A viable alternative to transfer molding for fine-pitch advanced IC packages gains acceptance. A dual chamber driven by an LPD pump provides continuous dispensing for maximum equipment usage.
By Prasad Nevrekar and Dan Crowley
 Figure 1. The basic dispense and molding processes. |
Wire-bonded semiconductor integrated circuits (ICs) are ultimately packaged by encapsulation, typically using transfer molding. Because of sheer volume – tens of billions of ICs produced each year – molding is the process of choice because it is the most cost-effective method for certain types of packages. However, liquid encapsulants are gaining acceptance for use in encapsulating advanced IC packages on rigid and flexible organic substrates. These applications include dam-and-fill applications in ball grid array (BGA), chip scale and chip array packages. As the market heads in the direction of fine-pitch interconnects (dual in-line package and pin grid array –> leadless ceramic chip carrier and plastic leaded chip carrier –> small-outline IC and quad flat pack –> BGA and chip scale package [CSP]), challenges have arisen in the ability to provide true production-capable assembly and packaging tools. Advanced IC packages (high I/O, high speed, high performance) also are being driven to achieve greater cost effectiveness (at less than one cent per I/O). One of the factors that affects cost is the overall performance of packaging and assembly equipment used to produce these packages. Equipment automation, flexibility, precision, fine-vision capability and cost of ownership are key factors that determine the viability and success of the introduction of newer technologies.
Limitations of Molding
Transfer molding has been used for a long time in high-volume, low-cost encapsulation and is therefore well-established as the process of choice for applications such as memory devices (dynamic random-access memory, static random-access memory, read-only memory, erasable programmable read-only memory and flash) and logic devices. However, for relatively low-volume, high-cost devices, such as microprocessors and application-specific ICs, where lot sizes are small because of smaller volumes, dispensing is a viable option. Molding has its limitations in the following areas:
 Figure 2. A dual-chamber LPD pump. |
Batch/lot size: The limitations of large batch volumes associated with transfer molding are addressed by continuous in-line dispense processes for low-volume runners.
Design changes: Because device designs of these products are changing constantly and rapidly, transfer molding is not as economically feasible and convenient as dispensing for low-volume production.
Product life cycles: Product life cycles are typically less than 12 months on most of these products. The transfer-molding costs associated with custom (product-specific) molds, combustion kits and mold refurbishing add up substantially with fast changes in product designs and product life cycles.
Wire-sweep issues: Dispensing (single part, continuous processing) may not be as fast as transfer molding (multiple part, batch processing). Nevertheless, it has distinct advantages for specific applications that are sensitive to the molding process, especially for fine-pitch, wire-bonded devices. The nature of molten epoxy flow (during the molding process) over fine interconnect wires results in wire sweep on rigid organic BGA packages. This causes shorting and subsequent package failure. Properly designed epoxy materials are available that are capable of flowing through the fine interstices between wires without wire sweep, typically 8 to 10 mil spaces between wires. The dispenser is capable of controlling the way epoxy flows by configuring different fill programs – one for flow around the wires and another for flow on the chip.
Market and Technology Drivers
With chip integration increasing at an alarming rate, the market is faced with challenges to handle fine-pitch interconnects. Although some are very skeptical that flip chip will be the ultimate method of interconnecting chip-to-board, experts agree that wire bonding will still dominate for at least the next decade. As long as wire bonding dominates, molding or liquid encapsulation of these chips will need to be addressed.
 Figure 3. Pump performance data. |
Current technology for chip-to-substrate interconnection can be broadly classified into three segments:
- a) Fine-pitch wire bonding to handle high I/O count while maintaining acceptable die size. Packaged in BGA rigid or flexible organic substrate æ dispensing is a more viable option.
b) High-volume, low I/O lead frame packages where transfer molding is preferred. - a) Flip-chip-in-package for very high I/O capability (e.g., high density interconnects) where wire bonds cannot be used because of the large number of interconnections placed in an area-array format. Packaged in BGA rigid or flexible organic substrate æ underfill is dispensed in the chip-to-substrate gap.
b) FC on board (DCA) for lower I/Os where form-factor contribution is critical because of size and weight requirements of the final product into which the chip goes. Underfill is dispensed in the chip-to-substrate gap. - TAB packaged in BGA and encapsulated; dispensing may be a viable option.
New technologies are emerging, such as stacked chips, which use two ICs stacked one on top of the other in several configurations. Wire bonding or combinations of wire bonding and flip chip can achieve the interconnections from chip-to-substrate. Some of these are currently being used in production.
Conventional Technology: Molding
Molding has been used for decades as an inexpensive way to encapsulate ICs for mechanical and environmental protection (Figure 1). Almost 90 percent of today's ICs are packaged using the transfer-molding process. The majority are mounted on metal lead frames and others are mounted on rigid and flexible organic substrates. Molding is a batch process that involves the loading of substrates (with ICs) in molds, then placing them in the mold press. Runners (or plungers) carry the heated molten epoxy and channel it over parts to be molded. The entire process cycle of pre-heat and epoxy transfer occurs in 100 to 120 seconds. The throughput of a mold press depends on the number of strips that are loaded into the press. For example, a four-strip mold will give twice the throughput compared with a two-strip mold. Selecting the number of strip molds depends on several factors, such as the customer's production setup, product mix and capacity.
Advantages:
- Throughput increases linearly with increasing strip molds (two- to four- to six- to eight-strip molds)
- No wire-sweep issues when two- or four-strip molds are used
- Short process cycle (pre-heat and transfer is about 120 seconds and is independent of batch size).
Disadvantages:
- Many plungers (runners) per mold (more parts per strip, more the number of plungers)
- Larger number of strip molds results in more plungers, longer travel distance for the epoxy melt to get to the parts, and greater wire-sweep effect
- Batch process
- High capital cost ($400,000 to $1,000,000)
- High consumable cost (two-strip mold is about $40,000 to $50,000)
- High annual mold rework cost (30 percent of mold cost, i.e., $12,000 to $15,000 per year)
- Large footprint (25 to 30 square feet)
- Large material wastage
- High maintenance costs.
Conventional Technology: Dispensing
Conventional methods of dispensing polymer materials use rotary or single-chamber linear positive displacement (LPD) pumps. These have limitations when used in high-throughput applications, such as IC encapsulation (area array CSP encapsulation, BGA dam-and-fill or cavity fill), board encapsulation, flip-chip underfill and die coating. Recent advances in the chemistries of these materials have mandated the need for specially designed pumps capable of dispensing these materials, specifically IC encapsulants. Material calibration frequency for these pumps is very high. Each calibration and subsequent weight/volume adjustment can take several minutes, thereby limiting the ability of these pumps to deliver high throughput.
A New Dimension in Dispensing
The desired end result for various dispense applications is customer-specific and dependent on numerous parameters, including quantity and height of material dispensed, material rheology (viscosity, filler, etc.), needle size and speed, and wire pitch. Pumps that dispense micro-quantities of liquid epoxies are evaluated based on their ability to accurately and precisely deliver the material at the flow rates needed for today's production.
 Figure 4. Dispensing vs. molding for fine-pitch advanced packages. |
Rotary pumps are typically used for dispensing liquids where accuracy is not critical and high flow rate is not required. Dispensing a high-viscosity, high-thixotropy dam in a BGA dam-and-fill application is a good example. This pump is robust enough to handle extremely viscous liquids. A hardened screw and body are used in such cases. For low- to very-low-viscosity liquids, a shut-off valve may be incorporated in the design to eliminate leakage. These pumps also can be used where small amounts of high-viscosity material need to be dispensed at a high rate, such as for dot dispense. Rotary pumps have the advantage of lower cost, ease of operation and low maintenance, and are less complex than linear pumps. Disadvantages, on the other hand, are lower volumetric accuracy, lower flow rates and high leakage probability for very-low-viscosity fluids.
LPD pumps are used where high volumetric accuracy or high flow rate are desired. These also can be used for a wide range of fluid viscosity. These expensive, and often complex, pumps can be more elaborate to maintain, depending on individual design.
A new innovation in pump technology, the dual-chamber LPD pump (Figure 2), offers true advantages that are associated with high-volume manufacturing. It is used in production where high volume (3,000 milligrams), high speed (> 300 milligrams per second), high accuracy (> 99 percent, 3s and precise dispense repeatability are required for specific applications, such as IC encapsulation in BGA dam-and-fill, BGA cavity fill, chip array and CSP encapsulation on rigid organic substrates.
 Figure 5. Dual LPD pump process capability data. |
The pump design provides continuous dispensing, such that the material is available at all times at its point of use. As a result, dispensing is nonstop until the cartridge empties out. It incorporates matched dual chambers, which allows the material to be delivered in very precise pre-programmed amounts, regardless of material viscosity. Dispense and recharge occurs simultaneously; when one chamber is dispensing, the other is recharging, resulting in zero refill time. Traditional single-chamber LPD pumps, on the other hand, have a material recharge time immediately following each dispense shot.
Interchangeable chamber sizes for application-specific needs offer flexibility to the user. The chambers require minimal maintenance – just periodic cleaning that generally takes less than 10 minutes.
 Figure 6. The combination of dual LPD pump design and dual conveyors delivers the highest throughput. |
Process control is a critical element in dispensing liquid epoxy materials because several variations could occur in material rheology that could affect the overall process. One such variable is material viscosity over time. The pump has to compensate for these variations, through closed-loop-control feedback, by checking the specified (programmed) dispense amount against the actual dispense amount. Pump-performance data and process capability for a typical fill encapsulation using the dual-chamber LPD pump are shown in Figures 3, 4 and 5.
Dual conveyors provide parallel processing for continuous and uninterrupted flow. As substrates are being pre-heated on one conveyor, material is being dispensed on the other conveyor that is already pre-heated. Continuous processing allows the dispenser to be in-line with upstream and downstream process equipment, as well as integrate with other equipment through a central robot. The combination of dual LPD pump design and dual conveyors delivers the highest throughput (Figure 6).
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Low-stress materials, improved rheology for better flow properties and new filler technology have enabled material suppliers to provide formulations that are easy to handle and quick to process. This has resulted in materials that have smooth flow through fine-wire spaces with no wire sweeps.
Summary
Dispensing is a viable option for specific applications – particularly for dam-and-fill encapsulation in fine-pitch advanced BGA packages. The benefits are many. A dual chamber driven by an LPD pump provides continuous dispensing for maximum equipment usage for high-throughput applications; true, precise and repeatable volumetric dispensing with best process capability; and interchangeable chamber sizes for application-specific needs. Parallel process conveyors eliminate lost time because of substrate pre-heat and post-heat, and substrate load and unload.
A large material reservoir additionally reduces material change frequency, improving material use and system downtime; up to 32-oz cartridge handling capability achieves significant reduction in material cost. Also, in-line configuration allows for equipment integration, and the process achieves better price-to-performance ratios for fine-pitch dispense applications.
References
- “The McClean Report,” IC Insights, 1999.
- Jennie S. Hwang, “What Else Can We Expect in 1999?,” Part 2 of a 3-part column series, SMT, January 1999.
PRASAD NEVREKAR has over 15 years of sales and marketing experience in equipment/materials for the semiconductor assembly and packaging industry. He has held positions with Advanced Polymer Solutions, Alpha Microelectronics Packaging Materials and MRSI.
DAN CROWLEY is vice president of sales at MRSI and has over 18 years experience in sales and marketing of capital in the semiconductor and electronic packaging markets, specializing in test, dispense and assembly. For more information, contact Dan Crowley at MRSI, 25 Industrial Avenue, Chelmsford, MA 01824; 978-256-4950; Fax: 978-256-5120; E-mail: [email protected].