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Unique combination of properties

The benefits of silicon-based materials in chip scale packaging
Unique combination of properties

Silicone is frequently used in the electronics industry as a generic term for a diverse family of silicon-based polymeric compounds, consisting of an alternating silicon-oxygen (i.e., siloxane) backbone with methyl substituents attached to the silicon to modify the polymer’s properties. In packaging applications, most formulations are based on PDMS (polydimethylsiloxane) which is currently used for adhesives, spacers, encapsulants and protective coatings. Have a look into physical properties, benefits, reliability and test results for CSP applications.

Silicones have a combination of properties which make them uniquely well suited to chip scale packaging, and their use can bring significant advancements in device reliability and longevity. These features include: flexibility/stress relief, thermal stability, moisture resistance, good electrical properties, low ionic impurity levels, low alpha particle emission, gas permeability and good compatibility with common substrates and processing techniques.

By improving performance and reducing failures of the IC package, silicone materials help satisfy the need for smaller, lighter and more reliable components. This in turn permits the development of smaller, faster-performing consumer and industrial electronics. Microelectronic gradesilicones are currently being used in the commercial production of Flash mem-ory devices, SRAM and DRAM chips. These low-outgassing formulations deliver JEDEC Level I moisture resistance, proven thermal cycling reliability, and die protection from alpha particle emissions released by solder. They can be designed into a reworkable package requiring no underfill, and having good mechanicalvibration dampin properties.
Moisture resistance
An important physical property offered by silicone materials is moisture resistance. Their hydrophobic nature and low ionic impurity mean they do not easily absorb water molecules, and their gas permeability allows any potentially trapped moisture vapor to rapidly escape, thus eliminat-ing a potential source of popcorning. Inaddition, the very low surface tensionand excellent wetting characteristics of PDMS helps create a void-free interface that contributes further to overall reliability. Testing of gold plated, copper-leadedCSPs has indicated that the reliability of these materials will survive Condition C thermal cycling (-65 to +150°C). Chip manufacturers using silicones have qualified their devices under JEDEC Level I moisture sensitivity standards, 85°C/85%RH bias testing and pressure cooker tests. As shown in figure 1, all four package sizes demonstrated good performance after 1000 hours and longer, which serves as an indicator of their reliability in Flash memory, SRAM, DRAM and other devices. The chips were constructed using silicone die attach adhesives and a silicone encapsulant.
Dielectric performance
Among the unique properties recognized in silicone materials when they were first used in electronics applications more than 30 years ago were low, stable dissipation factor values and permittivity under varying temperatures and frequencies. The low intermolecular forces of silicone change little over a wide temperature range, resulting in stable physical and electrical properties. Most silicones act as very good insulators, and the non-polar nature of the methyl groups in a PDMS molecule contributes to that. Volume resistivity values of silicone CSP materials are typically around 2 x 1015 Ohms/centimeter.
Figure 2 illustrates the dielectric constant and dissipation factor of a specific encapsulant, as an example of silicone’s electri-cal properties. The dielectric properties of most materials will degrade to some extent over time when subjected to high temperatures and frequencies, but the stability of silicone formulations helps ensure that the properties remain in an acceptable range throughout the life of the device.
Alpha emission
One performance feature which has been required of chip scale packaging materials is low alpha particle emissions, to prevent soft data errors in the chip, a particularly critical issue in DRAM applications. The main sources of these alpha emissions are two radioactive elements, uranium (U) and thorium (Th), which are naturally present in trace amounts in almost all materials. In DRAMs, the occurrence of soft errors is directly related to the number of bits per device. For large memories, even low levels of alpha particle radiation can create unacceptably high soft error rates.
Actual measurements of uranium and thorium particles can be used to calculate theoretical emission rates. The newest generation of silicone CSP materials has demonstrated an emission flux density of 4.4 x 10-5 alpha particles per hour, using a sample area of one square centimeter measuring 125 microns thick. To put the emission flux testing in perspective, this means only one alpha particle is emitted from the material approximately every 2.6 years, exceeding industry requirements by orders of magnitude.
These silicone materials also function as a shield from external sources of alpha particles in an IC environment, such as the lead in solder balls. This ability is a result of the high content of atoms heavier than carbon in silicon-based elastomers and composites. The „stopping power“ of a material – the energy loss of alpha particles per unit of distance as a function of their energy – increases with atomic number.
Compliancy
Latest CSP components are more susceptible to warping, cracking and delamination than previous devices. A typical property range for silicone and epoxy materials is provided in figure 3. The high elongation and low modulus of silicones compensate for CTE mismatches, preventing the stress from being transferred to the die and helping to prevent delamination during temperature cycling.
Silicone materials have proven critical in improving solder ball reliability, since the flexibility of the material relieves the stress of mismatched coefficients of thermal expansion (CTE) between the silicon die and the epoxy circuit board. Packages manufactured with silicone encapsulants and die attach adhesives in either a pad or an array of stenciled nubbins have been proven effective. Low modulus is also important in minimizing stresses in CSP designs. As shown in figure 4, silicone elastomers deliver very low modulus over a wide temperature range. Current material formulations show no transition in the operating temperature range of the devices, so the modulus remains fairly constant throughout that span. Also shown in the graph is one example of a flexible epoxy used in these applications, which has a glass transition (Tg) just below room temperature. This Tg results in a modulus increase of more than three orders of magnitude.
Rheology
One of the key properties for chip scale encapsulants is flow, and the rheology of silicon-based products can be modified from paste-like thixotropic materials to low-viscosity fluids. The primary material variables that affect flow time are viscosity and surface tension. PDMS has a constant surface tension of 22.6mN/m, which is significantly lower than organic polymers and results in improved flow properties. Viscosity, however, is a property which canbe manipulated by controlling certainformulation variables, permitting theformulation of a wide range of material viscosities.
Cure technology
While many different cure chemistries are available to give silicones the required performance for chip scale packaging, addition cure formulations are the most common. The crosslinking is rapid under high temperatures, with test samples heated to 150°C curing in less than three minutes. The addition cure system has several other advantages, including the absence of by-products, the ability to accelerate cure with heat and the availability of 1 or 2-part formulations. Addition cure chemistry also allows production of microelectronic grade materials which eliminate potential contamination and subsequent lead bonding issues caused by silicone migration or outgassing.
The use of non-microelectronic grade silicones in the initial development of chip scale packages showed evidence of silicone contamination on the leads during bonding. However, materials engineers have identified the root cause of silicone outgassing and migration, and current Dow Corning microelectronic grade formulations eliminate the low molecular weight siloxanes that were responsible. Successful removal of the volatile species has been confirmed via gas chromatography, field testing and manufacturing experience. Some of the next-generation low outgassing materials have been used in production for more than a year without report of contamination problems.
Outgassing and migration
One of the requirements for current CSP designs is for a silicone material to be stencil-printed in close proximity to wire-lead bonds. The small distance from the posts or nubbins to the lead bond window and gold-coated copper leads initially posed a challenge, as non-microelectronic grade silicones tended to migrate or creep. The resulting contamination on the wire-lead bond contributed to deformation and heel cracking during thermal cycle testing of the CSP. The subsequent development of microelectronic grade silicones has resolved migration issues by drastically reducing the volatiles content in the formulations, eliminating potential contamin-ation as shown in surface analysis techniques.
Well-suited for packaging
As the industry moves further into chip scale and wafer scale packages, the inherent properties of all materials will be re-examined. Silicones are key enabling materials for high reliability in CSPs, with ex-cellent electrical insulating capability, physical properties and processing characteristics for commercial mass production. Current trends of no-lead solders, higher temperatures and frequencies, and small-er/thinner packages bring new materials challenges to component manufacturers. Silicones offer a unique combination of properties that make them particularly well suited for device packaging appli-cations.
Dow Corning has been a world leader in silicon-based technology since the company’s inception in 1943. In electronics, its technology can be found throughout the manufacturing process, for example, with hyper-pure silicon for wafer fabrication, low-k dielectrics for semicon-ductors, microelectronics grade adhesives, encapsulants and coatings for IC packaging, and board level adhesives, conformal coatings and encapsulants. With facilities in North and South America, Europe, Australia, Asia and the Middle East, the company is a truly global supplier dedicated to customer service and product excellence in all parts of the world.
EPP 186
Zusammenfassung
In der Mikroelektronik sind Materialien auf Siliconbasis immer wichtiger geworden. Speziell im Halbleiterpackaging setzt man solche Formulierungen als Klebstoffe, Vergußmassen und Schutzüberzüge ein. Wir gehen auf die wesentlichen physikalischen Eigenschaften ein, die Vorteile sowie Testergebnisse und Zuverlässigkeit.
Résumé
Les matières à base de silicone occupent une place de plus en plus importante dans la microélectronique. Ces formules composées de colles, de produits de scellement et de revêtements protecteurs sont utilisées en particulier dans la fabrication de semi-conducteurs. Les principales propriétés physiques, les avantages de même que les résultats des tests et la fiabilité sont présentés.
Sommario
Nella microelettronica i materiali su base siliconica sono diventati sempre più importanti. Materiali simili vengono utilizzati come collanti, come masse fuse e come rivestimenti protettivi in particolar modo nel packaging di semiconduttori. Trattiamo più approfonditamente le sostanziali proprietà fisiche, i loro vantaggi, i risultati dei test e la loro affidabilità.
Part II (will be published in EPP Europe # 11/12) of this three-fold series on packaging materials deals with the application of silicone-based substances in this area. Part III (will be published in EPP Europe #1/2-2002) covers the issues of cost of ownership in chip-scale packaging.
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