Recent Advances in CSP Encapsulation

By Craig Mitchell, Tessera Inc., San Jose, California

Encapsulation is a vital element in the production of electronic packages. Beyond providing environmental and mechanical protection, encapsulants can, when properly employed, enhance the reliability of the product while protecting it from certain insidious and deleterious elements such as alpha particles.

To maximize an encapsulant's beneficial role, it is necessary to assure that the compound fully envelops the delicate elements of the micro assembly. The issues related to encapsulation become more pronounced„and perhaps even more important„as device packages move into the realm of chip scale and chip size. The materials and processes used to produce the µBGA® package are key to its successful and cost-effective manufacture.

Developing and Improving Encapsulant Matarials

As IC packages move to chip-scale, the need for improved encapsulant materials is a seemingly never-ending requirement. Dow Corning is meeting the challenge by developing encapsulants that will greatly improve the reliability of Tessera's proprietary µBGA® packages.

These materials exhibit the ability to extend device reliability. However, if the material is not properly dispensed to provide full coverage of important elements, the full advantages are not obtained. As a result, new processes are needed to assure complete encapsulant coverage.

New methods for encapsulation have been developed that accomplish these goals. The methods, which include both dispense-in-vacuum and pressure cure, provide essentially void free encapsulation. This article will discuss those advances.

Void-Free Encapsulation Methods

Many techniques have been employed to provide void-free encapsulation. Most traditional approaches to encapsulation place the assembly in a vacuum chamber after the encapsulant is dispensed. Remove entrapped air and to eliminate possible residual voiding. Processes where vacuum is applied after dispense can be time consuming and lead to contamination.

With Tessera's µBGA® chip-scale package, the device is encapsulated from the backside of the die. This means there is a possibility of contaminating the back surface of the die by vacuum as air bubbles escape and burst to the surface, splattering the encapsulant.

Splattering is a significant concern because it requires a step cure and an additional cleaning step. The step cure is used to minimize the degree of curing, allowing the material that has contaminated the backside of the die to be easily cleaned. (Figure 1 illustrates this concept.)

Processing at lower levels of vacuum within the chamber can reduce the potential for this type of contamination; however, this, also, is not without a tradeoff, since modifying the vacuum level downward can significantly extend process time within the vacuum chamber.

Pressure Encapsulation

Another technique developed at Tessera to ensure void-free encapsulation is the "pressure cure process." Pressure cure, a technique similar to molding, involves the use of pressure to collapse residual voids and temperature to cure the material in place.

Figure 1-Vacuum blurping can cause back side contamination.

Vacuum is applied after surrounding the die assembly with encapsulant.

Air bubbles soming to the edge burst spattering the back side of the die with encapsulation.

During this process, the encapsulant is dispensed from the backside of the die on three sides. As the material wets the edge of the die, capillary action pulls the material underneath the die in the space between it and the flexible polyimide substrate. Depending on the die size and viscosity of the encapsulant, a nominal waiting period is observed to allow the material to flow.

Encapsulant is then dispensed on the fourth side of the die, sealing it off. This process assumes that there will be an entrapped void. An additional wait period is observed to ensure that the encapsulant has flowed sufficiently and that the perimeter of the die has been fully sealed.

This is an essential element of this process, since it allows a cavity or void to be defined underneath the die. The strip of assembled and encapsulated ICs is then placed into the pressure oven where the pressure is ramped up to 100 psi. It is this pressure differential, between the entrapped void and the surrounding chamber, that allows the void to be collapsed as shown in Figure 2.

The temperature is then raised slowly to 80°C to cure (or mold) the material in place. Curing while under pressure prevents the void from reforming during the return to atmospheric conditions.

The pressure oven has been restricted to 80°C due to other heat-sensitive elements of the construction, such as the soldermask, which is used to seal the bond window. This is done because such material requires further processing and can be affected by excessive temperature exposure.

The pressure-cure process was originally developed to eliminate the necessity of vacuum degassing, the potential need for void removal by vacuum and to assure void-free encapsulation with any of the various elastomer pad configurations used in µBGA« package fabrication, such as the elastomer pad, segmented pad and nubbins (Figure 3). While the pressure-cure process provides these benefits, extended process time, batch processing constraints and pressure vessel safety codes are clear drawbacks.

Figure 2-The pressure differential forces encapsulant underneath the die.

With leads bonded and coverlay attached the package is ready for encapsulation.

Encapsulation is despensed around the part and begins to flow into the space between the chip and the flexible interposer.

The assembly is moved to a pressure vessel which pushes the encapsulant into the space between the nubbingns and collapses any bubbles.

Vacuum Dispense Processing

After some reflection, we felt that the advantages of both vacuum and positive pressure could be merged, eliminating their disadvantages while maintaining their benefits. This notion led to the development of the novel vacuum dispense process.

In the process, the dispensing operation takes place under vacuum. The basic concept of the process is to remove the air from the assembly, thus leaving nothing to be entrapped during encapsulation. Like the pressure-cure process, vacuum dispensing takes advantage of the pressure differential to force the encapsulant underneath the die and collapse residual voids.

Figure 3-Elastomer geometries for DRAM.

Split elastomer pad construction.

Segmented pad construction.

Elastomer post or Nubbin Construction.

As with many new processes, new equipment development was required. A prototype vacuum dispense machine was built at Tessera to verify that the concept worked.

Later, to minimize the development cycle for the vacuum dispensing equipment, Tessera worked closely with three equipment manufacturers who had experience in encapsulation and equipment integration: Asymtek, Camelot Systems and Phase 2 Automation.

All three vendors developed vacuum dispensers with the specific intention of providing manufacturable, void-free encapsulation of µBGA® packages. Since both Asymtek and Camelot engineered tools around existing platforms, they designed input, process and output chambers (Figure 4) to allow for increased throughput. The systems are set up to maintain the large process chamber under vacuum continuously. Thus the significantly smaller input and output chambers are cycled between vacuum and atmospheric pressure.

The input chamber attains vacuum readily, typically on the order of 3-5 seconds. The output chamber, which is the same size as the input chamber but is responsible for the application of the pressure differential, has a programmable pressurization rate to allow sufficient flow of the encapsulant to ensure void-free processing.

Single-Chamber Processing

In contrast, Phase 2 Automation designed a system that allows for vacuum, dispense and pressurization to take place in one small chamber. It's the smallest of the three systems and takes advantage of its size to facilitate reduced process time and increased throughput. The systems are also configured with magazine-to-magazine handlers and will be capable of reel-to-reel handling once pagination standards are set for the page length and width of µBGA® flex tape.

Design

Each of these novel tools, developed under Tessera license, was designed with the common automation options required for production. Each company worked with outside vacuum experts to assist in their design for continuous operation under vacuum, specifically with respect to the chambers, the motors, the vacuum gates, the vacuum pumps and the lubricants. This decision significantly reduced overall design schedules.

Figure 4-The new systems designed by Asymteck and Camelot offer input,
process and output chambers to allow for increased throughput.

Summary

Tessera has developed and implemented a unique and valuable method for reliable, void free encapsulation of µBGA® chip-scale packages. This process effectively neutralizes one of the more vexing problems facing electronics assemblers and enables the further advance of electronics miniaturization.

Craig Mitchell is a development engineer at Tessera. He joined the company from Manhattan College, Riverdale, New York, where he earned a bachelor's degree in electrical engineering. He is the author of many papers and several patents involving the packaging of chipscale electronics. He may be contacted at 408.383.3615 or by email at craigm@tessera.com.