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Piercing the Low Cost Die Attach Barrier

The emerging portable electronics market continues to fuel the demand for smaller, lighter, better performance, and low cost components. This trend will continue as each new generation of cellular phones, laptops, personal digital assistants (PDA) and smart labels demands new features and enhanced performance. The assembly technology of flip-chip has been an excellent low cost, high performance solution for many of these applications. However, standard flip-chip assembly using either solder or conductive adhesive faces a few critical challenges, such as precise die-to-substrate alignment, high solder reflow temperature, and complete underfill, etc, may not meet the requirement for many of the largest applications. For example, PET foil, a typical smart label substrate, cannot withstand the high temperature required for solder reflow in conventional flip-chip die attach. Nanopierce Technologies, Inc has recently developed a proprietary process, NanoPierce Connection System (NCS), offering an alternative for smart label application with both the cost efficient assembly and an improved reliability benefits.

Existing Flip-chip Process

First invented by IBM in the 1960s for mainframe computer applications, flip-chip technology has gained increasing attention in the development of packaging of mobile electronics in order to lower cost, increase the packaging density, improve the performance while still maintaining or even improving the reliability of the products. Basically, flip-chip technology has the following characteristics:

*The die is mounted onto a substrate (printed circuit board) with the active switching side face down;

*The electrical connection is made simultaneously for all contacts in a single step;

*One of the contact partners must have "bumps", i.e. raised areas composed of electrically conductive materials.

The main advantages of flip-chip technology are a small footprint size and a very thin package design. There are many ways to make the electrical interconnection between die bondpads and substrates in the flip-chip process. One of the most widely used methods is traditional soldering technology, as modified for direct chip mounting. In this case, eutectic Pb/Sn solders for the bumping and connection process such as Pb/Sn 37/63 are used. The efforts incurred for the underfill should by no means be underestimated. Typical materials are filled epoxy materials with different properties; especially with a curing temperature lower than the solder temperature used. New developments such as micro-soldering make it clear that this technology has not yet reached its limits.

Its disadvantages, however, are obvious. Due to relatively high temperatures, the selection of the substrate materials is limited to extremely high-grade materials, which is not exactly a favorable precondition for mobile electronics products.

Anisotropic Conductive Adhesives

Flip-chip die attach with anisotropic conductive adhesives (ACA) is another influential flip-chip bonding methods, which is gaining more attention recently. ACAs can provide electrical as well as mechanical interconnections between chips and substrate. The conductivity of the ACA is restricted to the Z-direction while maintaining electrical isolation within the X-Y plane. In addition, the ACA materials act as an encapsulant and seal the underface of the flip. This eliminates the need for an additional underfill step. However, ACA is often up to 10 times more expensive than nonconductive adhesive. Further, surfaces often require chemical treatment before the ACA is applied in order to achieve adequate bond strength. Moreover, the mating surfaces, i.e., the chip bond pads and the substrates, have to be deoxidized. The placement of the chip must be very accurate and only very limited tolerance is acceptable. Other disadvantages are high-pressure requirements during the device bonding process, which can damage the chip, and a highly sophisticated placement process for the expensive ACA materials (tape or film).

Flip-chip bonding with isotropic conductive adhesive is also a leading chip bonding technique. The process generally lends itself well to integration into a production line. Yet there are clear limits with respect to cost, especially for small components and pitch distances since the electrical connection is provided by isotropically conductive adhesive, i.e., it will conduct in any direction and therefore must be placed discretely at individual bond sites. The high precision requirements therefore, significantly increase the cost and complexity of this technology. In this process, bumps are also added to the bond pads of the chips. These are generally nickel/gold bumps or stud bumps. In addition, high temperatures and a long curing process are usually required. Another notable disadvantage of either chip bonding technique using conductive adhesives, either isotropic or anisotropic, is that immediate testability is often impossible because of the long curing time required.

NCS Process

NCS is a newly developed proprietary chip-level interconnection technology by NanoPierce Technologies. The technology overcomes many aforementioned disadvantages in conventional flip-chip technologies. It includes WaferPierce, a wafer level pretreatment, and a new die bonding process. The WaferPierce process involves embedding miniscule hard particles, such as diamond powder, in a conductive metal layer which is electrolessly plated onto Al or Cu bondpads.

The schematic drawing shown in Fig 1 illustrates the principles of the WaferPierce process. Conceptually, a comprehensive WaferPierce process consists of surface preparation of Al bondpads on wafer (cleaning and zincating), modified electroless nickel-particle co-deposition, a second electroless nickel-plating, and immersion gold treatment. The first electroless nickel (EN) plating step utilizes a modified composite electroless nickel plating method to co-deposit nickel and particles onto Al bondpads by mixing hard particles with the EN solution. After the co-deposition, a particle surface activation step is performed to ensure adhesion of the metal to exposed particle surfaces during the second nickel plating. The second EN plating step is a conventional electroless nickel plating process, which casts a layer of metal on deposited particles. Typically, the resulting NCS surface made by the WaferPierce process includes a co-deposited metal-particle layer (Ni1 + diamonds), an overcoat of the same metal (Ni2) used in the prior co-deposition, and a thin layer of immersion gold (Au). Fig 2 shows a plan view of a finished NCS surface on Al bondpad.

After the WaferPierce treatment, the wafer is diced by normal methods. Individual dies are then attached to a substrate in a "flipped" orientation using rapid curing nonconductive adhesive to mechanically bond the die to the substrate, resulting in a connection that is mechanically robust, chemically inert, and inherently electrically conductive.

Low Contact Resistance

The principal for NCS surface making low contact resistance contact is extremely simple. When a substrate metallization is pressed against a NCS surface on an Al or Cu bondpad, the hard and irregularly shaped particles on the bondpad can easily pierce into the mating substrate even with the presence of a nonconductive oxides layer and adhesive on the mating substrate surface. Fig 3 shows a microscopic view of such a piercing connection.

The NCS process presents many advantages over the conventional flip-chip processes. First, the NCS process is a low cost manufacturing process. The manufacturing process used to produce NCS contacts is a standard electroless plating slightly modified for particle deposition. The core process technology is well established and mature in the printed circuit board industry. The process is simpler and faster than conventional flip-chip process, such as soldering and conductive adhesive bonding. The material costs for NCS are low too since the diamond particles are commonly used as industrial abrasives and are readily available in consistent quality.

Second, resulting from its exceptionally short circuit pathway and piercing effect through any surface oxides, the contact resistance of NCS connection is low (7-15 milliohms). In addition, because the circuit path is short, parasitics are small.

Third, since the NCS material set (diamond, nickel, and gold) is inherently robust and stable, the NCS makes highly reliable contacts. The contacts made by NCS performed better on ultra thin flexible substrates in smart label application than conventional flip-chip processes under comparable conditions in various reliability tests, including temperature cycle test, mechanical test, and chemical tests. Other important benefits of the NCS process include no surface cleaning and deoxidization are necessary during die bonding and the process is lead free.

NCS for Smart Label

Smart labels are paper-thin radio frequency identification (RFID) devices with a semiconductor device (die) electrically connected to an antenna structure. The term RFID refers to electronic systems that use radio waves to exchange data between a data carrier (tag, transponder) and a write-read station. Typical applications for these components will be both the replacement of bar code labels in applications like luggage identification at airports or the identification of express mail parcels and some new applications like electronic ticketing and fare collection.

Not only does the smart label technology allow tagged products to be clearly and unambiguously identified, but also the data stored on the transponder can be read and written an unlimited number of times. The market for smart labels is growing explosively in the recent years and expecting to continue at an even faster rate. Many of the eight billion printed bar code labels currently used for express mail service and six billion used for airline baggage identifications in 2001 will be replaced by smart labels in the coming years.

The NCS and WaferPierce processes offer significant benefits in bonding transponder chips to antennas for RFID inlays, which have enabled NanoPierce Technologies to create a highly efficient RFID inlay production process:

*Assembly is fast, simple and cost effective;

*Low-cost substrates can be used (e.g. using PVC and aluminum);

*No additional cleaning required to remove contamination or oxide layers from the surface of the antenna contact pads;

*Low contact force, as a result suitable for ultra thin chip attachment for future generations of smart labels;

*Low contact resistance (7-15mW);

*Stable contact resistance over lifetime of the inlay, as a result stable read/write distance;

*No high temperatures needed during the entire pick and place process and the curing process.

In February 2002 NanoPierce founded a new subsidiary named ExypnoTech (exypnos, ancient greek: smart, clever) located in Rudolstadt in the province of Thuringhia, Germany, dedicated to the production of RFID components. After installation of the first semiautomatic equipment, high volume production will start in the third quarter of 2002.

New applications for RFID systems require innovative solutions for advanced packaging processes and manufacturing technologies. Standard flip-chip assembly using either solder or conductive adhesive may not meet all the requirements for smart label applications. NCS is a low-cost electrical connection system that can be used to assemble chips at low temperature under very low contact forces with low and consistent resistance. Using the NCS technology, smart labels can be produced cost-effectively and in high volumes. Once initial tests have been performed on semi-automatic flip-chip equipment, the next challenge will be to start up a completely automatic production line for smart labels.

by Dr. Michael E. Wernle, President, NanoPierce Card Technologies GmbH,Germany, Dr. Bin Zou, R&D; Scientist; and Dr. Herbert J. Neuhaus, President, NanoPierce Connection Systems, Inc, USA

(September 2002 Issue, Nikkei Electronics Asia)

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