Anisotropic conductive film

Last updated

Anisotropic conductive film (ACF) is an adhesive interconnect system that is commonly used in liquid crystal display manufacturing to make the electrical and mechanical connections from the driver electronics to the glass substrates of the LCD. The material is also available in a paste form referred to as anisotropic conductive paste (ACP), and both are grouped together as anisotropic conductive adhesives (ACAs). ACAs have more recently been used to perform the flex-to-board or flex-to-flex connections used in handheld electronic devices such as mobile phones, MP3 players, or in the assembly of CMOS camera modules.

Contents

History

ACAs developed in the late 1970s [1] and early 1980s, [2] with heat seal connectors by Nippon Graphite Industries, [3] and ACFs by Hitachi Chemicals [4] and Dexerials (formerly known as Sony Chemicals & Information Devices). [5] Currently there are many manufacturers of heat seal connectors and ACAs, but Hitachi and Sony continue to dominate the industry in terms of market share. Other manufacturers of ACAs include 3M, [6] Loctite, [7] DELO, [8] Creative Materials, [9] Henkel, Sun Ray Scientific, [10] Kyocera, [11] Three Bond, [12] Panacol, [13] and Btech. [14]

In the very early years, ACAs were made from rubber, acrylic, and other adhesive compounds, but they rapidly converged on several different variations of thermoset biphenyl type epoxy resins. The temperatures required were relatively high at 170-180C, however, and the market leaders Sony and Hitachi developed and released acrylic-based materials in the early 2000s that brought the curing temperatures down below 150C while keeping the curing times in the 10–12 second range. Further advances in the acrylic compounds used decreased the curing cycle times to below 5 seconds in many cases, which is where they remain as of this writing. Specification sheets are available at all of the manufacturers' sites listed above.

Current market

ACF continues to be the most popular form factor for ACAs, largely due to the ability to precisely control the volume of material, density of the particles in any sample, and the distribution of those particles within the sample. This is particularly true in the traditional ACF stronghold of display interconnects, but ACF has also seen strong growth out of the display industry and into areas long dominated by surface-mount technologies. The ability to make interconnections in a very small XYZ space has been the key driver in this expansion, helped by the ability under certain conditions to greatly lower cost either by the reduction of component counts or total material used.

ACPs are widely used in lower-end applications, primarily in the assembly of chips on to RFID antenna substrates. They are also used in some board or flex assembly applications, but at a much lower level than ACFs. While ACPs are generally lower cost than ACFs, they cannot provide the same level of control in adhesive quantity and particle dispersion as ACF. For this reason it is very difficult to use them for high-density applications.

Technology overview

ACF technology is used in chip-on-glass (COG), flex-on-glass (FOG), flex-on-board (FOB), flex-on-flex (FOF), chip-on-flex (COF), chip-on-board (COB), and similar applications for higher signal densities and smaller overall packages. ACPs are typically used only in chip-on-flex (COF) applications with low densities and cost requirements, such as for RFID antennas, or in FOF and FOB assemblies in handheld electronics. COG, in particular, also uses gold bumps to connect to the display. [15]

In all cases the anisotropic material, for example, a thermosetting resin containing conductive particles, is first deposited on the base substrate. This may be done using a lamination process for ACF, or either a dispense or printing process for ACP. The device or secondary substrate is then placed in position over the base substrate and the two sides are pressed together to mount the secondary substrate or device to the base substrate. In many cases this mounting process is done with no heat or a minimal amount of heat that is just sufficient to cause the anisotropic material to become slightly tacky. In the case of using a thermosetting resin containing conductive particles, the particles are trapped between prominent points, such as electrodes, between the substrate and the component, thereby creating an electrical connection therebetween. Other particles are insulated by the thermosetting resin. [16] In some cases this mounting step is skipped and the two sides go directly to the bonding portion of the process. In high volume manufacturing, however, this would lead to inefficiencies in the manufacturing process, so direct bonding is usually done only in the lab or in small scale manufacturing.

Bonding is the third and final process required to complete an ACF assembly. In the first two processes the temperatures can range from ambient to 100 °C, with the heat applied for 1 second or less. For bonding, the amount of thermal energy required is higher due to the need to first flow the adhesive and allow the two sides to come together into electrical contact, and then to cure the adhesive and create a lasting reliable bond. The temperatures, times, and pressure required for these processes can vary as shown in the following table.

Table 1: Common ACF Assembly Conditions

Assembly TypeAdhesive TypeTime(Sec)Temp (°C)Pressure
Flex-on-Glass (FOG)Epoxy10–12170–2002-4MPa▲
Chip-on-Glass(COG)Epoxy5–7190–22050-150MPa※
Chip-on-Flex (COF)Epoxy5–10190–22030-150MPa※
Flex-on-Board (FOB)Epoxy10–12170–1901-4MPa▲
Flex-on-Board (FOB)Acryl5–10130–1701-4MPa▲
Flex-on-Flex (FOF)Epoxy10–12170–1901-4MPa▲
Flex-on-Flex (FOF)Acryl5–10130–1701-4MPa▲

▲ Pressures for flex assemblies (FOG, FOB, FOF) are measured across the entire area under the bondhead.

※Pressures for chip assemblies (COG, COF) are calculated on the cumulative surface area of the bumps on the chip.

See also

Related Research Articles

<span class="mw-page-title-main">Adhesive</span> Non-metallic material used to bond various materials together

Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.

<span class="mw-page-title-main">Epoxy</span> Type of material

Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also collectively called epoxy. The IUPAC name for an epoxide group is an oxirane.

<span class="mw-page-title-main">Flexible electronics</span> Mounting of electronic devices on flexible plastic substrates

Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use.

<span class="mw-page-title-main">Thermosetting polymer</span> Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalyst. Heat is not necessarily applied externally, but is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

<span class="mw-page-title-main">Flip chip</span> Technique that flips a microchip upside down to connect it

Flip chip, also known as controlled collapse chip connection or its abbreviation, C4, is a method for interconnecting dies such as semiconductor devices, IC chips, integrated passive devices and microelectromechanical systems (MEMS), to external circuitry with solder bumps that have been deposited onto the chip pads. The technique was developed by General Electric's Light Military Electronics Department, Utica, New York. The solder bumps are deposited on the chip pads on the top side of the wafer during the final wafer processing step. In order to mount the chip to external circuitry, it is flipped over so that its top side faces down, and aligned so that its pads align with matching pads on the external circuit, and then the solder is reflowed to complete the interconnect. This is in contrast to wire bonding, in which the chip is mounted upright and fine wires are welded onto the chip pads and lead frame contacts to interconnect the chip pads to external circuitry.

<span class="mw-page-title-main">Thermal paste</span> Fluid used to maximize thermal contact

Thermal paste is a thermally conductive chemical compound, which is commonly used as an interface between heat sinks and heat sources such as high-power semiconductor devices. The main role of thermal paste is to eliminate air gaps or spaces from the interface area in order to maximize heat transfer and dissipation. Thermal paste is an example of a thermal interface material.

Araldite is a registered trademark of Huntsman Advanced Materials referring to their range of engineering and structural epoxy, acrylic, and polyurethane adhesives. Swiss manufacturers originally launched Araldite DIY adhesive products in 1946. The first batches of Araldite epoxy resins, for which the brand is best known, were made in Duxford, England in 1950.

<span class="mw-page-title-main">Powder coating</span> Type of coating applied as a free-flowing, dry powder

Powder coating is a type of coating that is applied as a free-flowing, dry powder. Unlike conventional liquid paint which is delivered via an evaporating solvent, powder coating is typically applied electrostatically and then cured under heat or with ultraviolet light. The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as household appliances, aluminium extrusions, drum hardware, automobiles, and bicycle frames. Advancements in powder coating technology like UV-curable powder coatings allow for other materials such as plastics, composites, carbon fiber, and MDF to be powder coated due to the minimum heat and oven dwell time required to process these components.

<span class="mw-page-title-main">Hot-melt adhesive</span> Glue applied by heating

Hot-melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed out of the heated nozzle is initially hot enough to burn and even blister skin. The glue is sticky when hot, and solidifies in a few seconds to one minute. Hot-melt adhesives can also be applied by dipping or spraying, and are popular with hobbyists and crafters both for affixing and as an inexpensive alternative to resin casting.

<span class="mw-page-title-main">Tape-automated bonding</span> Places a microchip on a flexible circuit board

Tape-automated bonding (TAB) is a process that places bare semiconductor chips (dies) like integrated circuits onto a flexible circuit board (FPC) by attaching them to fine conductors in a polyamide or polyimide film carrier. This FPC with the die(s) can be mounted on the system or module board or assembled inside a package. Typically the FPC includes from one to three conductive layers and all inputs and outputs of the semiconductor die are connected simultaneously during the TAB bonding. Tape automated bonding is one of the methods needed for achieving chip-on-flex (COF) assembly and it is one of the first roll-to-roll processing type methods in the electronics manufacturing.

Electronic packaging is the design and production of enclosures for electronic devices ranging from individual semiconductor devices up to complete systems such as a mainframe computer. Packaging of an electronic system must consider protection from mechanical damage, cooling, radio frequency noise emission and electrostatic discharge. Product safety standards may dictate particular features of a consumer product, for example, external case temperature or grounding of exposed metal parts. Prototypes and industrial equipment made in small quantities may use standardized commercially available enclosures such as card cages or prefabricated boxes. Mass-market consumer devices may have highly specialized packaging to increase consumer appeal. Electronic packaging is a major discipline within the field of mechanical engineering.

<span class="mw-page-title-main">Thermal management of high-power LEDs</span>

High power light-emitting diodes (LEDs) can use 350 milliwatts or more in a single LED. Most of the electricity in an LED becomes heat rather than light. If this heat is not removed, the LEDs run at high temperatures, which not only lowers their efficiency, but also makes the LED less reliable. Thus, thermal management of high power LEDs is a crucial area of research and development. It is necessary to limit both the junction and the phosphor particles temperatures to a value that will guarantee the desired LED lifetime.

<span class="mw-page-title-main">Thermal copper pillar bump</span>

The thermal copper pillar bump, also known as the "thermal bump", is a thermoelectric device made from thin-film thermoelectric material embedded in flip chip interconnects for use in electronics and optoelectronic packaging, including: flip chip packaging of CPU and GPU integrated circuits (chips), laser diodes, and semiconductor optical amplifiers (SOA). Unlike conventional solder bumps that provide an electrical path and a mechanical connection to the package, thermal bumps act as solid-state heat pumps and add thermal management functionality locally on the surface of a chip or to another electrical component. The diameter of a thermal bump is 238 μm and 60 μm high.

<span class="mw-page-title-main">Failure of electronic components</span> Ways electronic components fail and prevention measures

Electronic components have a wide range of failure modes. These can be classified in various ways, such as by time or cause. Failures can be caused by excess temperature, excess current or voltage, ionizing radiation, mechanical shock, stress or impact, and many other causes. In semiconductor devices, problems in the device package may cause failures due to contamination, mechanical stress of the device, or open or short circuits.


A stamped circuit board (SCB) is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or traces etched from copper sheets laminated onto a non-conductive substrate. This technology is used for small circuits, for instance in the production of LEDs.

<span class="mw-page-title-main">Decorative laminate</span>

Decorative laminates are laminated products primarily used as furniture surface materials or wall paneling. It can be manufactured as either high- or low-pressure laminate, with the two processes not much different from each other except for the pressure applied in the pressing process. Also, laminate can be produced either in batches or in a continuous process; the latter is called continuous pressure laminate (CPL).

An electrically conductive adhesive is a glue that is primarily used for electronics.

Titanium adhesive bonding is an engineering process used in the aerospace industry, medical-device manufacture and elsewhere. Titanium alloy is often used in medical and military applications because of its strength, weight, and corrosion resistance characteristics. In implantable medical devices, titanium is used because of its biocompatibility and its passive, stable oxide layer. Also, titanium allergies are rare and in those cases mitigations like Parylene coating are used. In the aerospace industry titanium is often bonded to save cost, touch times, and the need for mechanical fasteners. In the past, Russian submarines' hulls were completely made of titanium because the non-magnetic nature of the material went undetected by the defense technology at that time. Bonding adhesive to titanium requires preparing the surface beforehand, and there is not a single solution for all applications. For example, etchant and chemical methods are not biocompatible and cannot be employed when the device will come into contact with blood and tissue. Mechanical surface roughness techniques like sanding and laser roughening may make the surface brittle and create micro-hardness regions that would not be suitable for cyclic loading found in military applications. Air oxidation at high temperatures will produce a crystalline oxide layer at a lower investment cost, but the increased temperatures can deform precision parts. The type of adhesive, thermosetting or thermoplastic, and curing methods are also factors in titanium bonding because of the adhesive's interaction with the treated oxide layer. Surface treatments can also be combined. For example, a grit blast process can be followed by a chemical etch and a primer application.

<span class="mw-page-title-main">Chip on board</span> Method of circuit board manufacture

Chip on board (COB) is a method of circuit board manufacturing in which the integrated circuits (e.g. microprocessors) are attached (wired, bonded directly) to a printed circuit board, and covered by a blob of epoxy. By eliminating the packaging of individual semiconductor devices, the completed product can be more compact, lighter, and less costly. In some cases, COB construction improves the operation of radio frequency systems by reducing the inductance and capacitance of integrated circuit leads.

Adhesive bonding is a joining technique used in the manufacture and repair of a wide range of products. Along with welding and soldering, adhesive bonding is one of the basic joining processes. In this technique, components are bonded together using adhesives. The broad range of types of adhesives available allows numerous materials to be bonded together in products as diverse as vehicles, mobile phones, personal care products, buildings, computers and medical devices.

References

  1. ↑ http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=DTPSDS000039000001000244000001&idtype=cvips&gifs=yes [ dead link ]
  2. ↑ "Science Links Japan | Development History of the Anisotropic Conductive Film ANISOLM". Sciencelinks.jp. 18 March 2009. Archived from the original on 29 February 2012. Retrieved 18 October 2011.
  3. ↑ Nippon Graphite Industories, ltd. "Nippon Graphite Industories_index". N-kokuen.com. Archived from the original on 9 October 2011. Retrieved 18 October 2011.
  4. ↑ "Display Related Materials Index: Hitachi Chemical Co., Ltd". Hitachi-chem.co.jp. Archived from the original on 17 October 2011. Retrieved 18 October 2011.
  5. ↑ "Anisotropic Conductive Film (ACF) | Products | Sony Chemical & Information Device Corporation". Sonycid.jp. Archived from the original on 22 October 2011. Retrieved 18 October 2011.
  6. ↑ "3M™ Anisotropic Conductive Films". Solutions.3m.com. Retrieved 18 October 2011.
  7. ↑ "Welcome to the World of Loctite | Select your country". Content.loctite-europe.com. Retrieved 18 October 2011.
  8. ↑ "DELO Industrial Adhesives". Delo.de. Retrieved 18 October 2011.
  9. ↑ Creative Materials http://www.creativematerials.com
  10. ↑ Sun Ray Scientific http://www.sunrayscientific.com/ztach-datasheet-pdf/ Archived 23 June 2015 at the Wayback Machine
  11. ↑ Kyocera "Pastes for Flip Chip Assembly use, TAP/TNP Series" (PDF). 28 September 2005.
  12. ↑ Three Bond http://www.threebond.co.jp/en/product/series/adhesives/s_a3370.html
  13. ↑ Panacol http://www.panacol.com/products/adhesive/elecolit/
  14. ↑ Btech http://www.btechcorp.com/tag/z-axis-conductive-adhesive/
  15. ↑ Advance Information cdn-shop.adafruit.com
  16. ↑ "Anisotropic conductive film and method of making conductive connection".
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.