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4.3 Packaging Processes 65

4.3.3 Sealing Techniques

Most types of IC plastic packages are sealed as part of the transfer molding process.
Alternatively, premolded packages, in which the chip is placed in the package after
the transfer molding process, require a lid to be placed over the package opening.
Lids can be made from metal or preformed plastic and these are attached using a
polymer adhesive. Premolded packages are the most common type of plastic pack-
age for microsensors. In either case, these packages are not hermetic and moisture
will diffuse through the molding material and along the interface between the lead-
frame and the plastic. This moisture ingress is the main cause of failure in plastic
packaged ICs, usually through corrosion of metallized features. Moisture resistance
can be improved by encapsulating the die in silicone compounds prior to molding.
A variety of processes exist for sealing metal and ceramic packages once the die
has been mounted and the electrical connections made [6]. The suitability of these
processes will depend upon the nature of the package and the requirements of the
application. The simplest method of sealing is to simply use a plastic seal to attach a
lid to the package; this is generally known as epoxy sealing. This is a very inexpen-
sive approach but does negate the hermetic nature of these packages. Hermetic
packages require alternative sealing techniques that offer much greater levels of
resistance to moisture. No material is truly impermeable, but metals, ceramics, and
glasses possess permeability several orders of magnitude less than polymers.
Welding is the most reliable method for sealing hermetic metal packages and is
widely used in military applications. The higher capital cost of the equipment is jus-
tified by the improved yields and reliability. The welding process involves the appli-
cation of high current pulses resulting in localized heating of up to 1,500°C, thereby
fusing the lid to the package. Other techniques include electron beam and laser
welding, which is more attractive for larger packages and provides a noncontact
sealing method. Welding is also more tolerant of uneven surfaces and the process
does cause the outgassing of organic vapors, which can occur in soldering and glass
frit sealing. Welding cannot be applied to ceramic lids and is not cost effective for
high volume applications.
Alternative techniques, better suited to high volumes and suitable for use on
both metal and ceramic packages, are soldering and brazing. In the case of ceramic
packages a metal seal band should be incorporated on the substrate surface to facili-
tate the sealing process. Such a band can be formed by, for example, thick-film
printing. When soldering and brazing, attention must be paid to the process tem-
perature, which should be significantly lower than the temperatures necessary to
melt the seal around contact pins and affect the die mount. Seals formed with a
gold-tin eutectic braze are stronger and more reliable than their solder counterpart
and also avoid the use of flux. The eutectic of choice is usually applied in a preform
configuration that is placed between the lid and the package. Mechanical pressure is
then applied via spring clips or weights and the assembly heated in a furnace. Flat
surfaces are required on both the lid and package to ensure a reliable hermetic seal.
In addition to die mounting and the sealing of electrical interconnects, glass frits
can also be used to seal packages. The attractions of glass frits include their inert
nature, their electrical insulating properties, their impermeability to moisture and
gases, and the wide range of available thermal characteristics. Their main disadvan-
tages are their brittle nature and low strength. The seal design, choice of glass, and