Sealing/Hermetic Encapsulation

Sealing is the process of encapsulating a hermetic package, usually by capping or putting a lid over the base or body of the package.

The method of sealing is generally dependent on the type of package. Ceramic DIPs, or cerdips, are sealed by topping the base of the package with a cap using seal glass. Other ceramic packages such as sidebrazed packages, LCC's, and PGA's are sealed by covering the package with a combo lid through solder sealing. A sealing furnace (see Fig. 1) is used to expose the packages to the high temperatures needed to achieve proper sealing.

Seal glass, like any glass, is a supercooled liquid which exhibits tremendous viscosity when cooled below its glass transition temperature. A seal glass may be classified as vitreous or devitrifying. Vitreous glass are relatively transparent and can be reworked repeatedly without degradation in its properties. Devitrifying glass can no longer be reworked after crystallizing upon cooling. Fillers are added to seal glass to control the thermal expansion of the glass.

A typical cerdip glass sealing process consists of the following steps in its sealing profile: 1) organic burn-off/ougassing; 2) glass densification or softening of glass; 3) glass profile formation (around 350 deg C); 4) chemical bonding between glass and ceramic (around 420 deg C); 5) annealing; and 6) cooldown. Cooldown is more critical than heat-up because this affects the final properties of the glass more greatly.

Welding is used to join the metal header and metal cap of metal cans. A specially-designed welding machine is used to deliver the high currents needed to weld the metal header and metal cap at their interface. Figure 2 shows an example of a lid welding machine.

Common Sealing-related Failure Mechanisms: Seal Cracking; Package Cracking; Soft Errors; Pits and Voids; Incomplete Weld

Fig. 1. A Sealing Furnace

Fig. 2. A Lid Welding Machine

Front-End Assembly Links: Wafer Backgrind; Die Preparation; Die Attach; Wirebonding; Die Overcoat

Back-End Assembly Links: Molding; Sealing; Marking; DTFS; Leadfinish

See Also: IC Manufacturing; Assembly Equipment

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