Innovations in IC Packaging Adhesives
08/01/2005
UV B-Stage Technology Provides Process & Performance Advantages
BY MICHAEL KROPP AND ANDREW BEHR
The demand for increased product performance, along with reduced manufacturing costs drives IC packagers to search continuously for new materials. In recent years, much attention has been drawn to advancements in adhesives as a path to achieve both goals. IC package manufacturers use adhesives for a wide range of applications, including die, stiffener, heatsink, and substrate bonding. Polymer systems for organic adhesives can be divided into three broad categories: thermosetting, thermoplastic, and elastomeric. Historically, materials based on thermosetting polymers are most common for IC packaging applications. Among thermoset adhesives, film and paste delivery formats are widely used, but both present challenges. A new adhesive technology* has been developed that can be described as a UV B-stage, printable paste adhesive. A secondary thermal cure cycle yields fully crosslinked, void-free bonds. This adhesive is a 100% solid, thermosetting paste. The adhesive may be printed by either screen or stencil to enable high-volume manufacturing. UV B-staging immediately after printing “freezes” the adhesives in position, enabling a smaller package footprint design. The cured adhesive provides high bond strength and is moisture resistant, stable at high temperatures, and has demonstrated JEDEC Level 3 (260°C) reliability in commercial packaging applications. This adhesive technology has been used successfully in a number of packaging applications, including die and stiffener attaching.
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To gain a full understanding of the advantages of the new technology, it’s helpful to first take a look at existing paste and film adhesives. Paste adhesives are available in a variety of chemistries, but thermosetting materials are most common. Most commercially available thermoset adhesives contain solvents to reduce viscosity to a dispensable range. Bonding and thermal curing are performed immediately after dispensing. Proper thermal curing removes the carrier solvent from the adhesive matrix and subsequently crosslinks the resin, building both interfacial adhesion and cohesive strength. Because they flow, pastes tend to wet substrates well. However, a major disadvantage is the tendency to flow beyond the original application site.
A specialized subset of the solvent-borne thermosetting paste adhesives, commonly known as thermally B-stageable adhesives, has recently garnered attention. After dispensing, the adhesive is exposed to a specified thermal regime designed to evolve a majority of the solvent from the material without significantly advancing resin crosslinking. B-staging an adhesive permits the adhesive and substrate construction to be “staged,” or held for a period of time prior to the bonding and curing, without forfeiting performance. B-staging can significantly reduce the process that the bottlenecks experienced with dispensing and bonding traditional thermosetting pastes.
Film adhesives, on the other hand, are generally “preformed” (or B-staged), and are available in rolls, sheets, or die-cut shapes. Film adhesives enable a smaller adhesive footprint, but may have difficulty wetting substrates with topography.
Process Flow and Cost Comparison
Dispensing and curing often contribute significantly to the total cost associated with using paste adhesives. Paste adhesives generally require sequential dispensing, in which a substrate containing multiple bond locations is indexed under the dispensing head. Despite improvements in dispensing technology, this process remains a significant bottleneck. Some film adhesives have an advantage over paste adhesives in that they may be applied in batch- or roll-type processes that are conducive to high-volume manufacturing. However, the application of single die-cut or preform film adhesives can be more problematic. Printing is a cost-effective alternative method of applying paste adhesives that can provide significant increase in throughput. The rheology of the new adhesive has been optimized for conventional screen and stencil printing processes. Screen and stencil printing equipment is readily available and more cost-effective for high-volume manufacturing than syringe adhesive dispensers.
Bonding with traditional paste adhesives needs to be performed immediately after the adhesive is dispensed, necessitating an in-line dispensing and bonding work flow. With solvent-borne paste adhesives, disruptions in either the dispensing or bonding processes can have disastrous impact on yield and throughput. Substrates with UV B-staged adhesives may be stored for up to 4 months, reducing the threat of disruptions to the dispensing and bonding workflow. The new adhesive has the potential to allow substrate manufacturer to supply substrates with a pre-applied adhesive.
Curing of adhesives can add significant cost to the manufacturing process. Some paste adhesives require a 2-step cure process to initially remove the solvent and subsequently cure the resin. This may require up to 3 hours of oven time. If the solvent is not sufficiently removed from the adhesive prior to the resin crosslinking, voiding in the adhesive bondline may occur. Control of solvent evolution from adhesive curing may also be a significant environmental, health, safety, and cost consideration for IC packaging facilities. The 100% solid formulation used in the UV B-stage adhesive eliminates the cost and concerns associated with the curing of solvent-borne pastes.
Performance Comparison
Because of their rheological properties, paste adhesives are subject to some degree of sag and slump after dispensing, meaning that they may flow beyond the original application site. Additional flow may be induced during the thermal curing process. Compensating for adhesive flow in die attach applications may increase the distance required between the die and the bond pads - negatively affecting the overall package footprint. Another well-documented problem that may occur with thermosetting paste adhesives is resin bleed-out. In this case, a portion of the resin separates from the adhesive matrix and flows out onto the surrounding substrate, sometimes contaminating the adjacent bond pads. When using thermally cured solvent-borne adhesives, IC packagers often must account for these issues in their designs. Edge roll-up or paste overflow is another concern with paste adhesives. In this case, the adhesive flows up and over the top of the substrate, resulting in the top of the daughter component becoming contaminated by the adhesive. Bondline thickness control is another issue for traditional paste adhesives. If the bond thickness varies across the plane of the bond substrate, inclination may occur. In die bonding, this is termed “die tilt.”
UV B-staging an adhesive reduces or eliminates all of the traditional paste issues. By using UV irradiation for the B-staging process, precise control is achieved and the adhesive is “frozen” in place immediately after application. Precisely designed rheology, combined with UV B-staging, virtually eliminates the sag, slump, and flow-out experienced with traditional dispensed paste adhesives. Precise bondline control may be achieved without the use of large spacers. Unlike thermal B-staging, irradiating with UV energy eliminates the danger of advancing the thermoset reaction of the adhesive. UV B-staging occurs in seconds, while the thermal alternative is an order of magnitude longer process.
Film adhesives have their own problems. One inherent characteristic of film adhesives is that they are in a semi-solid state when applied to a substrate. Because they are “preformed,” film adhesives tend to flow less than paste during the bonding process, and this can result in a lack of conformability. The challenge of completely filling substrate topography with film adhesives has been the subject of much development. Bonds formed with film adhesives may exhibit voiding if unable to fully wet the substrate. The UV B-stage adhesive provides superior wetting because it is applied as paste and can easily wet out uneven surfaces.
Generally, film-based adhesives are more expensive than comparable paste adhesives on a material-cost-per-unit basis. The film-forming process, die cutting, waste, and handling can add to the final cost. The UV B-stage paste can provide a significant material cost advantage vs. film adhesives.
Stiffener and Die Bonding
Laminated high-density multilayer substrates for flip chip and wire-bond packages provide high density and electrical performance optimized for ASIC vendors (Figure 1). These cavity-down substrates are generally bonded to a heat spreader or stiffener with an adhesive. This metallic plane serves as an area for die attach, increases the rigidity of the package, helps prevent warping during assembly, and creates a flat surface for mounting a heatsink. Film adhesives have been traditionally used for attaching the stiffener to the multilayer substrate. In our testing, a variety of experimental and commercial film adhesives were investigated, and thermoset/elastomer hybrid films had superior performance.1 Despite the performance advantages of this type of hybrid film adhesive, a printable paste was required to enable cost-effective, high-volume manufacturing.
 Figure 1. High-density laminate with heat spreader attached. |
The evaluations of screen printing a thermoset/elastomer hybrid adhesive for stiffener attachment revealed potential for a simplified manufacturing processes. These evaluations showed that a 10x increase in parts per day is potentially achievable when changing from a die-cut film to a screen-printable adhesive.
Eliminating the high cost of films and reducing the number of process steps can help achieve significant cost-per-part savings. Other evaluations suggest this adhesive technology would offer similar advantages for die-bonding applications. An industry leader in IC packaging recently demonstrated the feasibility of using the adhesive for die attach on organic substrate-based packages.
Adhesive Printing and UV B-staging
The substrates in stiffener attach applications had 5 or 7 layers, and were composed of alternating copper and dielectric. The outer surface is an epoxy-based soldermask. The stiffener used in these studies is copper with a black velvet oxide surface. For the die-bonding experiments, the die were 6 × 10 mm, and the substrates were BT with soldermask. The adhesive formulations were printed on the substrate with a standard screen printer. The screens used were polyester mesh, 110 lpi, and with 0.5-mil HP emulsion at 22.5°. The separation speed was less than or equal to 20 mm/s. Metal stencils were used for some printing (Figure 2).
 Figure 2. Typical emulsion screen for stiffener attach application. |
After printing, the adhesives were B-staged with UV irradiation using a high-intensity UV processor with D or H bulbs. Exposure dosages were varied and the samples were run both with and without nitrogen inerting.
The shear thinning and thixotropy of the adhesives were optimized to provide optimal print quality. The adhesive for die bonding is non-tacky after B-staging, while the stiffener attach was designed to have a high tack level. The flow of the adhesive during substrate attach and curing can be controlled by the gel content of the B-staged component and the viscosity of the thermoset. The melt viscosity of the adhesive of the B-staged adhesives during cure were characterized and optimized using DMA and gel content measurements. The die and stiffeners were bonded using industry-standard placement times and pressures. Multilayer substrates attach to a panel of stiffeners prior to final cure and singulation (Figure 3). Final cures were 125° to 150°C for 30 to 60 minutes in batch ovens.
 Figure 3. Typical print decal for die attach application. |
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Shelf Life
The shelf life of the adhesive was determined to be in excess of 4 months at room temperature in the paste form. Aged materials were tested for both printing and adhesion performance and determined to be unchanged from new material. Perhaps more important, the adhesive exhibited several months of shelf life after UV B-staging. After 4 months, there was no degradation in the DSC profile. This opens the possibility for adhesives to be printed at one facility and bonded at another, eliminating the need to have adhesive handling and processing co-located with stiffener or die attach process equipment.
Alternative Curing Lamps and Photoinitiators
The UV curing process must allow for cure with various lamps and inerting capabilities. Experiments were run to investigate these variables, as well as several photo-initiators. The motivation for these studies was to investigate curing systems amenable to large-volume, in-line processing.
The degree of cure of the B-staged component was determined by IR using overtone bands. This method allows for measurement through a sample thickness representative of the adhesive in use. Fast throughput UV B-staging using high-intensity lamp and initiator combinations have been demonstrated.
The dosage required for curing the non-tacky die attach adhesive was determined by varying the belt speed on the UV processor to achieve different dosages. The adhesive was examined for tack after exposure (Table 1).
 Table 1. The adhesive was examined for tack after exposure. |
It is also important to know if the conditions during the UV cure affect the thermoset cure. Lamps that are too hot could lead to premature cure or loss of latency for the thermoset component. Data from DSC analysis showed that UV B-staging does not affect the final thermoset cure to any significant degree, so die placement, wetting, and final cure will be consistent.
Bondline and Adhesion Characterization
Void-free bondlines were obtained using optimized printing and bonding conditions. Evaluations using C-SAM show that less than 5% void content can be achieved.
The adhesion of the new adhesive was found to be superior to commercial film adhesives, even after PCT exposure. Samples were examined for failure of the substrate (delamination) and cohesive failure modes. Bondlines after JEDEC 3 preconditioning showed no delamination or blistering (Table 2). Good results for die attach were also obtained by a major IC packaging company (Table 3).
 Table 2. Bondlines after JEDEC 3 preconditioning showed no delamination or blistering. |
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 Table 3. Results of a die attach test. |
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Summary
The pattern-printable, UV B-stage adhesive technology described offers significant cost, process, and performance advantages over other types of paste and film IC packaging adhesives. The potential benefits of employing UV B-stage adhesive technology for both stiffener and die bonding have been documented. This technology may have utility in wafer-level and other IC packaging applications where paste and film adhesives are traditionally employed.
*Trademarks and brand names shown may be the property of their respective owners.
Acknowledgements
Ken Zieminski, Tom Norton, and Chris Nelson; Dan Foster, Kevin Koehler, Les Whitcome, William Gibson, Brian Curtis, Chuck Driver, Michael Holcomb, Sandy Walter, Duy Le-Huu, Victor Jonas, Donald Banks, and John Reagan for application and process development.
Reference
- W. R. Schildgen, C. T. Murray, “Material Set Comparison in Moisture Sensitivity Classification of Nonhermetic Organic Packages,” 35th International Symposium on Microelectronic Proceedings, September 2002, p. 679-684.
MICHAEL KROPP, Ph.D., senior research chemist, and ANDREW BEHR, semiconductor materials market development manager, may be contacted at 3M Electronics Markets Materials Division, 3M Center, Building 220-7E-01, St. Paul, MN 55144-1000; 800/567-1639, 8065.