Failure and Delamination in Microelectronic Packages

Abstract

Thin layers of dissimilar materials are used in most microelectronic components in order to achieve special functional requirements. Generally, the interface between two adjacent materials forms a weak link, not only because of the relatively low delamination strength, but also because of the existing mismatch in thermo-mechanical properties, such as Young’s modulus, coefficient of thermal expansion, hygro-swelling, and vapor pressure induced expansion. Residual stresses from the production processes and initial strains due to the changing thermal and humidity conditions together with acting mechanical loading form the crack driving factors for interface delamination. Failure of interface induces decreased reliability of microelectronic components. Nowadays, interfacial delamination forms one of the key reliability issues in the microelectronic industry and therefore is getting more and more attention. The analysis of a laminate structure with a crack along the interface is central to the characterization of the delamination toughness. The delamination toughness is highly dependent to temperature, moisture and mode mixity. In recent years, several studies were directed at the determination of the delamination behavior of Cu-EMC interfaces. In the event that moisture sensitivity was included in these studies, the temperature had to be limited to below 100oC. This limitation reduces the applicability of the toughness data obtained in a number of reliability studies of micro-electronic components. This is due to the fact that, in preconditioned (humid) micro-electronic components the interface delamination often occurs above this temperature limit. In many cases the damage is initiated during heating up in subsequent production steps, where temperatures can reach far above 100oC. Therefore the present research focuses on interface delamination measurements, especially interested in harsh environment (humidity combined with temperatures above the 100oC limit). In this thesis the thermal-mechanical properties of epoxy molding compounds in dry conditions are first investigated. The coefficient of thermal expansion and bulk modulus were measured via a dilatometer (PVT apparatus). DMA experiments in relaxation mode as well as in multi frequency mode were employed for obtaining the viscoelastic master curve and corresponding shift factors. Secondly, the thermo-mechanical properties of EMC during cure were studied. The volumetric contraction of the material during the curing period was measured via a PVT test. Furthermore the increasing shear modulus of the EMC because of the progressing cure was established through DMA experiments. For the dry state, the previous two-material characterization steps are sufficient to be able to interpret the measurement results of delamination tests via FEM simulation. For the humid state with T> 100 °C, in the delamination measurements the effect of (trapped) steam at the interface should be compensated by performing the measurements in a pressure chamber. Therefore, in a third step a pressure vessel surrounding the delamination test setup is designed and built. For the sake of simplicity the humid delamination measurements with T> 100 °C were only performed under full steam pressure (=relative humidity is 100%). The functionality of the setup has been verified by the measurement of the viscoelastic creep compliance of an EMC in dry state and to compare this with the result from measurements obtained from a commercially available measurement instrument. In order to be able to perform delamination measurements a mixed-mode bending setup is installed in the pressure vessel. Interfacial delamination measurements for a EMC-Cu lead frame interface (as obtained from a real production process) are subsequently performed for dry conditions as well as under pressurized steam conditions(= relative humidity 100%). A (I / II) mixed mode mechanical load is applied to the test sample, in which the initial stress state due to the manufacturing and the steam pressure (relative humidity 100%) is already present, in order to initiate and propagate the delamination. In conclusion: This dissertation shows that the temperature, the mix mode and the humidity under these conditions (T> 100 °C and 100% relative humidity) results in a significant effect on the delamination properties of interfaces. As far as known to the author, here for the first time a good insight on the impact of this harsh environment on the delamination properties is presented.