The use of anisotropic conductive adhesives (ACA) in flip chip interconnection technology has become very popular because of their numerous advantages. The ACA process can be used in high-density applications and with various substrates as the bonding temperature is lower than that in the soldering process. In this paper, six test lots were assembled using two anisotropic conductive adhesive films (ACF) and four different FR-4 substrates. FR-4 was chosen as it is an interesting alternative for making low-cost high-density interconnections. Some of the chips were thinned to study the effect on reliability. To study the effect of bonding pressure, four different pressures were used in every test lot. The reliability of the assembled test samples was studied in a temperature cycling test carried out between temperatures of −40°C and 125°C for 10 000 cycles. A finite element model (FEM) was used to study the shear stresses in the interconnections during the test. Marked differences between the substrates were seen. The substrate thinning and also the chip thinning increased the reliability of the test samples. From the FEM, it was seen that both decreased the shear stress in the adhesive, which is assumed to be the reason for the increased reliability. A significant difference was seen in the reliability between the ACFs. This was probably caused by differences in the conductive particle materials and the T g values and of the ACFs. In addition, the bump material used with the ACFs varied, which most likely affected the reliability of the test samples.
Purpose -The 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/approach -Four 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 mm. 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. Findings -The 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/value -The 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.
PurposeThe 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.Design/methodology/approachSix test series using two ACAs and an LCP substrate were made with varying bonding pressure. The ACAs had the same matrix and conductive particles. To lower the CTE of one of the adhesives silica had been added to it. The reliability of the test series was studied in a temperature cycling test. The purpose of these test series was to find the optimal bonding pressure for both the adhesives used. According to the results from these initial tests, further test series were made with both LCP and PI substrates. The reliability of these test samples was studied using a temperature cycling test and a constant humidity test. The adhesion strength of the joints was studied before testing.FindingsBoth substrates had excellent reliability during the temperature cycling test. However, the reliability of the PI substrates during the constant humidity test was markedly lower than that of the LCP substrates. Additionally, the adhesion strength of the adhesives on to PI substrates was clearly less.Originality/valueThe work shows how the substrate material used affects the reliability of flip chip joints. In addition, the work shows how the addition of silica to the ACA matrix affects the reliability of the joints.
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