APPLICATIONS TECHNOLOGY
 
Sealing elastomers in the semiconductor industry are used in hundreds of applications. Unique environmentsÛsuch as vacuum and plasma, and demanding requirements for contamination place great importance on the design phase of a seal application. 

This section covers traditional seal design concepts as well as specific considerations for the semiconductor industry.

 
 

SEALING APPLICATIONS
 
Sealing applications can be considered either static (no relative motion between the sealing surfaces), or dynamic (relative motion between the sealing surfaces). The shape of the gland is often selected based on the function of the components to be sealed or the available space for a seal. Trapezoidal glands are often used to retain the seal during assembly or maintenance. Additional information on compressive force, seal sticking, and installation and lubrication complete the ÏApplications TechnologyÓ section.
 
 

SEAL DESIGN & NONLINEAR FINITE ELEMENT ANALYSIS (FEA)
   
Seal Design Theory 

The use of an O-ring as a seal is mainly to prevent the transfer of fluid (liquid, solid or gas) between two or more regions. The components of the seal are the O-ring itself and the contact surfaces. The elastomeric O-ring relies on a compressive force acting on the O-ring to prevent the transfer of fluid between regions. Successful seal design ensures adequate seal compressive force while optimizing the destructive stress acting on the O-ring as a result of the compression or of the environment. 

Three Models for Characterizing Viscoelastic Behavior Are: 

1. Maxwell Model (dashpot and spring in series) 

2. Kelvin (Voigt) Model (dashpot and spring in parallel) 

3. Standard Linear Solid (dashpot and spring in series with a spring in parallel)

Incompressibility 

A material is incompressible if it exhibits zero volumetric change (isochoric) under hydrostatic pressure. Theoretically, PoissonÌs ratio is exactly one-half (0.5) and the bulk modulus is infinite (and det f = 1). 

Near incompressibility means that PoissonÌs ratio is slightly less than 0.5. 

Viscoelasticity 

Rubber exhibits a rate-dependent behavior that can be modeled as a viscoelastic material whose properties change with temperature and time. Features of viscoelastic materials are:

  • Under constant stress = creep

  • Under constant strain = stress relaxation 

  • During loading/unloading = hysteresis 

  1. Internal frictionÛrearrangement of molecular structure under load. 

  2. Strain-induced crystallizationÛ formation and melting of crystallized regions. 

  3. Stress softening (MullinÌs effect). 

  4. Structural breakdownÛthe breakdown of reinforcing filler/ polymer bonds. 

  5. Domain deformationÛdispersed inclusions contribute to hysteresis. 

Thermomechanical 

  • Temperature change causes thermal strains. 

  • Material properties change. Heat flow may occur.

NONLINEAR FINITE ELEMENT ANALYSIS (FEA) Rubber is a unique material. In its polymer form during processing, it behaves like a highly viscous liquid. After cross-linking (curing), rubber can undergo large reversible deformations. The unique prop-erties of rubber that require treatment different from traditional metal FEA are: 

  • Large deformations (over 100%). 

  • Load-extension (stress-strain) characteristics are definitely non-linear. 

  • Viscoelastic (spring and dampener) characteristics and time-and temperature-dependence. 

  • Nearly incompressible (volume does not change appreciably under stress). 

The finite element method is a technique for obtaining approximate numerical solutions to boundary value problems which predict the response of physical systems when subjected to external loads. The system or structure is characterized by many small individual pieces or elements which are connected at nodes. The solution of thousands of simultaneous equations for unknowns of displacements, rotations, or the hydrostatic pressure is obtained through a computer. 

FEA should be an integral part of the design and manufacturing processes. The advantages are numerous, including improved performance, faster time-to-market, optimal use of materials and verification of integrity before prototyping.
 
 

GLAND DESIGN
 

TYPES OF SEALSÛSTATIC  

Static Seals: Face (Flange), Radial (Piston), Crush 

Characterized by the absence of relative motion between sealing surfaces, or between the seal surface and a mating surface.

Static Face (Flange) Seal

Static Radial (Piston) Seal

Static Crush Seal

Gland Dimensions

Axial - Static Glands

 

Note: Gland diameter should provide for no greater than 3% to 5% stretch (based on nominal O-ring ID).

(IDGland - IDO-ring) / IDO-ring = Stretch

Gland Dimensions

Axial Vacuum - Static Glands

 

Note: Gland diameter should provide for no greater than 3% to 5% stretch (based on nominal O-ring ID).

(IDGland - IDO-ring) / IDOring = Stretch

Gland Dimensions

Trapezoidal Vacuum - Static Glands

Note: Gland diameter should provide for no greater than 3% to 5% stretch (based on nominal O-ring ID).

(IDGland - IDO-ring) / IDOring = Stretch

Gland Dimensions

Conical - Static Glands

Gland Dimensions

Tube Fitting Boss Seals

Gland Dimensions

Radial - Static Glands

Note: Gland diameter should provide for no greater than 3% to 5% stretch (based on nominal O-ring ID).

Types of Seals - Dynamic

Dynamic: Reciprocal, Rotary

Dynamic Reciprocal

Characterized by relative motion between the sealing surfaces. Reciprocal motion is most common in hydraulic cylinders or actuators, as well as in some types of valves. Rotary motion is common in many pump and valve applications.

 

Dynamic Rotary

Cycling (Open/Close) Seal

Cycling: Open-Close 

A hybrid dynamic static application where the relative motion occurs when the sealing surfaces are separated and then rejoined. 

Cycling (open-close) seal applications: slit valve, other valves

Gland Dimensions

Dynamic - Rotary Seals

Note: Gland diameter should provide for no greater than 3% to 5% stretch (based on nominal O-ring ID).

(IDGland - IDO-ring) / IDOring = Stretch

Gland Dimensions

Dynamic - Reciprocating Seals

Note: Gland diameter should provide for no greater than 3% to 5% stretch (based on nominal O-ring ID).

SPECIAL CONSIDERATIONS 

In many situations, it is very important to understand the anticipated response of an elastomer under a given compressive load. The compression/deflection curves for various elastomers are functions of the hardness (and bulk modulus) as well as the cross-sectional diameter. 

High-pressure applications may present unique challenges to elastomeric seals. Elastomers are essentially high-viscosity liquidsÛand as such will flow under extreme pressure conditions. In applica-tions where the clearance gap is larger than that recommended for a traditional O-ring seal, the user should consider backup rings. These flat or contoured rigid rings are inserted on the low-pressure side of the gland to prevent extrusion. Consideration of backup rings requires additional gland width to allow for elastomer expansion. In addition, chemical compatibility and ease of installation are other considerations when using backup rings.

Irregular, noncircular glands may present challenges for seal design. The chart at the right illustrates the minimum recommended bend radius for irregular glands. Smaller, sharper radii may result in excessive stress and seal damage.

SPECIAL CONSIDERATIONS 

In addition to compression, the stretch applied to O-rings in service (either to keep the O-ring in the gland during assembly or service, or to aid in the installation) can have an impact on the sealing performance. These effects can include: 

  • Reduction in cross section 

    As a seal is stretched, the cross section is reduced. 

  • Gow-Joule effect (elevated temperatures) 

    When an elastomer is stretched and the temperature elevated, the molecular chains will attempt to reduce the imposed stress. Excessive stretch can lead to premature cracking or seal failure.

A weight suspended from a rubber band will rise when subjected to localized heating - an indication of the Gow-Joule effect.

High-pressure applications may present unique challenges to elastomeric seals. Elastomers are essentially high-viscosity liquidsÛand as such will flow under extreme pressure conditions. In applications where the clearance gap is larger than that recommended for a traditional O-ring seal, the user should consider backup rings. These flat or contoured rigid rings are inserted on the low pressure side of the gland to prevent extrusion. Consideration of backup rings requires additional gland width to allow for elastomer expansion. In addition, chemical compatibility and ease of installation are other considerations when using backup rings. 

The surface finish of the gland can have a significant impact on the vacuum performance of seals. The following charts highlight terminology and processing techniques for specific surface finishes. Special care should be taken when sealing quartz componentsÛnot only due to the potentially irregular surface finish, but also to Ïlight pipingÓ effects on clear components in the sealing contact areas. Polymeric glands offer unique challenges if plastic deformation and creep of the sealing components are not taken into account.

INSTALLATION AND LUBRICATION 

Ensure that the seal gland is free of sharp corners or edges and sealing surfaces are free of contamination. Sharp edges can cause nicks in the seal surface which will lead to seal breakage or premature seal failure. Similarly, the installation of a seal over a threaded fitting can easily result in damage to the seal. Care should be taken to cover the threads before installing the seal. 

In trapezoidal applications, begin by inserting the seal in two (2) locations 180É apart. If the shape of the gland is not circular, begin by inserting the seal at the corners. Gently, apply downward pressure to the seal in several places until the seal is firmly seated. It may be necessary to apply a uniform compressive force through the use of a flat plate against the seal prior to installation in the equipment. Care should be taken to avoid ÏspiralingÓ the seal during installation. 

Radial sealing applications can result in excessive stretch being applied to the seal. The seal must not be stretched more than 50% of the materialÌs ultimate elongation during installation. Once seated, the stretch should be in the 3Ò5% range, as discussed earlier.

Lubrication of seal materials is often a necessary evilÛeither for the purposes of installation or vacuum pump-down performance. Most customers discourage the use of lubricants or foreign material in the gland area. If lubricants are to be used, make sure they are compatible with the seal material. All excess lubricant should be removed with a clean-room-grade wipe prior to installation.

 

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