermal stress and material strength, and was compared with experimental phenomena. Results showed the internal stress became extremely high as the width of the cavity narrowed down, due to the incompressible nature of the silicone and large bulk modulus, resulting in high risks of delamination or bulk crack. Whereas for the broad cavity, both bulk and shear modulus affected the thermal stress, leading to much lower stress. The employment of primer could enhance silicone adhesion and reduce delamination risks, yet also increase the risks of silicone bulk crack. The silicone encapsulant with a low modulus and a high strength, in the presence of primer, showed the lowest risks of internal defects after thermal cycles.