SOLDER BUMP AND ADHESIVE FLIP CHIP ADVANTAGES
BY JOACHIM PAJONK AND DAVID R. HALK
Cell phones, digital cameras, wireless and portable devices for end users, intelligent labels with RFID chips and high-powered components from medical electronics are all demanding more compact chips. Smaller and flatter packages and reduced assembly areas allow increasing function densities, while device dimensions remain the same or even decrease. Flip chip technology enables greater performance at a reasonable cost, especially for high-frequency wireless technologies. Components with increasing clock speeds require more power and produce increased waste heat. Form factor, performance and cost all favor flip chip assembly, which is expected to grow at a disproportionate rate of 27 percent per year.
Flip Chip Assembly
Solder bump and adhesive flip chip technologies both offer a cost advantage over traditional wire-bonding methods. Mid-range and low-end flip chip assembly are, therefore, competitively positioned compared to their chip and wire counterparts.
Adhesive technology is a promising high-volume bonding option for the future, where chips are bonded to the substrate using adhesive paste or film. A combination of pressure and heat produces the electrical connection and cures the adhesive permanently and with thermostability. The key advantages of adhesive technology are low process temperatures and full-surface mechanical connection to the substrate. Adhesive technology is forecast to grow about 10 percent annually through 2007.
With adhesive technology, curing occurs at approximately 150°C — a considerably lower temperature than required for soldering. With soldering technology, process temperatures increase with the transition from eutectic to lead-free soldering. Adhesive assembly allows the use of more cost-effective substrate materials that are not highly refractory. The adhesive wets the entire surface, so there is no need for underfill material. This eliminates a cost and risk factor, because the adhesive takes over for the underfill function and also fixes the chip to the substrate.
 Figure 1. Nonconductive adhesives (NCAs), primarily dielectric liquids, have been used to mechanically mate flip chips to circuitry. |
Adhesive assembly material options include anisotropically conductive adhesive (ACA) in paste form or as a film (ACF), in addition to nonconductive adhesive (NCA) (Figures 1 and 2). Isotropically conductive adhesive (ICA) is not suitable because it is not applied to the entire surface, bringing back the requirement for underfill.
 Figure 2. Isotropic conductive adhesives (ICAs) must be applied only where needed, because they conduct in all directions like solders. |
Adhesive flip chip bonding, a relatively new assembly technology, is the subject of intensive development work by material and equipment manufacturers aiming to increase reliability, enable economical mass production and create new adhesives and processing procedures. Manufacturing throughput, for example, is increased through parallel curing during the bonding process. This is achieved by connecting a curing station to the bonder, and capacity is adjusted to bonder performance and curing duration. Continuous reel-to-reel applications benefit from the economics of this parallel production. New developments will produce adhesives that even with a no-pressure curing process will apply the necessary adhesive force for a reliable connection between die and substrate. Finally, equipment suppliers are enabling previously unattained levels of productivity through dedicated machine platforms for flip chip assembly.
An example of a typical, high-volume flip chip application is chipcard module production. Chipcard modules traditionally were wire bonded, but now are produced with flip chip technology. In this reel-to-reel process, a die with gold bumps is bonded onto gold-coated substrate pads with NCA, and cured under pressure and heat. Manufacturing on an existing production line currently produces 7,000 components per hour. With this dedicated flip chip assembly equipment, throughput can be easily increased to 8,500 or more components per hour.
Solder Bump Technology
Bump technologies have seen a marked change in the last few years. In 2000, electroplated gold bumps led the field, followed by solder bumps and C4 bumps (IBM technology). Today, 70 percent of flip chip assemblies involve solder bump technology — a figure that is set to grow by 25 percent per year. For newly emerging adhesive technologies, such as gold bumps, an annual increase of 16 percent is expected (Figure 3).
 Figure 3. Flip chip technology is expected to grow at a rate of about 27 percent per year from 2003 through 2007. Source: Tech Search. |
The drivers of flip chip solder bump technology include not only high-frequency modules because of their broadband requirements, but also demanding applications with large numbers of I/Os such as processors with more than 700 pins, graphics modules, chipsets, programmable logic devices, field-programmable gate arrays and large ASICs with between 300 to 700 pins. Their performance features will continue to require solder bump technology and to justify the high substrate costs.
While the high-end applications set the pace for flip chip technology, greater market volume is expected in the future for mid-range and low-end applications. Joint sales of these types of packages are expected to soon be eight times those of the high-end components.
Mid-range packages with 75-700 pins include: Multichip modules, flip chip ball grid arrays, tape carrier packs and chip scale packages. Until now, these packages were formed predominantly with chip and wire technology. For performance reasons, flip chip technology is increasingly preferred in this area.
The key requirement in the low-end segment with 2 to 75 pins is the form factor for chip scale , flip chip lead frame and matrix lead frame packages. Some of these components are already made using flip chip technology, but wire bonding is still in use.
Until now, the high price of substrates and the elaborate and costly underfill step have prevented flip chip technology from penetrating the mid-range and low-end segments of the market more strongly. But this is about to change.
Flip Chip Industry Changes
Recent progress in four areas will ensure additional growth of flip chip technology in all of its forms:
Infrastructure. More companies in the semiconductor industry are offering bumping services, leading to increased capacity for solder and gold bumps. This, in turn, ensures lower prices for bumped wafers.
Substrates. The number of substrate manufacturers and the range of substrates available, including FR4 and flex substrates, and high-density interconnect substrates also are increasing.
Materials. Progress in materials technology is leading to new adhesives and underfills with more favorable material and processing characteristics that help reduce costs.
Machine Technology. A host of manufacturers offer a growing range of flip chip production equipment, the most recent development toward dedicated high-volume flip chip production that maintains precision and valuable process times.
These four development trends will ensure that the benefits of flip chip technology, such as small form factor, better thermal management and more favorable high-frequency characteristics, will open up the mid-range and low-end segments and make them more economical.
Flexible Machine Technology
To solve the many and varied tasks of flip chip assembly and promote the use of flip chip technology across the board, new ground must be broken. For example, production equipment must simultaneously satisfy many criteria, including: handling diverse flip chip processes, high process security and high productivity, as well as high-precision, reproducible results and ease of use.
The prerequisite for a specialized flip chip assembly platform geared to the future is the capacity for a variety of processes. It must be able to manage both classic soldering processes, including flux dipping and the new and diverse adhesive processes. While soldering processes operate primarily on individual substrates, adhesive processes are divided into individual and continuous substrates. For this, the machine requires a transport system for individual substrates, one for continuous substrates and for chip-to-wafer bonding, chuck integration. In addition, to process continuous material, an I/O spooler system with integrated spacer-tape handling is necessary.
Other critical factors in high-volume production are the necessary reliability and process security. To increase throughput, therefore, it is not enough to speed up an existing machine by increasing axis acceleration and reducing process delay time.
Dual Bonding
To satisfy future requirements of the flip chip market, new machine concepts must be geared to reliability as well as high-volume and high-precision bonding. One company's* workable approach is to execute the individual bonding steps in parallel and to work with a dual bonding system in one machine. For dual bonding, the machine** provides two flip modules, two gantries (each fitted with a bond head and substrate camera), two slide fluxers and two up-looking cameras.
From an eject position, the tools of two flip units, in turn, pick up the die from a wafer. After the flip process, the die is taken over by the pick-and-place tool, wetted in the slide fluxer, then measured relative to the tool by the up-looking camera, before being bonded precisely onto the substrate. An “anti-collision arbiter” ensures that each axis knows where the other one is and controls its movement according to this initial condition. The dual bonding system enables parallel bonding, leading to throughput of as many as 10,000 units per hour (dry cycle) — double the throughput with the same process times and a high 10 µm at 3 sigma precision.
Dedicated flip chip equipment should also be prepared for new kinds of tasks. If, for example, future flip chip processes require a heated bonding tool, this should be incorporated into the machine concept and development roadmap. Another advantage of a versatile dedicated flip chip platform concept is the possibility of integrating a dispenser on the machine input side, to work independently of and in parallel with the bonding step. For future high-volume flip chip assembly tasks, ACP, NCP and solder paste dispensing capability is required. Spray fluxing capability should also be a consideration.
Conclusion
Solder bump technology will continue to dominate the high-end segment of the flip chip assembly. In the fastest-growing mid-range and low-end segments, for reasons of cost and performance, both solder bump and adhesive technology will become more attractive. Which of the currently available adhesive process will ultimately prevail is not yet clear.
For the users, this means that a standard flip chip bonder alone will no longer be sufficient to cope equally with proven and future tasks of flip chip assembly. What is called for is a highly flexible flip chip bonder, which can be integrated into production lines based on both solder-bump flip chip and adhesive technologies. Such a production line could include the following modules: flip chip bonder with integrated dispenser and subsequent curing, inspection and testing module with a reject identification function.
*Datacon
**8800 FC Quantum
JOACHIM PAJONK, product manager, may be contacted at Datacon Semiconductor Equipment GmbH, Radfeld, Austria. DAVID R. HALK, general manager, may be contacted at Datacon North America Inc., Seven Neshaminy Interplex, Suite 116, Trevose, PA 19053; (215) 245-3052.