Page 100 - Power Electronics Handbook

Smartpower 93

MCM-Si uses a silicon wafer as the substrate with aluminium or copper
conductors. Silicon dioxide may be formed to give an inorganic dielectric
layer. The techniques used to form the systems are very similar to those used
in integrated circuits, and very fine lines are possible primarily due to the
smoothness and flatness of the silicon surface. Generally costs are the highest
of all the MCM technologies.

3.53 Packaging
A variety of packages are used for smart power devices, most of the lower
power packages being very similar to those used for integrated circuits, a few
of these being described here (Iscoff; Maliniak, 1996). The TO type of package
was a natural development from the transistor case. Various sizes are in use,
such as TO-3, TO-5, TO-8, TO-99 and TO-100. The number of pins in these
packages generally varies between 8 and 14.
Flat pack encapsulations also come in many varieties. Figure 3.14 shows
typical structures. The die is bonded to the base by eutectic solder. Gold
plated leads are embedded in a glass frame and the lid is sealed with a low
temperature glass frit. A metal flat pack uses kovar for most parts except for
the walls between the ring and base, which are glass. The leads pass through
this. A glass package is usually made from borosilicate and consists of a one-
piece base and ring assembly in which the kovar leads are sealed. The lid can
be made of glass, ceramic or metal and is attached by a low melting point
sealing glass. The package has good thermal and mechanical properties.
Ceramic flat packs are similar to metal devices. The base, ring and lid are
now made from alumina or, for higher power dissipation, from beryllia.
Figure 3.15 shows a dual-in-line package which can be made from metal
or ceramic. The chip is placed in a cavity and bonded by a glass frit. The
leads also pass through glass frit seals in the package. The ceramic lid is
initially metallised and then brazed or solder sealed to the body, or a glass frit
can again be used to connect the two together.
With the ever-increasing density of boards, surface mount technology
(SMT) is becoming a necessity, although surface mount packages are not
viable for higher powers or large sizes (Flaherty, 1993; Wilson, 1995). Figure
3.16 shows a few examples of SMT packages. The small outline integrated
circuit package (SOIC) has gull-winged leads which extend out from two
sides of the package. The leads are close together and the package is thin so
that it takes up relatively little room. The package is self-aligning during the
soldering process, the viscosity of the solder causing the leads to be pulled
into place on its pads. Because the solder joints are visible they can be
inspected and manual or automatic soldering can be used.
Placing pins on all four sides of the package increases the number of pin
outs. Plastic leaded chip carrim (PLCCs) can have gull-winged leads or J
leads, as in Figure 3.16. J leads take up less mom than gull-wings and give
a high positioning accuracy with low lead deformation. The package is again
self-aligning, but since the solder joints are formed under the body of the
package they cannot be inspected and special tools must be used for
replacing components.
The butt style package has straight leads, which are soldered into
individual puddles of solder on the pads. The package has low deformation