If you’re developing, producing or selling IoT devices, it might be a good idea for you to get acquainted with the term anisotropic conductive film, or ACF for short. You don’t hear much about it. However, it plays a big role in the manufacture of small printed circuit boards (PCBs), especially when IoT products are made of rigid-flex circuits.
ACF serves as the interconnection between the rigid and the flex circuit. There are other methods for performing this interconnect, but ACF is gaining momentum in the market due to its flexibility and reliability. Connection between the rigid and flex circuits is made using a thin film of polymer and tiny conductive polymer balls. This process requires the right tools and the right levels of temperature, since both circuits can only take a certain amount of temperature.
Two other considerations must be taken into account during manufacturing. First, manufacturing engineering must assure that when performing the interconnect that current capacity is successfully transferred to the rigid circuit, and vice versa to the flex circuit. Second, size and composition of the polymer balls and film must be carefully scrutinized and, in effect, make sure the ACF material is optimized.
It also takes seasoned PCB manufacturing veterans to properly control the ACF process, largely due to the temperature cycles involved. For utmost reliability, assurances must be made to avoid increasing the temperature too much because the conductive particles of polymer have certain gold and nickel content. This makes ACF susceptible to certain temperature cycle changes. If the process engineer doesn’t properly define temperature cycles suitable for polymer ball size and composition, then the ACF interconnect bonding will either fail or result in poor reliability with a high probability of latent failures in the field.
As for its flexibility, ACF offers IoT device OEMs several applications. It can be used with chip-on-board, which means a bare die or chip can be mounted on the board with the ACF process. It can also be used on chip-on-flex, which is similar to chip-on-board, but the bare chip is mounted on the flex circuit. There is also chip-on-glass and flex-on-glass, as well as other applications.
Typically, the process here is to heat up the anisotropic conductive film tool to a specific temperature range. Then, aligning of the flex and rigid board takes place where the ACF bond has to take place. Subsequently, the film is placed between the rigid and flex circuits, along with certain pressure and temperature to make sure of proper adhesion to correctly make the solid joint.
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