Reliability Of Flip Chip Joints Research Articles

Effect of substrate material and thickness on reliability of ACA bonded flip chip joints

PurposeThe purpose of this paper is to investigate the effect of substrate material and thickness on the thermal cycling reliability of flip chip joints assembled with anisotropic conductive adhesives (ACA).Design/methodology/approachFour test lots are assembled using three different substrates. Two of the substrates are made of FR‐4. The thicknesses of these substrates are 600 and 100 μm. The third substrate is made of liquid crystal polymers (LCP) and is flexible. With the thicker FR‐4 substrate two test lots are assembled using both normal and two‐step bonding profiles to study how the bonding profile affects the deformation of the substrate. Four different bonding pressures are used to study the effect of pressure on reliability and the failure mechanism of the ACA joints. The reliability of the test samples is studied using a temperature cycling test.FindingsThe reliability of the test lot with the LCP substrate is considerably better than that of the test lots with the FR‐4 substrates. Additionally, the thinner FR‐4 substrate has better reliability than the thicker FR‐4 substrate. The failure mechanisms found varied among the test lots. The effect of the two‐step bonding process on the deformation of the substrate is found to be minor compared with the effect of the glass fibres.Originality/valueThe work shows that the thermal cycling reliability of ACA flip chip joints is markedly influenced by the thickness and material of the substrate. It is also seen that the substrate used influences the failure mechanisms formed during thermal cycling testing.

Effect of RCC on the Reliability of Adhesive Flip Chip Joints

A need for higher packaging density and functionality has increased the use of new packaging technologies, which has also caused demand for higher interconnect densities on printed circuit boards (PCBs). Sequential build-up (SBU) processes can be used to meet these demands. In the SBU process, additional dielectric and conductor layers are formed on a core board, which is typically made of FR-4. Microvias are formed on these layers to achieve an electrical connection between them and the core board. Resin-coated copper foil (RCC) is the most widely used dielectric layer in the SBU process. The effect of RCC on the reliability of flip chip joints with anisotropically conductive adhesive film (ACF) was studied. Two substrates were used. The difference between the substrates was RCC laminated on the other substrate. The reliability of the test samples was studied using a temperature cycling test and a constant humidity test. The reliability of the substrate with the RCC was found to be better in both tests. Failure mechanisms were studied after the tests, using optical and scanning electron microscopes. After the temperature cycling, several of the test samples made with two highest bonding pressures showed delamination, which has probably caused the failures. In addition, failures occurred during the changes in the test temperature. These were probably caused by warping of the flip chip package. No delamination was found in the test samples with the lowest pressure. The failures in these series were probably caused by relaxation of the adhesive matrix and by too low deformation of the conductive particles. Several cracks had formed on the FR-4 substrates without the RCC during the temperature cycling. In addition, air bubbles were found in the test samples with the FR-4 substrates without the RCC. Since RCC is a pure resin system, it has a high coefficient of thermal expansion, which may cause problems, especially when large components are attached to it. However, in this study, the RCC was found to increase the reliability of the flip chip joints made with ACF during both temperature cycling and constant humidity testing.

The effects of chip and substrate thickness on the reliability of ACA bonded flip chip joints

PurposeThe purpose of this study is to investigate the effect of chip and substrate thickness on the thermal cycling reliability of flip chip joints assembled with anisotropic conductive adhesives (ACA) on FR‐4 substrates.Design/methodology/approachFour test lots were assembled with two substrates and two test chips. The thicknesses of the substrates were 710 and 100 μm and the thicknesses of the chips were 480 and 80 μm. To study the effect of the bonding pressure each test lot contained four test series bonded with four different bonding pressures. The reliability of the test samples was studied using a temperature cycling test.FindingsThe reliability of the test lots varied widely during the test. The test lot with a thin substrate and thin chip demonstrated considerably better reliability than the other test lots. In addition, the test lots had different failure mechanisms. After the test delamination was found in every test lot except the one assembled with the thin chip and the thin substrate.Originality/valueThe work shows that the thermal cycling reliability of ACA flip chip joints can be markedly increased by using thinned chips or reducing the thickness of the substrate.

Reliability of ACA bonded flip chip joints on LCP and PI substrates

Purpose – The purpose of this study is to investigate the reliability of flip chip joints made with anisotropic conductive adhesives (ACA) on flexible polyimide (PI) and liquid crystal polymer (LCP) substrates.

Anisotropic conductive adhesives with enhanced thermal conductivity for flip chip applications

In this paper, we present the development work of anisotropic conductive adhesives (ACA) with particular emphasis on the enhanced thermal conductivity of ACAs for flip chip applications. To ensure good thermal conductivity, we incorporated silicon carbide (SiC) fillers in the ACA formulation. For the characterization of modified ACAs with enhanced thermal conductivity by loading different content of SiC fillers, measurements of thermal conductivity and thermomechanical analyses such as dynamic scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA) were performed. The current carrying capability and reliability of flip chip joints using conventional ACA and novel ACA with enhanced thermal conductivity were compared to investigate the role of ACA as a new thermal transfer medium in the flip chip assembly. We developed a new ACA with high thermal conductivity which can improve reliability under high current stressing. This paper is part of a broad study of thermally conductive ACAs for flip chip packaging and other electronic packaging technologies.

Effect of bonding pressure on reliability of flip chip joints on flexible and rigid substrates

In this study the effect of bonding pressure on the reliability of adhesive flip chip joints was evaluated. Two different substrates were used: rigid FR-4 board and flexible liquid crystal polymer (LCP) board. The reliability of the test samples was studied in a temperature cycling test, carried out between temperatures of -40 °C and 125 °C. To determine the exact time of a failure the resistance of each test sample was measured using continuous real-time measurement. A clear difference between different substrates was observed in the test. Cross sections of the samples were made in order to analyse the failure mechanisms.

Mechanical loading of flip chip joints before underfill: the impact on yield and reliability

The underfill-facilitated migration from ceramic to lower cost laminate substrates has become a powerful enabler of direct chip attach by offering lower cost, greater electrical functionality, and a smaller system footprint over comparable packaging technologies. Once underfilled, flip chip on laminate has proven extremely reliable even in severe automotive environments. However, between the process steps of reflow and underfill cure, unprotected flip chip solder joints assembled to laminate boards are susceptible to damage and breakage if mishandled. Here, the survivability and long-term reliability of flip chip joints was studied over a range of applied strains. Mechanical loading of joints was applied via beam deflections of populated, but nonunderfilled, laminate boards. Electrical continuity was monitored before and after testing to determine when the load applied to the flip chip exceeded the joint fracture strength. The propensity for solder joint fracture was then calculated as a function of solder bump size and also as a function of strain rate. Analysis of the mechanical properties of solder revealed assembly strategies which reduce bump damage and eliminate yield loss during the process steps leading up to underfill cure. Both strained and unstrained units were then underfilled and cycled between −50 and +150 °C. While mechanical damage was evident in bump cross-sections of strained flip chip assemblies, the fatigue lives of underfilled solder joints were found to be independent of the size of mechanical loads applied before underfill.