Can you guess what does this picture represent?
This week’s picture is taken from anisotropic conductive adhesive (ACA). The picture above shows a single metal coated polymer particle, which has been trapped between bump on a silicon die and ENIG copper pad on a printed circuit board (PCB).
ACAs have been widely used in electronics, especially, when soldering is not a viable option. For example, in display attachments and radio frequency identification (RFID) tags ACAs have been the leading interconnection technology. ACAs consist of polymer binder mixed with conductive particles. The polymer binder is inherently an insulator. To obtain electrical conductivity the binder is filled with electrically conductive fillers, for example with metal particles or metal coated polymer or glass particles. In ACA the concentration of the particles is low and the adhesive conducts only after the attachment process. In addition to the conductive particles, other fillers are often added to ACA to make it mechanically more stable.
There are two types of ACAs, pastes (ACP) and films (ACF). In the ACA attachment process ACP or ACF is placed between the attached components, for example a silicon die and a PCB (Chip-on-board, COB) or a flex circuit and a PCB (flex-on-board). The ACA interconnection is established by applying pressure and heat simultaneously to the interconnection area. When the temperature is raised, the adhesive matrix transforms into low viscosity fluid, which allows excess adhesive to flow from the joints and fill the spaces around the contacts, forming a physical connection between the parts to be attached. The conductive particles are trapped between the contacts and deform forming an electrical connection. As a result, electrical conduction is restricted to the z-direction and the electrical insulation in x-y directions is maintained. During cooling the ACA matrix contracts and a stable, low-resistance connection is formed. A schematic presentation of an ACA flip chip joint is shown below.
ACAs are used, because they have several advantages compared to underfilled solder interconnections. ACA process is lead and flux free and no cleaning or underfilling after the attachment process is required. Furthermore, the process temperature is lower than that needed in soldering, so heat sensitive or non-solderable substrate materials can be used. Additionally, the ACA process is typically very simple and has low cost. It enables also very high interconnection density. On the other hand, the electrical conductivity of ACA interconnections in markedly lower than that of solder interconnections.
Although the ACA process is quite simple, it may be challenging to achieve interconnections with good quality and reliability. This is true especially when the attachment differs from a typical ACA interconnection. For example, when a very large component is attached, soft, printed or flexible substrates are used, or very low bonding temperature is needed. For good quality interconnections is it critical to optimise the bonding process and to find suitable ACA material considering both matrix material and conductive particle type, material, size and concentration. Often, a systematic study is needed to find suitable material and bonding parameters combination. The differences between different ACA materials can be huge, so it is useful to compare several materials.
With suitable parameters very good reliability can be achieved with ACA interconnections, even for very demanding applications and use conditions. Especially, for new flexible and stretchable, printed and low temperature electronics ACAs offer lots of interesting possibilities. Contact us for more information about ACA technology and its possibilities for different applications.