ACF-Heat Seal Bonding
Semi-automated anisotropic conductive film heat seal bonding for precise assembly of highly sensitive microelectronics in large area image sensor panels.
ACF-HSB Method for the production of large image sensors
Co-developed with Getronic Engineering AG
In cooperation with our partner Getronic Engineering AG, CrossTEQ has co-developed a specific, semi-automated ACF heat seal bonding process for the production of highly sensitive, large area X-ray image sensor panels in medical technology. In the developed process, flexible printed circuit boards (Flex PCBs) with a conductor pitch of only 0.150mm and a conductor width of only 0.100mm are contacted to an amorphous silicone-based base plate.
About ACF Heat Seal Bonding Technology
Anisotropic conductive bonding (ACF-HSB for Anisotropic Conductive Film Heat Seal Bonding) is a bonding technique for the precise contacting of flexible conductive tracks to a base medium, such as an LCD panel. By using flex films and anisotropic conductive adhesive (in which the electromagnetic properties depend on the direction, whereas with isotropic materials the electromagnetic properties are the same in all directions), ACF-Heat Seal Bonding can be used to join almost any material in a visually appealing way and to achieve a uniform distribution of adhesive force without thermal stress or degradation of the material.
Why ACF-Heat Sealing?
Anisotropic conductive (ACF) heat seal bonding has an extremely high bonding quality, which is why there is a strong shift towards this bonding technique in the manufacturing industry. The explosion in the use of displays, as well as new developments in production technology such as vision-assisted automatic alignment and fully automated Anisotropic Conductive Foil modules, have recently paved the way for widespread use of this technique.
Connection properties of ACF-HSB vs. HSC
Heatseal connections have a relatively high resistance and a low maximum current density. The electrical resistance is in the range of 2-10 Ω for PCB connections and 10-100 Ω for LCD connections. Insulation resistance is in the 109-10 Ω range. ACF can withstand temperatures up to +80 to +120 °C while for HSC the effective temperature comparison is -40 to +60 °C.
Cross Assembly's assembly experts specialize in ACF heat sealing of electronic components to sensitive sensor panels.
Contact us with any questions about HSB bonding technology or your options with CrossTEQ's ACF Heat Seal Bonding process.
ACF-HSB Process
Before the connection, an insulating adhesive film separates the conductive particles. When a heating element presses the top and bottom circuit boards together with the adhesive in between, the adhesive flows and the conductive particles are trapped, resulting in an electrical connection.
With anisotropic heat sealing, compression only occurs in the direction of the Z-axis, which is why an electrical connection is only made in this direction. Due to the low filler content (1-5 %), a short circuit between adjacent conductive paths cannot take place.
The adhesive
The adhesive is usually a blend of thermoplastic and thermosetting (also known as thermoset) adhesives to get the best of both properties. Thermosetting adhesives require a higher temperature and longer cure time than thermoplastic adhesives. Due to their higher melting temperature needs, polyamide film is most commonly used for this purpose.
The conductive particle
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Massive conductive particles
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Solid plastic particles coated with conductive material
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hollow plastic particles, which are covered with conductive material
The most commonly used are solid graphite particles, gold particles and gold-plated plastic particles. Graphite particles are sharp, which can be an advantage if one of the materials to be joined has a thin insulating oxide layer. The disadvantages are that the particles are not elastic, which results in higher resistance.
In addition, graphite particles are hygroscopic. Attracted moisture can affect the adhesive matrix and cause corrosion between the contacts.
Since gold is not hygroscopic, it is sometimes preferred over graphite. Contact resistance is also lower than with graphite particles. Gold-plated plastic particles are compressible, which offers two major advantages:
The contact resistance is lower because a larger surface area comes into contact with the upper and lower webs. The particle also acts like a spring; a small relaxation of the adhesive is compensated for by an expansion of the particle, resulting in a particularly secure bond.
Anisotropic conductive film (Anisotropic Conductive Film)
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If the ACF consists of three layers (the adhesive and two protective layers), one layer is peeled off before the first process step. Positioning of the adhesive and the protective layer on the bonding surface, with the protective layer facing the thermode.
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The material is pre-sealed: It is heated to 80 °C for a few seconds to bond it to the display or PCB.
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The last protective layer is removed.
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The Flex-connector is placed on the ACF and aligned.
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Final sealing is performed at a temperature of 150-180 °C in the adhesive for 10-30 seconds.
ACF-Flex connection design
The flex connector traces are typically 12.5 or 25 µm thick passivated copper or solder or tin coatings.
At CrossTEQ we use gold-plated contacts because they are the most reliable.
Most flexes have three layers, with a thin capton protective layer on the bottom of the flex to minimize stress on the joint.
Display Design
In general, the contact areas should be kept as small as possible because the display areas on the displays should be as large as possible compared to the outer dimensions of the display. In most cases, an area along the entire length of the display is available for the connections. The width of the connector can vary. Normally, this width is 3 or 3.5 mm.
Positioning and final alignment
We use a microscope to achieve the necessary magnification and good contrast during active positioning. Alignment can be performed on special markings (such as crosses, thick tracks, curves, etc.) on the tracks themselves.
The alignment of the parts can also be done passively (preferably with reference holes). It is important to position the holes as close as possible to the display (PCB) to achieve maximum accuracy.
Do you have questions about HSB bonding technology or your options with CrossTEQ's ACF Heat Seal bonding process?
Our assembly experts specialize in ACF heat sealing of electronic components to sensitive sensor panels and are happy to help.
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