Anisotropic Conductive Adhesive Flip Chip Research Articles

Reliability of Anisotropic Conductive Adhesive Flip Chip Attached Humidity Sensors in Prolonged Hygrothermal Exposure

Sensor components may markedly differ from typical silicon chips. Consequently, versatile attachment methods are required for their interconnections. Anisotropic conductive adhesives (ACA) are interesting materials for attachments of sensors due to their versatility. In this study reliability of ACA attached humidity sensors was studied in hygrothermal conditions. The reliability of the interconnections was found to be excellent even under prolonged exposure showing that high reliability can be achieved with ACA materials in sensor applications.

Reliability of adhesive joined thinned chips on flexible substrates under humid conditions

Thin chips are an interesting option for reducing the thickness of an electronics package. In addition to the reduced size, thinned chips are flexible and can dissipate more heat than thicker ones. Joining of the thin chips can be done using several different techniques. Of these, anisotropic conductive adhesives (ACA) are an interesting option as they have several advantages, such as low bonding temperature and capability for high density interconnections. The reliability of ACA flip chip joints under thermal cycling conditions has been found to increase when thinned chips are used. However, the effect of humidity has not been fully explored. In this study the reliability of thinned chips (50μm) under humid conditions was investigated using thin flexible substrates. Seven test lots were assembled with thinned chips using two different ACA films and liquid crystal polymer (LCP), polyimide (PI) and thin FR-4 substrates. A high humidity and high temperature test was used to study the reliability of the interconnections. A finite element model (FEM) was used to analyse the stresses in the test samples during testing. Several failures occurred during the test and significant differences between the substrates were seen. Additionally, bonding pressure was found to be a critical factor for the reliability under the humid conditions.

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.

Homogenization of delamination growth in an ACA flip-chip joint based on micropolar theory

Abstract In the present paper the delamination mechanism of a typical internal structure of the anisotropic conductive adhesive (ACA) interconnect for electronic packaging is modeled on the basis of micropolar theory and computational homogenization. The interface is treated as a finite Representative Volume Element (RVE), across which the macroscopic deformation is expressed in terms of regularized strong displacement and rotational discontinuities. For the microstructure of the RVE, the micro-macro kinematical coupling is considered as a Taylor series expansion in the regularized macroscopic discontinuities, and, connected to that, a discontinuous fluctuation field representing the microstructural variation is included to describe delamination on the microlevel. As to the microlevel delamination modeling, on the basis of the discontinuous fluctuation field, a damage coupled to slip and dilation formulation is used to model the interface degradation. The constitutive relations are established in a thermodynamic setting, where the interfacial free energy involves internal variables of damage and plastic deformation. The parameters of the interface are calibrated so that a predefined amount of fracture energy is dissipated in mode I. In the numerical example, the response of a planar interface is considered when it is subjected to the basic modes I-II and also the non-conventional rotational discontinuity mode. Case studies on fracture and geometry parameters have also been carried out. Finally, an uncoupled thermomechanical analysis of a microsystem involving a representative ACA microstructure has been made for the understanding of the microscopic delamination during a thermal cycling procedure.

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.

Formulation and characterization of anisotropic conductive adhesive paste for microelectronics packaging applications

There has been a steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. In this paper, several anisotropic conductive adhesive (ACA) pastes were formulated, which consist of diglycidyl ether of bisphenol F or diglycidyl ether of bisphenol A as polymer matrix, imidazoles as curing agents, and different sizes of silver (Ag) powders or gold (Au)-coated polymer spheres as conductive particles. The effects of ACA resin and different curing agents, as well as different conductive particles, on flexible substrate of the flip-chip joint were studied. The results show that the size and type of different conductive particles have very limited influence on an ACA flip-chip joint. The ACA resin as well as the curing agent can affect the reliability of the joint. The same results can be applied for the failure analysis of ACA flip-chip technology.

Thermal Cycling Reliability and Delamination of Anisotropic Conductive Adhesives Flip Chip on Organic Substrates With Emphasis on the Thermal Deformation

One of the most important issues whether anisotropic conductive film (ACF) interconnection technology is suitable to be used for flip chip on organic board applications is thermal cycling reliability. In this study, thermally induced deformations and warpages of ACF flip chip assemblies as a function of distance from neutral point (DNP) and ACF materials properties were investigated using in situ high sensitivity moire´ interferometry. For a nondestructive failure analysis, scanning acoustic microscopy investigation was performed for tested assemblies. To elucidate the effects of ACF material properties and DNP on the thermal cycling reliability of ACF assembly, Weibull analysis for the lifetime estimation of ACF joint was performed, and compared with thermal deformations of ACF flip chip assembly investigated by moire´ interferometry. Results indicate that the properties of ACF have a significant role in the thermal deformation and reliability performance during thermal cycling testing. Therefore, optimized ACF properties can enhance ACF package reliability during thermal cycling regime.

Correlation of material properties to reliability performance of anisotropic conductive adhesive flip chip packages

The anisotropic conductive adhesive (ACA) is a promising solder alternative candidate that shows potential for further pitch reduction. Although much work has been published on ACA joint behavior, study on correlation of material properties with reliability performance is still lacking. The main objective in this study was to identify the impact of material properties on reliability, so as to engineer highly reliable microelectronics assemblies. Four representative ACA materials (both film and paste types) with diverse properties were selected. Material properties were characterized as close as possible to "stress test" conditions so as to allow more accurate correlation predictions. Reliability performance was obtained by assembling test chips of 200-/spl mu/m pitch onto BT-substrates, then subjecting them to reliability tests. Correlation analysis was conducted and key material properties that contributed to good reliability performance were identified. Findings indicated that the best properties for high reliability assemblies were: high adhesion strength after subjecting to "stress aging", low coefficient of moisture expansion (CME) and low elastic modulus (E).

Theoretical Analysis of RF Performance of Anisotropic Conductive Adhesive Flip-Chip Joints

The RF performance of a single flip-chip joint using anisotropic conductive adhesive (ACA) was modeled using three-dimensional finite difference in time domain (FDTD) methodology. The effects of the number, the distribution of ACA conductive particles, as well as the ACA resin, on RF transition of the joint were studied. The results show that the number and the distribution of conductive particles as well as the adhesive resin have limited influence on the RF performance of an ACA flip-chip joint. Based on the FDTD simulation results, an equivalent circuit model of a single ACA flip-chip joint was also proposed.

Effect of nonconducting filler additions on ACA properties and the reliability of ACA flip-chip on organic substrates

We investigated the effect of nonconducting fillers on the thermomechanical properties of modified anisotropic conductive adhesive (ACA) composite materials and the reliability of flip chip assembly on organic substrates using modified ACA composite materials. For the characterization of modified ACAs composites with different content of nonconducting fillers, dynamic scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA) were utilized. As the nonconducting filler content increased, CTE values decreased and storage modulus at room temperature increased. In addition, the increase in the content of filler brought about the increase of T/sub g/(DSC) and T/sub g/(TMA). However, the TGA behaviors stayed almost the same. Contact resistance changes were measured during reliability tests such as thermal cycling, high humidity and temperature, and high temperature at dry condition. The reliability results were significantly influenced by CTEs of ACA materials, especially at the thermal cycling tests. Results showed that flip chip assembly using modified ACA composites with lower coefficients of thermal expansion (CTEs) and higher modulus by loading nonconducting fillers exhibited better contact resistance behavior than conventional ACAs without nonconducting fillers.