Stress Relaxation in Plastic Molding Compounds

Viscoelastic materials for plastic encapsulated molding compounds invariably exhibit a time-dependent stress response to an imposed constant strain, which is called stress relaxation. Stress relaxation tests with molding compounds used to encapsulate microcircuits have been performed to measure the time dependent non-linear constitutive relation between stress and strain as a function of temperature and imposed strain. In an effort to improve the design process, a methodology using short time stress relaxation tests can be used to provide long-time design information. The goal of this study is to characterize the non-linear viscoelastic behavior of the encapsulated molding compound during environmental conditions that microelectronics devices have possibly experienced in their lifetime in order to help in developing new design including material selection and process and to inform that it is necessary to include the effect of curing shrinkage as well as viscoelastic behavior for better estimation for stress and deformation such as warpage in device. Introduction The main advantages of plastic-encapsulated microelectronics (PEM) are low cost, compact size, light weight, and ease of processing [1, 2]. In the past, PEMs have been used in commercial and telecommunication devices, which have a large manufacturing base. With major advantages in cost, size, weight, performance, and availability, plastic packages have reached 97% of the market share of worldwide microelectronics sales, although they still encountered formidable challenges in gaining acceptance for use in government and military applications. In fact, it was only in the early 1990s that the industry dispelled the notion that hermetic packages, such as ceramic types, were superior in reliability to plastic packages, in spite of their low production and procurement volumes [3]. One of the main trends in plastic packaging is to move toward thinner packages with fine-pitched leads. While reducing package thickness results in lower die stresses, it can also lead to greater warpage after molding. Most concerns of all are the bending and twisting of packages caused by molding induced stresses because they can affect back-end process steps, such as trim and form and may eventually reduce production yield due to non-coplanarity, which makes the packages difficult to be mounted on printed circuit boards [4]. Excessively warped packages may also lead to tensile stresses on the die surface that, in the presence of flaws could lead to die cracks due to the nature of the brittle material [5, 6]. Post-molding warpage is often used as an indicator of residual die stress when developing new molding compounds [7] and to indicate whether the molding process is stable. A PEM consists of many different materials. Some properties of commonly used materials in PEM packages are presented in Table 1. Due to the unsymmetrical geometry, material construction, and CTE mismatch of different parts of the package, thermal stress and deformation, such as warpage, can occur inside the package, while it is being manufactured and surface mounted or being used [8, 9]. Table 1 Some Properties of Commonly Used Materials in PEMs [10, 11] Coefficient of Thermal Expansion (ppm/oC) Elastic Modulus (1010dynes/cm2) Property Component Material Type 25oC 215oC 25oC 215oC Glass Transition Temperature (oC) Copper 16 18 16 18 119