March 1998
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Silicone Materials for Chip-Scale Packaging
Device reliability benefits from the flexibility, thermal stability and moisture resistance of silicone CSP materials.By Ann M. Norris, Dow Corning, Midland, Michigan and Marlene E. Gladstone, Dow Corning, Fremont, California
Silicone materials possess a balance of properties which make them uniquely well suited to chip-scale-packaging applications, bringing significant advances in device reliability and longevity. This combination of features includes, flexibility/stress relief, thermal stability, moisture resistance, low ionic impurity levels, high permeability to gases, high overall purity and good compatibility.
The Tessera µBGA&3174; package design (Figure 1) requires a compliant layer as part of the construction, and a new product line of electronic grade silicones is an excellent fit, offering low glass transition temperature (Tg = -120 °C ) and low modulus.
The thermal stability and moisture resistance of these materials is a key to the enhanced reliability and service life of semiconductor devices. Their ability to protect chips and other electronic components from heat, moisture and contaminants also expands the number of applications in which they can perform. (See Table 1)
By improving performance and reducing failures in chipscale packaging, these silicone materials help satisfy the overall industry need for smaller, lighter and more reliable components. This, in turn, permits the development of smaller, lighter, faster-performing consumer and industrial electronics.
Commercial Formulations
Dow Corning has developed a family of products for this industry, including an encapsulant, structural spacer and wet die-attach adhesive. All three are one-part, solventless silicone elastomers, which together form an essential component of the µBGA package design.A pressure-sensitive, dry die-attach adhesive for affixing the die to the elastomer spacer is also under development. The spacer and die-attach materials offer controlled
outgassing. Together, these products have shown the potential to improve the temperature cycling reliability of the µBGA chip-scale package.
Dow Corning 6810 Chip Scale Encapsulant is a self-priming liquid that cures to a tough elastomer, and is used to seal and protect semiconductors and device leads from moisture, temperature and contaminants. The flexible encapsulant provides stress relief for delicate leads and solder connections, absorbing stresses from thermal expansion differences between components, leads, and substrates. It is chemically compatible with most device surfaces and creates an opaque barrier to UV light.
Dow Corning 6910 Chip Scale Spacer is a non-flowing paste used to form a structural spacer between the die and flex circuit. The cured elastomer absorbs stress from thermal cycling and decouples CTE mismatches between die and substrate. Like the encapsulant, black pigmenting of the spacer creates a protective barrier for UV-sensitive devices. A key feature is the formulation's controlled volatility, which minimizes lead contamination. This is critical to obtaining reliably high quality ultrasonic welding of the leads.
Figure 1 - Dow Corning's new electronic grade silicones were developed especially for Tessera's µBGA package.
In the die attach and spacer applications, non-electronic grade silicone materials were initially used, but they did not provide the required low outgassing and could actually contribute to package failure by contaminating wire leads prior to bonding. A number of sophisticated analytical techniques have been employed including SEM-EDX, ESCA, GPC, GC, TGA and IR spectroscopy to identify the specific chemical species responsible for the contamination, and they have been removed from the new product formulations.
Dow Corning 7910 Chip Scale Die Attach Adhesive is a nonflowing, thixotropic paste used to affix the die to the cured spacer. The spacer and die attach formulations deliver high purity, low mobile ion content, and low alpha-particle content, with low outgassing/controlled volatility to minimize the potential for contamination of leads.
These silicone products cure to tough, protective elastomers that remain flexible under a wide range of conditions, absorbing the stress from thermal contraction and expansion.
They show negligible physical property changes at high and low temperatures and deliver stable dielectric properties over a wide range of frequencies and temperatures (dielectric constant <3.2). The typical dissipation factor is <0.001.
Modulus vs. Temperature
Figure 2 illustrates the outstanding flexibility and thermal stability of two silicone formulations as compared to one type of flexible epoxy. The Tg of the epoxy increases its modulus by more than three orders of magnitude in the center of the operating temperature range, which would cause stress on the package during thermal cycling.
Figure 2 - Modulus vs. temperature (silicone=compliance and thermal stability)
In the graph, the typical silicone material is one in which the modulus increases at the melt point (Tm). In the enhanced silicone formulation, the melt point has been eliminated; therefore no thermal transitions occur in the operating range of -75 °C to +150 °C. Because stress is a function of modulus and CTE, the lower modulus of the silicone elastomers reduces stress on the components.
Temperature and Moisture Resistance
The thermal stability of these silicone materials is a significant advantage over organic products, allowing reliableoperation over a wider temperature range. The specially developed silicones remain flexible at both high and low temperatures and avoid stress on the package.
Unlike rigid materials, the formulations cure to pliable elastomers with the ability to relieve stress on components and substrates which have different CTEs. Testing of the encapsulant in the gold-plated, copper-leaded µBGA package has indicated that the reliability of the system will survive Condition C thermal cycling (-65 °C to +150 °C).
The unique moisture resistance of these materials can be characterized in three ways:
- Their hydrophobic nature means they don't absorb water easily and their electrical properties remain constant.
- Low surface tension allows the uncured silicone to fully wet-out the surface during application, creating an
interface that's free of voids. This prevents moisture from condensing in air gaps and causing corrosion or popcorning.
- The permeability of silicone allows any trapped moisture vapor to rapidly escape during processing, further reducing the chances of popcorning and loss of adhesion.
The relative moisture absorption of silicones is far lower than epoxy materials (Figure 3). Silicone's inherent moisture insensitivity significantly improves device reliability, allowing manufacturers to achieve JEDEC Level 1 moisture sensitivity standards.
Manufacturing/Processing
This new family of one-component materials requires no mixing, and the products have good self-priming adhesion. In addition, the solventless formulations do not shrink, and the excellent wetting of these silicone materials helps create a void-free interface and reduce the potential for moisture contamination. The spacer material is supplied frozen, and after thawing, can be applied with a spatula and stencil printed. The encapsulant, also supplied frozen, can be dispensed using conventional liquid dispensing methods.Epoxy materials for these applications tend to trap moisture during processing of a typical overcoated package. Depending on the process conditions, it may require anywhere from minutes to hours for the moisture to escape. In contrast, the permeability of the silicone polymer releases any trapped moisture vapor within seconds at the thickness used in this application.
Environmental Characteristics
Unlike products based on conventional solvents, these formulations contain no VOCs, ODCs, or HAPs, and they release no by-products during cure. Cure is achieved via an addition reaction upon exposure to heat. This is a neutral cure system that is adaptable to a wide range of processing conditions. By avoiding the use of solvents, these products help reduce emissions and contribute to a healthier environment.
| Property | Result | Benefit |
| Hydrophobic | Low Moisture Content | Preserves Dielectric Properities |
| Low Surface Tension | Void-Free Interface | No Corrosion |
| High Permeability | Rapid Vapor Escape | No Popcorning |
| Inherent Silicone Properties | Moisture Insensitivity | JEDEC Level 1 Reliability |
Figure 3 - Modulus vs. temperature (silicone = low moisture uptake)
Summary
The advantages of silicone materials for CSP applications include flexibility/stress relief, thermal stability, moisture resistance, low ionic impurity levels, high permeability to gases and high overall purity.The thermal and moisture protection of these materials is a key to enhanced reliability and service life of semiconductor devices. By improving performance and reducing failures in chip-scale packaging, these silicone materials help satisfy industry needs for smaller, lighter, faster and more reliable electronic components.
Ms. Gladstone is the project leader for chip-scale packaging at Dow Corning. She joined the company in 1985 after receiving a bachelor's degree in chemical engineering from Washington University, St. Louis, Missouri. In 1989, she relocated to the regional office in Fremont, California, and has been involved with chip-scale packaging since 1995. She may be contacted at 510.490.9302, fax 510.490.0656. Dr. Norris is the group leader for electronics product development at Dow Corning, Midland, Michigan. She joined Dow Corning in 1980 with a bachelor's degree in chemistry from the University of Wisconsin at LaCrosse. She later left the company to earn her Ph.D. in materials science from Virginia Polytechnic Institute and State University in 1987, rejoining the company after graduation. Contact her at 517.496.7047, fax 517.496.7084, usdcctx5@ibmmail. com.