Understand Wire Bond Loops and ACF
Two major PCB technologies are gaining more attention on the manufacturing floor. Those are wire bonding loops and anisotropic conductive film or ACF, both are especially associated with smaller PCBs. The industry is placing greater emphasis on these smaller boards and rigid-flex circuits for a growing number of PCB applications in the industrial, mil/aero, medical, and IoT sectors.
As shown in Fig. 1, the wire bonding process creates a loop or wire arc between the bare chip and substrate or PCB surface connection. Here, there can be a series of failure modes that can arise that OEMs need to be aware of. Wire bond loops demand precise pull testing, well-trained technical personnel and wire bond pull tester for reliable bond joints.
ACF serves as the interconnect between the rigid and the flex circuit and 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 levels for an optimal process since both circuits can only take a certain amount of temperature.
For more detail on these major areas, check out our recently published articles:
- Get linked up with anisotropic conductive film for your IoT devices
- Failures in IoT PCB Wire Bond Loops
In the meantime, here are some tips and hints to get you started:
- Wire bond loops can break from different segments of the wire length.
- One of the weakest points is at the tip of the loop where it’s been formed.
- Another is a mid-span break in or around the middle of the wire.
- When the ACF interconnect is performed, current capacity must be successfully transferred to the rigid circuit and vice versa to the flex circuit.
- Proper ACF process control demands seasoned PCB manufacturing veterans to maintain correct temperature cycles.
- ACF can be used with chip on board, chip on flex circuit, and chip on glass.
A clear understanding of wire bond loops and ACF can help you to plan your next microelectronics projects involving smaller boards that demand close scrutiny on how your circuitry will be laid out.