ACF bonding makes reliable connections in ADAS applications
ACF has been used for years for creating electrical conductive adhesive bonds between flexible and rigid circuit boards, glass panel displays, and flex foils. The process essentially means heating and cooling an adhesive containing conductive particles under pressure. ACF bonding is one of the best methods for bonding fine pitch connections, and is now being used for a growing number of new automotive safety and driver assistance systems. The advent of autonomous driving systems and increasing use of displays and camera’s will only increase this trend, since it will require even more communications and sensor applications.
ACF bonding basics
ACF bonding is the process of creating electrical conductive adhesive bonds, with anisotropic conductive adhesive/film, between flexible and rigid circuit boards, glass panel displays, and flex foils. This interconnection technique is mostly used for connecting to printed circuit boards (PCBs).
The ACF material comes in reels and has three functions: electrical connection, insulation of the adjacent terminal, and adhesion.
Figure 1. ACF bonding, basic principles
Anisotropic conductive adhesives contain small, spherical particles that, when compressed and heated, form an electrical connection between parts. The conductive material in the adhesive can be foil, flex, or paste. The conductive particles are distributed homogeneously, maintaining consistent particle density and thickness.
Before bonding, the particles are separated by an isolating matrix of adhesive. The parts to be joined are first brought together with the adhesive in between and tacked, in a step called ACF laminating.
A heating element (thermode) then presses the top and bottom circuit board together with the adhesive in between, causing the adhesive to flow and trapping the conductive particles, resulting in an electrical connection. The particles that are trapped between the conductors form a conductive interface between the pads on the two mating surfaces and conduct only in the Z axis.
Figure 2. Position of the thermode, flex, and adhesive in the ACF process
The joint is stabilized by subsequent cooling and full curing of the adhesive while still in the compressed condition. Because of the low filler content, there is no short-circuiting between adjoining tracks.
[continues]
Figure 3. Scanning electron microscope image of a single hollow conductive particle compressed between two conductive tracks.
The scanning electron microscope (SEM) image shows one hollow conductive particle squeezed between two conductive tracks. The actual particle size is about 5 microns (μm). Normally there are between 100 to 1000 particles involved in one connection.
The adhesive usually consists of a mixture of thermoplastic and thermohardening (also called thermoset or duroplast) glues, to get the best of both substances’ properties.
The conductive particles can be either massive conductive particles, massive plastic particles coated with conductive material, or hollow plastic particles coated with conductive material. They are most frequently made of graphite, gold, or gold-plated plastic, each of which has specific advantages and disadvantages.
Graphite particles are sharp, which can be a benefit if one of the materials being connected has a thin isolating oxide layer. One disadvantage is that that the particles are not elastic, causing higher resistance. Graphite particles are also hygroscopic (they absorb moisture from the air), which can influence the glue matrix and cause corrosion between the contacts.
Gold is not hygroscopic, and contact resistance is lower than graphite particles. However, gold can be more expensive than carbon.
Gold-plated plastic particles are compressible, so contact resistance is lower, because there can be more surface contact between the upper and lower track. The gold-plated particle also works as a spring: a small relaxation of stress in the glue is compensated by an extension of the particle, resulting in an extra safe connection.
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 500 μm (0.002 to 0.020 inches). 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.
Figure 4. 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.
[continues]
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.
Figure 5. Comparison of pitches achievable with available interconnect options
Larger pitches are generally considered to be between 500 and 1000 μm (0.020 to 0.040 inches) and fine pitches are usually defined as between 50 and 500 μm (0.002 to 0.020 inches).
With ACF, the problems limiting very fine pitches, in the 50-100 μm 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 200 μm 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 6. (Figure 6 appears at the top of this page and shows 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 10 newton/centimetre (N/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 miniaturization 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.
Jan-Bart Picavet is product engineer for Hot Bar technology, at Amada Miyachi Europe, https://www.amadamiyachieurope.com/
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
