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Polymers for encapsulation: Materials Processes and Reliability
PolyurethanesPolyurethane was first made available by Otto Bayer in the late 1920's in Germany. The early study of polyurethane was simply based on di-isocyanates and diols or polyols. However, recent work has bee focused on the use of intermediates which are low molecular weight polyethers with reactive functional groups such as hydroxyl or isocyanate group able to further crosslink, chain extend or branch with other chain extenders to become higher molecular weigh polyurethanes.
Diamine and diol are chain extended with the prepolymer (either polyester or polyether) to form polyurethanes wit urea or urethane linkages, respective! The morphology of polyurethane is well characterized. Hard and soft segment from disocyanates and polyols, respectively, are the key to excellent physical properties of this material.
Polyurethane Polymerization- Bases are more widely used than acids as catalysts for polyurethane polymerization. The catalytic activity increases with the basicity. Amines such as tertiary alkylamines and organic metal salts, such as tin or lead octoates, promote the reaction of isocyanate and hydroxyl functional groups in the polyurethane system and accelerate the crosslinking. However, the hydrolytic stability of the polyurethane can be affected by the catalyst used.
UV stabilizers are usually added to reduce the radiation sensitivity of the material. In addition, polyurethane has unique high strength, high modulus, high hardness and high elongation. It is one of the toughest elastomers used today. High performance polyurethane elastomers are used in conformal coating, potting and in reactive injection molding of IC devices.
PolyimidesThe polyimides are one of the fastest growing categories of electronic polymers. They were first developed at DuPont in the 1950's. During the past 20 years, there has been tremendous interest in this material for electronic applications.
The superior thermal (stability up to 500 °C), mechanical and electrical properties of polyimide have made its use possible in many high performance applications, from aerospace to microelectronics. In addition, polyimides show very low electrical leakage in surface or bulk. They form excellent interlayer dielectric insulators and also provide excellent step coverage which is very important in fabrication of the multilayer IC structures.
Polyimides also offer excellent solvent resistance and ease of application. Most polyimides are crosslinked products of aromatic diamine and dianhydride. However, by changing the diamine and dianhydride substitutes, one will derive a variety of high performance polyimides.
Polyimide Composition-Siemens of Germany developed the first photodefinable polyimide material. However, Ciba Geigy has a new type of photodefinable polyimide which does not require a photoinitator. Both of these photodefinable materials are negative resist type polyimides. A positive resist type polyimide, which reduces the processing steps in IC fabrication, has recently been reported by Sumitomo Bakelite, Hitachi Chemical and other suppliers.
An interpenetration network (IPN) of two types of polyimides is used to achieve the positive-tone material. Hitachi has developed an ultra-low coefficient of thermal expansion (CTE) polyimide which has some potential in reducing the thermal stress of the silicon chip and the polyimide encapsulant. The rigid rod-like structure of the polyimide backbone structure is the key to preparing the low CTE polyimide.
By simply blending a high and low CTE polyimide, one will be able to achieve a desirable CTE encapsulant which will match the CTE on the substrate and reduce the thermal stress problem in encapsulated device temperature cycling testing.
The affinity for moisture absorption due to the carbonyl polar groups of the polyimide, a high temperature cure and the high cost of the polyimides are the only drawbacks that prevent their use in low cost consumer electronics applications.
Preimidized Polyimides-These polyimides cure by evaporating solvent and may reduce the drawbacks common to a high temperature cure of the material. Advances in polyimide syntheses have reduced both the material's moisture absorption and dielectric constant by incorporating siloxane segments into the polyimide backbone.
Silicone-Polyimide (New, Modified Polyimides)-Combining the low modulus of the siloxane, and the high thermal stability of polyimide, the siloxane-polyimide (SPI) copolymers were first developed at General Electric. SPI copolymers have become very attractive IC device encapsulants.
Silicone-polyimides are fully imidized copolymers and are soluble in low-boiling solvents, such as diglyme, which reduces the high processing temperature and eliminates the outgassing of water during the normal polyimide imidization (cure) process. The high processing temperature and outgassing of water are the polyimides' main drawbacks. The SPIs feature good adhesion to many materials, and eliminate the need for an adhesion promoter. These materials have potential as IC device encapsulants, interlayer dielectrics and for passivation in microelectronic applications.
ParylenesParylene, (polypara-xylylene), was first developed by Union Carbide Corp. The process uses a thermal reactor to first vaporize (at 150 °C, 1 torr pressure), and pyrolyze (at 680 °C, 0.5 torr pressure) the di-para-xylene then polymerizes the dimer into polymer at room temperature. This room temperature deposition is a very attractive encapsulation process, especially for temperature sensitive, low glass transition substrate materials. Parylene deposition provides an excellent conformal step-coverage and conformal film with thickness ranging from 2 µm to 50 µm.
The high cost of the starting dimer and the deposition equipment may prohibit its widespread use in consumer electronic applications. Nevertheless, it is a unique conformal coating material with potential coating applications.
BenzocyclobutenesThe high performance benzocyclobutene (BCB) polymers were recently developed by Dow Chemical Co. The crosslinking process is carried out by the thermal rearrangement of the dicyclobutyl monomer to form the reactive intermediate orthoquinodimethane, which can polymerize with the unsaturated functional group (Figure 3).
Figure 3-Synthesis of benzocyclobutene.
Since it is based on the thermal rearrangement process, BCB requires no catalyst and there are no byproducts created during the curing process. BCB has excellent physical, chemical and electrical properties which make it suitable for use in microelectronics applications, similar to polyimides. With its low dielectric constant (2.7), low moisture absorption (<1%) and good adhesion properties, BCB offers potential as an IC passivating encapsulant and as interlayer dielectrics for multichip module applications.
Sycar (Silicon-Carbon) Polymers- Hercules (Wilmington, DE) recently developed Sycar, a new class of siliconcarbon materials (Figure 4). This class of material consists of a backbone of the siloxane (-Si-O-Si-O-) structure and cross-links the silicon backbones with hydrocarbons. These hydrocarbons provide excellent mechanical and solvent properties, yet maintain the silicone-like electrical properties.
Bis-Maleimide TriazineBT (bis-maleimide triazine) is a new resin used instead of FR-4 to prepare PWBs. The triazine polymer is produced mainly by Mitsubishi Chemical Co. in Japan.
PolycyclicolefinsBF Goodrich recently developed a new class of cyclicolefins based on the principle of polynorbornene chemistry. A transition metal catalyst is used to provide a tightly-controlled polymerization of the monomers to saturated polymers with excellent Tg (>35 °C), a low dielectric constant (2.45), low moisture (<0.1) and a low CTE (50ppm/°C). All the drawbacks of polyimide may be alleviated by this new material.
Furthermore, the material has isotropic physical properties in x, y and z directions which are lacking in other high performance materials such as polyimides, BCD, etc. BF Goodrich is marketing these materials under the trade name of Avatrel (Table 2).
No-Flow UnderfillsUnderfill encapsulants are polymeric materials used to reduce the shear stress of the solder joints between the chip and the substrate generated by thermal mismatch.
The encapsulant not only provides dramatic fatigue life enhancement with minimal impact on the manufacturing process flow, but also extends its use to a variety of organic and inorganic substrate materials resulting in a 10-100X improvement in fatigue life compared to an unencapsulated package. 1, 2
Therefore, the underfill encapsulation has been key to the development of flipchip DCA technology.
Off-Chip Packaging TechniquesOff-chip packaging and encapsulation techniques consist mainly of cavity-filling, saturation and coating. These are described as follows.
Cavity-filling processes consists of molding, potting and casting.
MoldingMolding is the most cost-effective and high performance way to package ICs in plastic. It involves injecting a polymeric resin (one of the thermosetting molding compounds) into a mold and then curing
The process involves the following steps: (1) The molding compound is preheated until it melts and the resin flows through runners, gates and finally fills up the cavities. (2) The resin is then cured and released from the mold to predetermined shapes. The exact control of the mold pressure, viscosity of the molten molding compound, and the delicate balance of runners, gates and cavity designs is very critical for optimizing the molded plastic.
Figure 4 - Sycar resin structure.
Since the shear stress of the IC's molded component can cause wire bond sweep, device passivation cracks, top-layer metallization deformation and multilayer oxide and nitride cracks, improved molding compounds and processes can eliminate the damage to the molded IC devices. Transfer, injection and conformal moldings are some of the current molding processes. With the new advances of low stress molding compounds, techniques such as the new transfer molding, aperture plate molding, and reactive injection molding are in production use and provide economic ways to encapsulate and package ICs.
PottingPotting is the simplest way to encapsulate devices. It involves placing the electronic component within a container, filling the container with a liquid resin and then curing the material as an integral part of the component.
Polymeric resins (epoxies, silicones, polyurethanes, etc.) are usually used as potting materials. Containers such as metal cans or rugged polymeric casings made from high performance engineering thermoplastic polymers to enhance the effectiveness of the encapsulant.
The adhesion between the potted material and the casing is essential, however, in achieving a long-lasting reliable package. Rugged, machine insertable components, such as surface mounted chip carriers, DIPs, SIPs, molded and potted packages and discrete components are highly desirable components for the automated manufacturing processes.
CastingCasting is similar to potting, except the outer case is removed after the polymer cavity-filling process and curing are completed. No heat or pressure is applied in the process. However, this labor-intensive casting process is not commonly used compared to potting.
Saturation Coating ProcessesSaturation coatings consist of impregnation, dip, conformal and surface coatings. Impregnation coating is performed by the saturation of a low-viscosity resin to the component which also includes a thin film coated on the component surface. This process is usually used with a cavity filling or conformal-coating process.
Dip Coating-Dip coating is performed by dipping the component into an encapsulating resin. The component is then withdrawn, dried and cured. Coating thickness is usually a function of resin viscosity, withdrawal rate and coating speed. This process also depends on the resin reactivity, curing rate and curing temperature. The dip-coating process is widely used in glass-laminated printed circuit board and optical fiber coatings.
Conformal and Surface Coatings- Conformal and surface coatings are common techniques used in IC device encapsulation. They include spin coating, spray coating and flow dispensing of the encapsulant onto the component.
Suitable rheological properties of the encapsulant such as dynamic viscosity, yield stress, storage modulus and loss modulus are critical in obtaining a good flow dispense, especially in hybrid IC encapsulation, where the encapsulant tends to runover from the substrate and wick onto the leads of the hybrid devices. Surface-mounted components on PWBs are routinely encapsulated by the conformal coating process.
ConclusionsThe rapid development of IC technology has created a critical need for advanced polymeric materials employed as device interlayer dielectrics, passivation layers, encapsulants and packages.
Recent advances in high performance polymers, such as improved silicone elastomers and ultra-soft silicone gels, low stress epoxies, high performance flip-chip underfills and low CTE and photo-definable polyimides have provided polymers which are compatible with the deep submicron VLSIC and USLIC technologies.
Dr. Wong is a professor in the School of Materials Science and Engineering and research director of the Packaging Research Center at Georgia Tech and is also on the Editorial Advisory Board of Chip Scale Review. He can be reached at 404. 894. 8391, fax 404. 894. 9140, firstname.lastname@example.org
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