Encapsulation wire bonding

Polyphenylene sulphide has been used commercially to encapsulate ceramic capacitors for several years and mica capacitors encapsulated in Phillips Ryton BR61B have recently received the British Standard 9000 approval (BS9070-N-002). More recently, attempts have been made to encapsulate wire-bonded integrated circuits, which is one of the most difficult applications to successfully mould with a thermoplastic material because of problems with wire-bond sweep. By paying particular attention to the melt viscosity of the PPS and having careful control of the moulding parameters it has been shown that it is possible to mould successfully both TTL and linear operational amplifier type devices. 

The earliest kinds of interconnection involved solder joints later, wire bonding and dual in-line (DIP) phenolic-molded packages were developed. In fact, the DIP continues to be the most commonly used package. In a DIP (see Figure 1) the IC chip (shown here encapsulated) is connected by wire bonding to the two rows of... 

Figure 1 - Edge Effect of Device Passivation (Due to the exposure of the wire bond pad and the passivation material, this access area for wire bonds must need encapsulation after wire-bonding interconnection).

Epoxy molding compounds, used to encapsulate microelectronic devices, contain bromine to provide flame retardancy to the package. This bromine, typically added as tetrabromo bisphenol-A or its epoxy derivative, has been found to contain many hydrolyzable bromides. These bromides, along with the presence of chloride impurities, are detrimental to the life of the electronic component. Bromine especially has been suspected (proven) to cause wire bond failure when subjected to moisture and/or high temperatures. With the addition of a more thermally and hydrolytic stable bromine compound, flame retardancy does not have to be compromised to increase the device reliability. Stable brominated cresol epoxy novolac, when formulated into a microelectronic encapsulant, increases the reliability of the device without sacrificing any of the beneficial properties of present-day molding compounds. 

The assembled stacks are then attached to a leadframe or substrate again with conductive epoxy or wire bonds, and encapsulated with epoxy. Alternately, they may be packaged in thin, small-outline packages (TSOP) or EGA packages. If multiple die are used on each layer, as many as 64 die may be incorporated in one stacked module." ... 

FIGURE 10 Schematic illustration of a discrete LED lamp. The LED chip is attached to a reflecting cup on an electrical lead. A wire bond connects the LED to a second electrical lead. The chip is then encased in an encapsulant. [From Haitz, R. H., Craford, M. G., and Weissman, R. H. (1995). In Handbook of Optics, 2nd ed., Vol. 1, McGraw-Hill, New York. With permission.]... 

Prior to the discussion of encapsulation techniques, the IC assembly sequence will be reviewed. The wire bonded IC device will be used as an example of a common IC package. This IC packaging example will provide a better understanding of the packaging sequence and encapsulation process. Let us assume that the IC devices have gone through the completed fabrication process and are still in a wafer form. These devices... 

Die bond and wire-bond attach suffer from the problem that this method of chip attach is difficult to repair, particularly if the chip is encapsulated. 

CMp-on-Boaid (COB). In COB assembly, a back of a bare (unpackaged) IC is attached directly onto a PWB, wire-bonded, and then encapsulated with a polymer. The die bondpad pitch is generally in the range of about 0.175-0.25 mm and IC placement must be very accurate. 

D ects. A defect is an unacceptable deviation from a norm. As an example, a bond wire could be missing in an IC.This defect may in turn be due to a problem (the root cause ) in a wire-bonding machine, such as it misfeeding wire. Other deviations may not be considered defects, but rather acceptable variations. For example, the plastic used to encapsulate a component may have significant variations in color that are stiU considered acceptable. 

P.L.H.M. Cobben, R.J.M Egberink, J.G, Bomcr, EJ.R, Sudholler, P. Bergvcld, and D,N. Reinhoudt, Chemically modified ion-sensitive field-efifcct transistors application in flow injection analysis cells without polymeric encapsulation and wire bonding, Anal Chm. Acta, 248 (1991) 307-313. 

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