Angela Kessler

Advanced viscoelastic material model for predicting warpage of a QFN panel

Warpage is a critical issue for a QFN panel molding process. Much work is done in the past to predict the warpage of a package during cooling down from molding temperature. However, until now, warpage could not always be predicted well, even if the viscoelastic behavior of the molding compound is taken into account. It was for example observed that the cooling velocity affected the warpage after cooling down. Because of this reason, the mechanical behavior of the molding compound was investigated in more detail. In this research, the mechanical properties of the molding compound are determined. It turned out that the properties are highly dependent on time and temperature. A complete viscoelastic model of the model compound is achieved by combining DMA and dilatometric test results. The model is implemented in the finite element software ABAQUS. In this study, our advanced model is compared with elastic calculations which are normally done. A validation experiment is performed in which simulation results are compared with experimental warpage data of a double layered beam, consisting out of a layer of molding compound and a layer of silicon. This beam is cooled down from a temperature above Tg to room temperature with different cooling rates. In the meantime warpage is measured and compared to simulation results. Finally, the advanced material model is used for calculations on a QFN-panel.

Effect of postcure and thermal aging on molding compound properties

Thermosetting polymers are widely used in electronic industry as encapsulants of electronic devices. It is well known that properties of polymers and polymer-based composites like molding compounds are highly dependent on conditions like: temperature, time, humidity, degree of cure etc. These effects are investigated extensively in the past. Surprisingly, the effect of postcure and thermal aging on the thermomechanical properties of molding compound is scarcely studied. Some studies are devoted to this topic but are not systematically carried out. The main conclusion of previous research is that the glass-transition temperature increases at increasing postcure time. Also weight loss during aging is reported. Since thermomechanical properties determine mainly the reliability of electronic devices it is essential to have knowledge on the effect of post mold cure treatment and thermal aging. In this research the influence of postcure and thermal aging is studied in a systematic way. It turns out that postcure and thermal aging treatment causes an increase in Tg, a change in viscoelastic behavior, an increase of the rubbery modulus, and ongoing shrinkage of the molding compound. The change of these properties is attributed to the oxidation of the molding compound at high temperatures.

Effects of Molding Compound Cure on Warpage of Electronic Packages

Warpage is a critical issue for an electronic package molding process. Much work is done in the past to predict the warpage of a package after the cooling down from the molding process. However, there are many material models for the molding compound to predict warpage: Elastic and viscoelastic models are used, and there are even groups which use cure-dependent viscoelastic models. In this paper, we compare results obtained with the different models with each other. In our group we investigated the mechanical properties of a molding compound in detail. From this research it is concluded that the viscoelastic properties highly depend on the chemical conversion level. Furthermore, dilatometric studies showed that the bulk modulus and the coefficient of thermal expansion are independent on time. The thus obtained model which includes cure shrinkage is implemented in the finite element software ABAQUS by making use of user-subroutines (UMAT). The model is used in a theoretical study on the warpage of a double layered beam consisting out of a layer of molding compound and a layer of copper. This beam is cooled down form a temperature below Tg to room temperature with different cooling rates. Warpage of the beam is calculated with different models and a comparison is made. It turned out that there is a big difference between the different approaches.

Laminate Chip Embedding Technology — Impact of Material Choice and Processing for Very Thin Die Packaging

Laminate chip embedding tailored to very thin dies is an attractive package technology for the integration of logic and power dies. In this paper we report on a new leadframe based laminate chip embedding technology. Best benefit from this technology can only be achieved if experience from silicon front-end, assembly and packaging, as well as from printed circuit board research is combined and consequently applied. We demonstrate the importance of close interaction between Si-frontend and assembly and packaging technology on three examples of applied materials and process research: First, we show in a comparative study that glass fiber filled resin systems result in superior package reliability as compared to filled systems without glass fibers. Second, we present a novel approach to avoid lamination voids by process simulation based on the Monte-Carlo method. Third, we describe the mechanism of laser induced chip metallization damages during contact opening. We also present measures on how to avoid these damages. Our results show that careful selection of laminate materials and chip back-end-of-line materials are the key success factors for stable processes and reliability.