A closer look at ACF benefits

Article By : Jan-Bart Picavet

ACF bonding is an alternative for new systems compared to zero insertion force connectors, board-to-board connectors or soldering.

« Previously: Why ACF bonding makes reliable ADAS connections

ACF bonding is the most cost effective option for a growing list of automotive applications. ACF has been used for a long time for vehicle navigation systems, sensor input devices and car keys. Recently, the bonding method has been used in newer organic light-emitting diode (OLED) rear car lighting systems, as well as for advanced driver assistance systems, such as lane departure warning systems, frontal collision warning systems, intelligent parking assist systems, and driver monitoring systems.

These systems typically use cameras with fine pitch connections, usually defined as those with centre-to-centre spacing between the leads of a component of between 50 and 500μm. Safe and durable connections are critical, since the cars may go into an emergency stop if there is a problem with a connection, an unacceptable outcome.

ACF bonding is a good option for these new systems compared to other available options, which include zero insertion force (ZIF) connectors, board-to-board (BTB) connectors or soldering. The main reason for selecting ACF bonding is cost reduction–the price of these other connectors is high compared to ACF. The following figure compares ACF’s significant cost, volume and dimensional advantages.

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__Figure 1:__ *Cost comparison of alternative connection technologies*

In addition, ZIF or BTB connectors require a good deal of space and they must be positioned more to the centre of the board, whereas ACF use less space and can be placed on the edge. Locating connectors on the outside of the board allows users to include more additional components in the centre of the board. The other option, soldering, can be difficult at the fine pitches required. There is a risk of shorts if there are problems guaranteeing the amount of pre-tinning.

A closer look at ACF benefits

Smaller pitch is possible: As noted, the general trend is towards finer pitches and more connections, as automotive dashboards use more and more complex displays. These systems feature high definition cameras requiring more connections in the same amount of space to process and display data.

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__Figure 2:__ *Comparison of pitches achievable with available interconnect options*

Larger pitches are generally considered to be between 500 and 1000μm and fine pitches are usually defined as between 50 and 500μm.

With ACF, the problems limiting very fine pitches, in the 50-100μm range, do not occur from the glue or the equipment, but from thermal expansion. The newest ACF interconnects allow a fine pitch solution, a minimum of 150 to 200μm pitch for film on board (FOB).

Best option for electrical connections to glass and flex foils; ACF is by far the best option for making connections to glass and flexible displays. With the huge increase in general automotive communications and connectivity to the outside world, the technique is being called on for touchscreen displays and connecting antennas for near-field communication (NFC) applications, as depicted in Figure 3.

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__Figure 3:__ *ACF bonding technology employed on an automotive dashboard.*

Flux-free process: Not only is there an increase in vehicle communications systems, there is also an increasing number of sensors being placed inside cars for monitoring. When making a connection to a sensor, it is critical to avoid any contamination. ACF is flux-free, eliminating the contamination risk of flux used in soldering connections. In addition, no cleaning is required after the process, unlike soldering, which requires flux application, reflow and then part cleaning. When using ACF for very fine pitches, one cleans the parts first, and then performs the ACF bonding steps; no further cleaning is required, which results in time and money savings.

High reliability interconnection: ACF bonding produces a very durable and reliable connection. For example, heat and humidity tests show that conductive resistance remains low and stable. Adhesive strength is typically 10N/cm immediately after bonding. ACF interconnections also withstand drop tests, while mechanical connectors have higher failure rates.

ACF bonding is an interconnection technology with the highest connection quality. A shift is taking place towards the ACF bonding interconnection technique, supported by the market demand for higher input/output counts, further interconnection miniaturisation and weight, and the explosive increase of cameras, displays and antennas in automotive applications.

New developments in production equipment, for example, vision-supported automatic alignment, full automatic ACF laminating module and full process monitoring have recently opened up the path for widespread usage of the ACF technique by significantly lowering interconnection costs.
For example, Amada Miyachi Europe recently developed an ACF bonding system suitable for ACF bonding of OLEDs: for example, as BMW used on an OLED rear light for its M4 coupé. Other similar projects are now under development. The Amada Miyachi Europe ACF bonding systems are designed according to automotive standards and include vision-supported automatic alignment, automatic ACF laminating, final bonding, full process monitoring and data logging/communication options.

This article first appeared on EDN Europe.

« Previously: Why ACF bonding makes reliable ADAS connections

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