PACKAGING TECHNIQUES

Once the IC has been produced, it requires a housing that will protect it from damage. This damage could result from moisture, dirt, heat, radiation, or other sources. The housing protects the device and aids in its handling and connection into the system in which the IC is used. The three most common types of packages are the modified TRANSISTOR-OUTLINE (TO) PACKAGE, the FLAT PACK, and the DUAL INLINE PACKAGE (DIP).

Transitor-Outline Package

Transistor-Outline Package. The transistor-outline (TO) package was developed from early experience with transistors. It was a reliable package that only required increasing the number of leads to make it useful for ICs. Leads normally number between 2 and 12, with 10 being the most common for IC applications. Figure 1-19 is an exploded view of a TO-5 package. Once the IC has been attached to the header, bonding wires are used to attach the IC to the leads. The cover provides the necessary protection for the device. Figure 1-20 is an enlarged photo of an actual TO-5 with the cover removed. You can easily see that the handling of an IC without packaging would be difficult for a technician.

Figure 1-19. - Exploded TO-5.

Figure 1-20. - TO-5 package.

The modified TO-5 package (figure 1-21) can be either plugged into [view (A)] or embedded in [view (B)] a board. The embedding method is preferred. Whether the package is plugged in or embedded, the interconnection area of the package leads must have sufficient clearance on both sides of the board. The plug-in method does not provide sufficient clearance between pads to route additional circuitry. When the packages are embedded, sufficient space exists between the pads [because of the increased diameter of the interconnection pattern, shown at the right in view (B)] for additional conductors.

Figure 1-21A. - TO-5 mounting PLUG-IN MOUNTING

Figure 1-21B. - TO-5 mounting EMBEDDED CAN(LEADS PLUGGED IN)

Flat Pack

Many types of IC flat packs are being produced in various sizes and materials. These packages are available in square, rectangular, oval, and circular configurations with 10 to 60 external leads. They may be made of metal, ceramic, epoxy, glass, or combinations of those materials. Only the ceramic flat pack will be discussed here. It is representative of all flat packs with respect to general package requirements (see figure 1-22).

Figure 1-22. - Enlarged flat pack exploded view.

After the external leads are sealed to the mounting base, the rectangular area on the inside bottom of the base is treated with metal slurry to provide a surface suitable for bonding the monolithic die to the base. The lead and the metalized area in the bottom of the package are plated with gold. The die is then attached by gold-silicon bonding.

The die-bonding step is followed by bonding gold or aluminum wires between the bonding islands on the IC die and on the inner portions of the package leads. Next, a glass-soldered preformed frame is placed on top of the mounting base. One surface of the ceramic cover is coated with Pyroceram glass, and the cover is placed on top of the mounting base. The entire assembly is placed in an oven at 450 degrees Celsius. This causes the glass solder and Pyroceram to fuse and seal the cover to the mounting base. A ceramic flat pack is shown in figure 1-23. It has been opened so that you can see the chip and bonding wires.

Figure 1-23. - Typical flat pack.