The thermally enhanced ball grid array (TEBGA) electronic packaging under thermal cycling and thermal loading was investigated numerically. Two-dimensional finite element analysis by ANSYS was used for calculating the temperature distribution and thermal stress on the symmetric and diagonal cross sections of TEBGA. The thermal failure based on the peel and shear stresses at interfaces of TEBGA took place at the interface between the heat sink and epoxy moulding compound. The Tasi-Hill failure criterion was modified to predict the failure at various interfaces in TEBGA package. The TEBGA geometric parameters, including the thickness of the heat sink, the thickness of the adhesive layer between the heat sink and the die, and the thickness of the reinforcing copper ring, were varied to assess their effects on the failure mode of TEBGA. The results showed that for a TEBGA under thermal cycling, the stress values were reduced for thicker adhesive layers and thinner heat sinks; for a TEBGA under thermal loading, the die-to-ambient thermal resistance of TEBGA decreased for thinner adhesive layers and thicker heat sinks. The slimmer heat sink of extruded plate type can dissipate more heat and can reduce the stress values. Proper choice of geometric parameters of TEBGA package can prevent its failure at interfaces and furthermore, improve the reliability of electronic packaging.