Flip chip reliability and design rules for SIP module
Authors: Morard Adrien , Riou Jean-Christophe , Pares Gabriel 
Published: November 2017

The first part of this work is dedicated to the study of “system in package” (SiP) solutions based on different substrates, namely organic or silicon. Generally speaking a SIP is composed by several active and passive components stacked on an interposer. Benchmarks done by Safran have demonstrated that in terms of substrate, embedded die technology leads to several advantages compared to 3D TSV or TGV based packaging approaches. The benefits leaded by this substrate is the possibility to embed some Surface Mount Technologies, bare chips or integrated passives devices (IPD) banks directly above or below the stacked active components. This way, top and bottom surface of the substrate can be used to integrate several heterogeneous dies side by side while using low profile flip-chip assemblies on the C4 side. Finally, in this kind of 3D architecture, this embedded technology enable a gain of integration, without using costly TSV connections. Substrates of high quality allow a reduction of interconnection pitches leading to very aggressive integration.

Secondly, a 3D stack with 3 levels of components, as described above, means to, at least, 2 or 3 REACH compliant sequential assembly processes, depending on the needs. In order to consider all the solutions for an optimized integration and a high reliability, this work focused on the study of a simple SIP, which includes the top die assembled by flip-chip. For the flip chip hybridization, copper-pillars technologies are studied in the case of both organic and silicon interposers. The aim of this study is to understand in depth both processes and to obtain information on the reliability aspect after thermal cycling stress of the flip chip assembly.

Thirdly, we built many silicon test chips with different characteristics with a dedicated daisy chain test vehicle. The different parameters are: chips' thicknesses (50 to 200 μm), chips' sizes (2 to 8 mm), bump structures (diameter), and the pitches of the interconnection (from 50 to 250 μm) and the number of interconnection rows. Designs were chosen in order to fit real operational configurations. Moreover, these configurations are interesting to build a comprehensive model in order to understand the failure mechanisms. These chips are then stacked by flip-chip on the silicon and on the organic substrate. We are also designing the two configurations of substrates. Only the production of the organics part is outsourced.

Fourth, with all these configurations we will be able to fit the thermo-cycling test results with thermos-mechanical simulations done by finite elements. 3D models will take into account the different geometries in order to understand and quantify the various key parameters. The analysis will mainly focus on 3D interconnections. Design rules based on the results will be derivate. The aim is to obtain dimensional criteria based on stress versus deformation responses. Information obtained will be exploited for designing the future functional SIP.

Fifth, in order to assess the electrical behaviors of this 3D architecture, signal integrity aspect will be considered as well. As for the design, the migration from an existing 2D electrical design to a 3D architecture design will be studied keeping the signal transmission without any degradation.

The ultimate aim of this work is to define mechanical and electrical design rules that can then be used in functional SiP modules.