Typical Defects and Prevention of Ceramic-to-Metal Sealing — A Different Process, Different Challenges

1. The Three Most Common CTMS Defects

The GTMS in the previous article relies on direct chemical bonding between glass and metal; CTMS (ceramic-to-metal sealing) is different — ceramics and metals cannot bond directly and must be joined indirectly through a two-step “metallization + brazing” process. With more process steps, the defect types differ accordingly. The three high-frequency defects:

Metallization layer peeling — the sintered metallization layer adheres insufficiently to the ceramic and peels off during brazing; the seal fails outright.

Brazing voids — the brazing filler metal fails to flow fully between the metallized ceramic surface and the metal housing, leaving voids. Voids reduce the effective sealing area and cause local stress concentration, becoming leak hazards.

Ceramic cracking — cracks appear in the ceramic after brazing and cooling, directly destroying hermeticity and insulation performance.

2. Root Causes and Prevention of the Three Defect Types

The three defect types correspond to three lines of prevention, with root causes and countermeasures matched one to one:

Peeling prevention — the root causes are an improper metallization paste formulation, insufficient sintering temperature, or substandard ceramic surface roughness; the countermeasure is metallization layer quality control. Metallization is the “soul process” of CTMS: it determines whether the ceramic can be brazed. After sintering, the metallization layer should be uniform in thickness and form a chemically bonded transitional interface with the ceramic. Xuri’s HTCC ceramic series products, such as the SMT-11*9, all follow this standard metallization process.

After sintering, a nickel layer must be plated as the brazing transition layer — nickel is well wetted by common brazing fillers such as the Ag-Cu eutectic.

Void prevention — the root causes are insufficient filler flow or uneven placement; the countermeasure is brazing process control: control the filler quantity precisely according to the sealing area and pre-place it uniformly (as filler foil or rings); braze in a vacuum or reducing atmosphere to prevent oxidation of the metallized surface from impairing wetting.

Cracking prevention — the root cause is the large CTE mismatch between ceramic and metal, which leaves excessive residual stress after cooling; the countermeasure is dual control of structure and process: use low-expansion alloys such as Kovar for the intermediate transition ring, and control the brazing temperature and cooling rate.

The prevention of the three defect types has different focal points — peeling is governed by metallization, voids by brazing, cracking by structure — but the essence is the same as for GTMS: only when every process parameter stays within its window can the defect rate remain consistently under control.

[Glossary]

1. Metallization: the process of forming a metal layer on the ceramic surface so that the ceramic can be brazed

2. Brazing Void: microscopic voids formed where the brazing filler fails to fill the joint completely, reducing sealing reliability