The development of Anisotropic Conductive Paste (ACP) to make solder metal connection

Abstract

Recently, the various handy devices including the cellular phone and the lap top PC have proceeded rapidly to miniaturization and high performance therefore the semiconductors are required to high performance and high density. As for the flip chip package as well, it is developed into the miniaturization and light-weight in each companies. Accordingly the assembly processes using Pre-applied material (PAM) such as Anisotropic Conductive Paste (ACP) and Non conductive paste (NCP) are developed frequently. PAM is adapted to the process which Under-fill is applied on the substrate prior to bonding of the IC which is different from the process using Capillary Under-fill (CUF). PAM is divided into a number of types according to the bonding condition (bonding energy, control factor, etc.) and the composition materials. When we use PAM, we need to choose Resin and connection type in consideration of the specifications of the package: However, there are problems for these combinations. Compression Contact by conventional ACP and NCP is difficult to pass severe reliability test because the bump and the pad are easy to leave by the thermal expansion of film. In the case of ultrasonic bonding, it is unsuitable for the multiple pin packages since it deteriorates the chip and the stabilization of the connection. Metal connection by Au bump and Solder pre-coat has high reliability, furthermore it is suitable for the multiple pin packages, but production cost is high because a solder precoat is necessary. We investigated advanced ACP which contains solder to solve for these problems. This ACP makes Alloy solder connection, thus connection reliability will be high. It can be applied for the multiple pin packages by thermal bonding. And it is not necessary to form pre-coat solder on the pad. This paper presents the development of the effect of diameter and loading level of solder for connectivity and insulation, in addition we compared to Advanced ACP and conventional ACP, NCP. At first, alloy connection was confirmed using ACP with solder particle. The Chip was torn off after bonding, we confirmed melting solder at the surface of Bump and Pad. Solder alloy layer was confirmed between Bump and Pad by observation of cross-section. As these results, we concluded that the alloy layer could be formed due to the intervention of the solder. Then we evaluated the effect of diameter and loading level of solder to form alloy layer. When Diameter of solder is small, the quantity of existing solder between bump and pad was large. In addition, when solder loading level is high, the quantity of existing solder was much on pads. Especially the melting scars at all bumps were observed with solder loading level of more than 12vol%. The insulation specification was evaluated by HHBT at next stage. In consequence, ACP with solder particle could get good insulation as for a 50um pitch as well by controlling the size distribution of the solder. After that we compared to conventional ACP and NCP under the bonding condition of 2sec. When we measured Electrical resistance value, only ACP with solder particle could get normal value. As a result, good alloy connection and insulation were obtained by controlling the diameter and the quantity of the solder, furthermore ACP with solder particle have more possibility with short time bonding than conventional type. Finally we evaluated reliability test using test vehicle, and it was passed all tests.