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

 

Cracking

• Radial crack Glass:  Glass CTE higher than the pin

• Circumfuncial crack:   Glass CTE lower than the pin

• Meniscus crack:   Meniscus is too high,  Compression forces from the metal housing are too high

• Planner crack at seal mid-point Seal is designed too long, Mismatch of CTE too large

 Bubbles

• 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