Reliability of ACF Interconnections on FR-4 Substrates

Abstract

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.

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

“…Bonding time and temperature were chosen according to the ACF manufacturer's recommendation and results from earlier studies with the same materials [18][19][20]. A bonding pressure of 110 MPa was chosen, since it had been found to give good results for the same adhesive in other studies.…”

“…The test was performed To determine the exact time of a failure during accelerated environmental testing, the resistance of the test samples was measured using continuous real-time measurement with a National Instruments data logger system [18]. The voltage was measured every 60 s during the constant humidity test.…”

“…The highest shear stress in all structures were found in the middle of the corner joint. This is a typical place for the highest shear in ACA structures [18,24]. The middle point in the ACF layer in this joint was used as the point at which the values for the stresses were calculated.…”

Study of Structure and Failure Mechanisms in ACA Interconnections Using SEM

“…The cracking of the metal plating may separate it from the polymer core and reduce the amount of conductive path between the pad and the bump (Yim & Paik, 1998). Another problem with too high bonding pressure is elastic stress formed in the chip or in the substrate (Frisk et al, 2010;Lai & Liu, 1996). In some cases, the high temperature used during the bonding process may soften the substrate material used and it will deform markedly during the bonding process.…”

“…Deformation of substrate in ACA interconnections was studied using cross-sections and SEM (Frisk & Kokko, 2006;Frisk & Cumini, 2009;Frisk et al, 2010). SEM has proven to be a very effective method for such studies as the different materials and their interfaces may be clearly seen.…”