Chip-on-board Technology

Hybrid Technology The trend in electronics is to continue to integrate more and more functions and numbers of components into a single, smaller assembly. Hybrid circuit technology is a key method of increasing packing density, usually involving a mixture of active and passive components. Fig.1 shows a 40 mm x 40 mm package with over 3000 connections. After many years of development, hybrid and chip-on-board or COB technology has reached the stage where considerable savings in space and cost are achievable. This article reviews the most important features of these technologies, together with the manufacturing equipment necessary for their practical use. Fig. 1: Hybrid technology in a 40 mm x 40 mm package. 1 During the various manufacturing stages the interconnects and some of the passive components such as resistors are deposited onto a ceramic substrate. When designing the layout a number of important aspects such as track width, proximity of tracks to bonding pads, bondability, loop heights, heat dissipation and so on must be considered. In thick film technology the functional interconnects, tracking and resistors are created by printing various pastes onto the different levels of the substrate. In thin film technology the interconnects and tracking are deposited galvanically onto the ceramic substrate, resistors and other passive components being added using printing and soldering techniques. Once all passive and surface mount devices have been mounted the chips are placed onto the substrate

using die bonders, the electrical connections between chips and substrate being made with gold or aluminum wire bonders and the whole unit being then mounted in a package. Hybrid technology enables large numbers of chips and miniaturized passive components to be integrated into a very small area. If standard surface mount technology were used it would occupy up to 20 times the area used with hybrid techniques. Manufacturing hybrid circuits requires complete familiarity with wafer fabrication technology as well as chip assembly and bonding techniques. Often smaller companies do not have this detailed knowledge and it is also relatively expensive to manufacture hybrids in small batches. Nevertheless there are a number of applications such as medical technology, aerospace, military, automotive and communications where hybrid circuits are indispensable. Chip-on-Board technology Over the years considerable effort has been devoted to developing methods of utilizing the benefits of hybrid technology but at lower cost. Results show that the tried and tested printed circuit technology is still the best available for complex circuitry, if certain improvements are implemented which allow bare die to be placed and bonded easily and reliably (Fig. 2). Fig. 2: Principle of COB Technology COB_E2.doc 2(9) COB was first used in applications such as digital clocks and watches where a single chip per board is used. Since then its use has spread and it is now widely used in areas such as video cameras, pocket calculators, telephone cards and smart cards. It is also used increasingly in more complex assemblies such as printer modules with upwards of 5000 LEDs, with their associated driver IC's and in sophisticated data processing circuits such as the 32 bit HP 9000 computer which incorporates 22 IC's and a modem on the board. The historical development of COB Technology is shown in Fig. 3

Simple chip (transistor) Complex chip (integrated circuit) Simple hybrid component Complex hybrid component Chip-on-Board-Technology Fig. 3: Historical Development of COB Today state-of-the-art multi-chip COB systems with over 100 chips on a single board have been developed and COB is being used to produce most of the electronic assemblies in entertainment equipment in Japan. In some areas COB has already replaced SMD technology. Cost analyses show that often the DIP packages cost up to three times as much as the chips that they contain. By eliminating the packaging with COB technology significant cost savings are achieved, particularly when producing large batches. Table 1 compares the space requirements of a chip measuring 3 x 3mm within different packages. It shows that requirements vary by up to a factor of 20. There are also significant differences in the height or thickness of the final product. Area Chip COB CC 24 SOP 24 DIP 24 mm 3 x 3 5 x 5 11.8 x 11.8 15.4 x 10.24 31 x 15.24 mm² 9 25 139.24 157.70 472.44 In Europe after a slow start COB technology is becoming more widespread but is still less prevalent than in Japan or the United States where it is widely used, in particular for applications requiring high packing density within a small area and the lowest possible package height. Table 1: Space requirements for different packages Manufacturing COB assemblies COB_E2.doc 3(9) The manufacturing processes for Chip-on-Board modules and hybrids are very similar, the main differences being in the different basic materials used and the packaging. With COB technology a PCB rather than a ceramic is used for the substrate. The bare die are encapsulated or glob topped rather than being assembled in a metal package as in hybrid technology. Compared to conventional SMT assembly both COB and hybrid assembly require fewer manufacturing stages as shown in Fig. 4.

COB or Hybrid assembly Conventional SMD assembly Wafer 1 Wafer Test, dice, clean 2 Test, dice, clean Die bond onto board or ceramic 3 Die bond onto leadframe Wire bond, test 4 Wire bond Seal off bond area 5 Mould, Stamp Add connectors, test 6 Surface treat leads, test Finished product 7 Pack and Ship 8 Unpack, prepare for assembly 9 Assemble using SMD or through-hole machines 10 Solder 11 Cleaning 12 Test Printed circuit boards or PCB's are made from a number of different materials such as phenolic resins, polyurethane, polyamide resin, silicon, epoxy, Teflon and more. Teflon exhibits particularly good resistance to high temperatures whereas polyurethane is used when the product is subjected to extreme temperature variations as in automotive electronics. In applications where minimal thermal expansion is required at very high temperatures Teflon is preferred. Finished product Fig.4: Comparison of manufacturing stages. COB_E2.doc 4(9) The printed circuit tracks used in COB can be normal copper tracks but the bond pads on the board require special preparation. Usually they are built up from a copper base covered with a 2 to 4 micron layer of nickel on which a 0.1 to 0.2 micron layer of gold is deposited. Today bond pads of less than 100 microns with a pitch of less than 100 microns are state of the art.

minimal unevenness no distortions heights maintained to +/- 0.01mm surface roughness maintained to +/- 1 to 2µm (depending on wire diameter) optimum bond surfaces: Al-Wire Copper foil Nickel (2 µm) Gold (0.1 µm) Gold Wire Copper foil Nickel (2 µm) Gold (1 to 2 µm) The bond surface must be completely free of contaminants. The chips are bonded to the board using silver epoxy pastes which are cured and degassed at around 150 C. The problem of conducting away the heat generated by power devices is solved by bonding the chips onto a metalic plate integrated into the board. During final assembly the cooling plate will be fixed to cooling fins or the main casing. Table 2: Printed circuit board prerequisites for COB Electrical connections between the chip and the PCB are usually made with gold or aluminum wire. Aluminum wire bonding has the great advantage of being executed at normal room temperature but takes around three times as long as gold wire bonding. Nevertheless aluminum wire bonds are more reliable when the product is subjected to high temperatures or large temperature variations. To bond reliably with gold wire requires a bond temperature of at least 120 C. At higher temperatures many of the materials used for printed circuit boards go soft and the bond pads pull away from the base material. When bonding with gold wire onto aluminum bond pads on the chips there is a danger of the bonds degrading if the end product is subjected to elevated operating temperatures. The degradation is caused by the growth of Kirkendall pores and can lead to the bond pulling away from the bond pad completely. The decision on which wire to use, gold or aluminum, is made depending on the specific application and the ambient temperature under which the product must operate. After wire bonding the chips are coated or encapsulated using various processes. Silicone compounds that harden at room temperature, epoxy compounds and other materials are used. Additionally the chip can be covered with a plastic or metal cast. Finally the COB element will be mounted in a package and electrically connected using soldering, bonding or crimping techniques. COB_E2.doc 5(9) Aluminum wire bonding When high bond quality is required aluminum wire will normally be used. Bonding speed is slower when compared to gold ball bonding but the end product is often cheaper since the surface treatment of the materials is less expensive.

Fig. 5a and 5b show two views of a memory module COB product bonded with aluminum wire. Fig. 5a Fig.5b Aluminum wire bonding is a friction welding process. In this process two pure metals are pressed together at a pre-defined pressure and then, using a transducer, ultrasonically vibrated until the friction-bond occurs. The amplitude of the ultrasonic vibrations is usually 1 to 2 microns. Two views of a COB technology memory module COB_E2.doc 6(9) The welding process is divided into three distinct phases: firstly the surface cleaning phase, secondly the breakdown of the oxide layers and thirdly the joining together of the pure metals. The metals are pressed together until there is less than one atomic lattice distance between them and the resulting weld is characterized by high quality and extreme stability. Chip metalization is usually aluminum or an aluminum alloy 0.8 to 2 microns thick and is particularly suited for aluminum wire bonding. The ideal bond pad on a PCB consists of a layer of copper covered with a 2 to 4 micron layer of nickel on which a 0.1 to 0.2 micron layer of gold

has been deposited. The gold surface protects against impurities and chemical reactions that could occur during the manufacturing process. It does not effect the bonding process as it is pushed aside during the cleaning phase and the friction weld is made between aluminum and nickel. Evaluations show that aluminum-nickel bonds are best where stability, reliability and good conductivity are required, particularly at elevated temperatures. When the PCB layout is designed a number of aspects such as pad size, pitch and so on must be considered. To avoid problems during bonding it must be ensured that the PCB's are not mechanically distorted and that the top surface is plane. Aluminum wire bonding is a pure ultrasonic welding process done at room temperature. It is absolutely essential that the board is held fast during the welding process to prevent movement or vibrations which could destroy the bond area. This is best achieved by holding it in a vacuum chuck during bonding. It is also essential that the adhesion of the copper tracks to the board in the area around the bond pads is optimized since a lateral movement of 1 micron will adversely effect the bond process. The surface homogeneity of the PCB tracks is another factor. If the thickness of the nickel layer varies and goes below 0.5 microns the bond quality is unstable and bondability may be reduced to zero. The roughness of the copper surface in the bond area should be controlled to less than 2 microns, this being the maximum deviation that can be compensated for by the ultrasonic vibrations. Gold wire bonding Unlike aluminum wire bonding, gold ball wire bonding can not be done at room temperature. A temperature of at least 120 C is necessary to achieve acceptable bond quality. There are the same pre-requisites as with aluminum bonding: the PCB must be plane in order to avoid surface temperature variations and loss of ultrasonic power during the bond process. The bond surface is built up from a 1 micron thick nickel layer covered with a 1.5 to 2 micron thick gold layer. The manufacturing costs of the PCB are higher than for aluminum wire bonding because of the amount of gold used but the bonding speed is about three times faster. This is because the low temperature used for aluminum bonding requires more ultrasonic power and a very precise work holder to clamp the PCB, which has an effect on the overall throughput. Also the loop speed is influenced by the type of tool used - wedge with aluminum wire and capillary with gold wire. COB and packaging technology COB_E2.doc 7(9) The chip is separated from the outside world by a protective package which also provides the electrical connections. Standard packages are available commercially in a limited number of forms and the number of connecting pins is also standardized. This can mean that one is compelled to select a much larger package if only one extra electrical connection is required which increases the

size and cost unnecessarily. Chips with over 100 pins usually require expensive packages and sometimes the package geometry makes bonding more difficult which can result in damaged die. Special ASICs are usually produced in only small quantities with a corresponding increase in the difficulty of selecting a suitable package. The greatest difficulties lie in manufacturing custom specific packages with the largest possible pin counts. COB technology offers the best solution in this case as a specific board design with the requisite number of interconnections can be generated in a very short time using today s technology. See fig. 6a and 6b Fig. 6a Fig. 6b Chip-on-Board packaging technology The number of bond connections and the interconnections required can be properly accounted for allowing very small runs of 10 or more to be manufactured economically. After wire bonding the chip and all the bonds are encapsulated as discussed earlier in this article. The resulting package is perfectly matched to the requirements and cannot easily be copied which is often an advantage with small runs of ASICs where intellectual property needs to be protected. A further benefit is that passive components and/or other chips can be integrated into the same package. The advantages of this packaging method is firstly the miniaturization that would not be possible with standard packages that are often 10 or 20 times larger that the die themselves. Secondly the cost of the standard packages with high pin count ASICs is often higher than the cost of the die themselves. Wire bonders for COB Our research shows that 90% of the products being manufactured with COB technology require a board size of 100mm x 100mm and less than 100 die per board. Wire bonders for COB must therefore fulfill the following minimum requirements: Fig.7: Wire-bonder for COB COB_E2.doc 8(9) Bond area of 100mm x 100mm, minimum Storage of at least 200 reference images for pattern recognition Programmable focus to accommodate multiple chip heights Up to 4 programmable light sources to process chips with different surface contrast A fine touch-down mode to allow for uneven board surfaces Wire control facility to detect wire loss Large Z-axis movement to allow bigger components such as capacitors to be accommodated The capability to bond at 60 when wedge bonding

A vacuum chuck to hold the board Sufficient clearance around the bond tool to allow bonding in deep packages A flexible transport system able to handle 25mm to 150mm board edge lengths Die Bonders for COB Among the important features of die bonders for COB are: Bond area of 100mm x 100mm, minimum Storage of at least 100 reference images for pattern recognition with ink dot and edge recognition capability Programmable focus to accommodate multiple heights Programmable light sources to accommodate chips with different contrasting surface Chip input from wafers, waffle packs and Gel-packs Programmable eject system for at least four different die sizes At least four automatically selectable die collets The ability to print or dispense adhesive as required, selectable and interchangeable under program control A flexible transport system able to handle 25mm to 100mm board edge lengths Fig. 8: Die-bonder for COB Figures 7 and 8 show wire and die bonders from F & K Delvotec suitable for COB applications. Conclusion The requirements for COB boards outlined above must be considered at the design stage. Once these fundamental techniques have been mastered and are under control, COB technology offers a number of benefits in handling, packing density, reliability and in reduction of manufacturing costs when compared to SMD assembly. The author Dr. Farhad Farassat has worked for Delvotec since 1977 and is president of F&K Delvotec Bondtechnik GmbH. Dr. Farassat holds many European and International patents and is widely recognized as one of the leading experts in bonding technology. References COB_E2.doc 9(9) - COB, the technology of the future?: Dr. Farassat, Fellbach 1990 - Trägersubstrateinfluß beim Bonden von COB: Dr. Lang, NIM Berlin der SLV Hannover, - Miniaturization and the increased availability of bare chips is accelerating the adoption of COB Technology in Japan: Havao Nakahara