Typical Defects and Prevention of Glass-to-Metal Sealing — From “Verified Quality” to “Manufactured Quality”

1. The Three Most Common GTMS Defects

The previous two articles discussed how to verify that sealed parts are qualified — hermeticity testing and reliability testing are both “after-the-fact gatekeeping”; consistently producing qualified parts relies on manufacturing process control. GTMS (glass-to-metal sealing) is the most classic packaging process, so let us first look at what defects arise in production and how they are prevented. The three most frequent:

Glass cracking — cracks appear at the sealing interface or in the glass insulator; this is the most common GTMS failure mode. Cracks degrade insulation performance and, in severe cases, develop into leak paths.

Bubbles and voids — bubbles or voids appear inside the glass or at the sealing interface. Bubbles reduce insulation strength and, in severe cases, link up into channels that cause leaks.

Sealing leakage — poor bonding at the glass-metal interface leaves microscopic gaps, ultimately showing up as hermeticity failure.

2. Root Causes and Preventive Measures

Each of the three defect types has a clear root cause, and prevention proceeds along three corresponding lines:

Crack prevention — the root causes are improper CTE matching or overly rapid cooling; the countermeasure is dual control of materials and cooling: matched seals use Kovar + borosilicate glass (CTE ≈ 5.3×10⁻⁶/°C). This is also the standard combination long adopted by classic GTMS products such as the TO46 and TO56 headers.

Bubble prevention — the root cause is gas in the raw materials that is not discharged in time; the countermeasure is dual control of the sintering atmosphere and the process: sintering is performed in a reducing atmosphere so that the gases released by the decomposing glass batch are fully discharged, and the pre-sintering degassing step must never be skipped.

Leak prevention — the root causes are improper control of the metal surface oxide layer or insufficient glass wettability; the countermeasure is surface condition control: before sintering, the Kovar housing is degreased, acid-pickled and pre-oxidized to enhance the wetting of the metal by the glass and their chemical bonding.

The essence of defect prevention is process control — only when every process parameter stays within its window can the defect rate remain consistently under control; and whether the prevention is effective must ultimately be verified by the hermeticity testing described in the first article.

[Next Article Preview] Next article: typical defects and prevention of ceramic-to-metal sealing — also defect control, but what makes CTMS different?

[Glossary]

1. CTE (Coefficient of Thermal Expansion): the coefficient of thermal expansion — the linear expansion per 1°C of temperature rise, in units of ×10⁻⁶/°C; the core parameter for matching sealing materials

2. Wettability: the ability of molten glass to spread over a metal surface, determined by surface tension and interfacial chemical reactions

3. Pre-oxidation: a process step that forms a controlled oxide layer on the metal surface before sealing to enhance chemical bonding with the glass