Common Glass-to-Metal Seal Defects: Causes and Solutions

The following guide provides a list of some commonly encountered problems with glass-to-metal seals and possible causes.


  • Radial crack Glass: Glass CTE higher than the pin
  • Circumferential crack: Glass CTE lower than the pin
  • Meniscus crack:Meniscus is too high,Compression forces from the metal housing are too high
  • Planar crack at seal mid-point Seal is designed too long,Mismatch of CTE too large


  • Uniform throughout glass: Sealing temperature is too high, sealing time is too long, the glass density is lower than normal
  • At the interface: Furnace atmosphere is not optimum, Possible metal cleaning problem, Possible preform cleaning problem, The metal is out-gassing, Possible reaction between the glass and metal.
  • Single big bubble: Glass contamination
  • Bubbles at bottom of the seal: Glass could be reacting with fixture
  • Bubbles at the top of the seal: The sealing furnace atmosphere is not optimum

See-Through Seals

  • See-through seals: Low weight glass preform, Glass preform diameter is too small for the housing, glass melt density is higher than normal

High Meniscus

  • High meniscus: Glass preform weight is too high, Glass melt density is lower than normal

Low Pin Pull Strength

  • Low Pin Pull Strength: Not enough bonding, Metal requires pre-oxidation and/or coating

Glass Not Wetting

  • Glass not wetting: Too much oxidation on the metal, Metal requires pre-oxidation, Metal not clean,Furnace atmosphere not optimum, Choose a different metal or glass

Low Breakdown Voltage

  • Low breakdown voltage: Inclusion(s) in the glass, Glass composition has a high dielectric constant, Seal design is not optimum
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