March 1998
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Encapsulation Trends: Pushing the Envelope for the Next Generation of CSPs
Chip-scale packages are helping to define the next level of sophistication in encapsulant materials, processes and systems.
By Ron Iscoff, Editor
Encapsulants and the equipment used to dispense them have become one of the fastest growing, most sophisticated areas of semiconductor packaging. As chips become more complex-and more costly-the need to protect them from moisture, via the package and its encapsulant, has taken on a new urgency.Newly emerging markets, flash memory, for example, are demanding greater levels of equipment automation for CSPs, lower-stress encapsulants, higher adhesion and anti-popcorning materials-in short, a quantum leap in encapsulant processing.
"In general," said Dr. Guna Selvaduray, San Jose State University, "the encapsulant must serve as a very effective moisture barrier; it must neither absorb nor should it permit the transfer of moisture through it. Adhesion is also a major reliability item," Dr. Selvaduray added, "and the correct adhesion will help reduce moisture permeating through the package. The 'ideal' encapsulant will also adhere perfectly to all the materials commonly used in packaging."
The CAM/ALOT 3900 system, designed for use with the µBGA package, is representative of the new generation of encapsulant dispensing equipment.
Vacuum Dispensing
The recent progress in vacuum dispensing of encapsulants for CSPs represents a major breakthrough in the field of encapsulation. Vacuum dispense offers a host of benefits. First of all, it has been proven to totally eliminate voids, which commonly occur during encapsulation under the silicon die. Vacuum dispensing also reduces or eliminates settling time for both underfill and liquid encapsulation applications, noted Dr. Charles Bauer of TechLead Corp., Boulder, CO.Employing patented technologies developed at Tessera for the µBGA® package, three companies-working closely with Tessera-have developed systems specifically for dispensing encapsulant in a vacuum for use with Tessera's packaging technology.
The commonality of systems manufactured by Asymtek, Camelot and Phase 2 Automation is based on their use of the Tessera patents, which cover the dispensing of the encapsulant in a vacuum for the µBGA package. Beyond that, each system offers several unique options (Figure 1).
Figure 1 - New liquid dispensing systems have been developed for the critical encapsulation step in the assembly of Tessera's µBGA package.
Asymtek
The Asymtek entry, known as the C-718-VE system (Figure 2), combines the company's proven Century Series platform with a specially-designed vacuum chamber capable of pulling vacuum to 5 x 10 2 millibar (3.8 x 10 2 torr). Standard features on the Asymtek machine include:- An integrated vacuum system, which features a rotary-vane vacuum pump, digital vacuum level display and PLC for automated or manual vacuum control.
- A fully-enclosed, CE-approved platform with three-axis motion control, Pentium®-based computer, emergency stop and safety interlocks, integrated lighting and 24 I/O ports.
- A Cognex vision system for automated pattern recognition
- A DP-2000 linear pump designed to deliver fluid with high accuracy and high flow rates
Asymtek is working with "select strategic customers who have made a commitment to developing the µBGA encapsulation process," Carbin added. The keys to the Asymtek system, he said, are a user-friendly interface, a closed-loop dispense process and a true positive displacement pump which doesn't vary the volume of material dispensed, based on changes in temperature, viscosity or other outside factors.
Figure 2 - The Asymtek C-718-VE dispensing system features an integrated vacuum system which includes a rotary-vane vacuum pump, digital vacuum level display and a Cognex vision system for automated pattern recognition.
Camelot Systems
Camelot Systems of Haverhill, MA, has entered the µBGA package encapsulation market with the CAM/ALOT 3900 (See Figure 3). The system is comprised of three separate vacuum chambers: pre-dispense, dispense and post-dispense. The multiple chambers enable the dispense chamber to remain under constant vacuum not requiring pump down each time a µBGA strip is processed."With low viscosity (<7500 centipoise) flexible epoxies becoming available, the newly-developed vacuum systems will
Figure 3-The CAM/ALOT 3900 system employs separate vacuum chambers for pre-dispense, dispense and post-dispense.
certainly play a role in ensuring the 100% void-free flow of material around and/or under the die in a timely manner," according to Camelot System's Chris Lawing. (See his article in this issue.)
"Heating the flexible tape substrate and the encapsulation material at the time of dispense will ensure a positive flow of encapsulant under the die," he added. "Overall," said Lawing, "the new vacuum dispensing systems with multiple chambers will offer higher throughput than with single chamber systems. Higher yields will be facilitated," he added, "because the dispensing program, heat profiles and vacuum profiles can be modified easily to accommodate the package type (nubbins or pad-type elastomer) and the type of encapsulant to be dispensed."
Phase 2 Automation
Phase 2 Automation of Fremont, CA, has produced the third entry in vacuum liquid dispensing systems for encapsulants. Dr. Barry Saffaie, chief operating officer, said Phase 2 began the design of a new encapsulation and dispensing tool with "the most intuitive method of addressing both the voids and process time."With 16 years of motion control and automation experience, and without the encumbrances of an existing design, our new products division was able to begin implementing the Vacuum Liquid Dispensing Systems (VLDS) design using the latest technology in hardware and software," he added.
Dr. Saffaie said the company aimed its focus on equipment design which utilizes today's latest technology and tools to develop a system that's easy to operate and maintain. "Our method of recipe creation, for example, is non-text-file based, which eliminates tedious x,y,z coordinate editing."
Another benefit of the Phase 2 Automation's design is in the software, which is able to manipulate primatives easily, for example, and nudge and fine-tune recipe or pattern quality after initial dispensing. The major hardware and software components of the equipment are off-the-shelf and field proven, according to Dr. Saffaie.
Dr. Daniel Tracy, an industry consultant with Rose & Associates, Los Altos, CA, noted that improved throughput is high on the "wish list" for dispensing liquid encapsulant materials for BGA and CSP applications. At Semicon Japan, Sumitomo Bakelite "prominently displayed its granulated molding compound technology, which offers process and handling advantages over pellitized molding compounds," Dr. Tracy reported.
The development of encapsulants with low moisture absorption and minimal package warping are perhaps the key requirements for CSPs. The CSP form factor is "particularly susceptible to popcorning," Dr. Tracy noted. "Since they lack mold release agents, and, therefore, offer better adhesion to substrates, liquid encapsulants with high filler loading are particularly good bets for the CSP's smaller form factor."
Figure 4-The Phase 2 Automation Vacuum Liquid Dispensing System is available in two versions, the VLDS-100, semi-automated system, and the VLDS-200, which is fully-automated.
Materials
Co-polymer and silicone-based liquid encapsulants are the most important developments in IC packaging, particularly for flip-chip and generic CSP applications, said TechLead's Bauer. Materials for encapsulation that offer high productivity and reduced cost will be the favorites for CSPs, added Dr. Mak Shinohara of Ciba-Geigy, Los Angeles. Low stress, and a high glass transition temperature to provide good temperature cycle resistance are now mandatory, with low viscosity to enable fast, void-free dispensing. (Editors note: Certain prominent CSP encapsulants, such as silicones, have very low Tg and represent an exception)Carl Lebtonen, president of Silicon Coast Associates, Tustin, CA, agreed that low stress is a key asset of encapsulants. Easy cure properties are also paramount and compatibility is important. "For example, if flip-chip technology is used, the encapsulation material must be compatible with the underfill material," Lehtonen said. "And last, but certainly not least," he added, are materials for CSPs that match the base material's CTE. One material to do everything is now a thing of the past," he observed.
Leadframe-based rigid plastic packages have become thinner-1.4 mm, 1.0 mm and 0.7 mm-affording perhaps only 25 µm to 40 µm of protection to the die, depending on the method of interconnection, said Joel J. Camarda, an industry consultant in San Francisco. "Molding compound composition, adhesion, molding flow and general moldability have become critical factors-not all of which are mutually complimentary."
Chip-size and near-chip-size packages have further reduced the plastic environmental barrier to a mere "blush," with reliance on the chip passivation to provide major environmental protection, said Camarda.
Batch-level Operations
As the packaging operation moves closer to the wafer end with batch-level CSPs, processing will have to take place in a cleanroom environment without contaminating other equipment or product in the room. This requirement will be imposed on both the equipment and the encapsulant, said David Francis, industry analyst at III Inc., Montara, CA.Equipment, too, will need to offer quick changes for maximum productivity. "The use of electronic control systems should permit programs to be downloaded as a new part is being loaded into the machine. By bar coding carriers, trays and cassettes, the encapsulation machine should be able to read the incoming product information, download the required process setup and make whatever changes are needed to encapsulate the product," Francis added.
Conclusion
Encapsulation of CSPs demands equipment that is highly automated, flexible and able to provide void-free processing. Encapsulants have to be better than ever before with lower stress, no popcorning, appropriate viscosity and a CTE that meets the requirements of the given package structure, whether compliant or rigid.Suppliers today are working to meet the new challenges in both equipment and encapsulants as future generations of devices are readied to move off assembly lines.