Adhesive Bonding Technology

Introduction
A number of joining processes exist in the industry today that vary from bolting and riveting to welding, adhesive bonding and so on [TAP07]. Out of all these joining processes, bonding is by far the most universal joining technique. Bonding is the joining of similar or dissimilar material surfaces using a substance which adheres to the two material surfaces that need to be joined.
Adhesive bonding technology is a widely established method for joining metals, ceramics, plastics, composites and other substrates as well. It offers great flexibility for design and can be integrated easily for mass production applications. With the advancement of polymer chemistry, a variety of adhesives have been developed with higher adhesive strength

1 State of the Art
1.1 Adhesives in the Electronic Industry
There are a variety of adhesives used in the market, each with different bonding mechanisms, different raw materials that are used in its production and so on [PIZ03]. They also differ based on the method of curing, its molecular structure or the nature of its application. The adhesives find a variety of applications in the electronic industry, they are used in die attachments, solder replacements, flip chip attached devices and even in surface mount technology. One interesting application is the use of adhesives in flexible circuits, where adhesion is retained even if the circuit is flexed [LIC11].
The Figure 2.1, depicts the classification of the adhesives into three main categories based on conductivity. They are non-conductive adhesives, thermally conductive adhesives and electrically conductive adhesives. The description of each of these category of adhesives is provided below [HAN15].

1.1.1 Non-Conductive

This is achieved through a pure mechanical contact and the application of a compressive force [LIU99]. The quality of the conductivity would therefore depend on the mechanical contact pressure or the compressive force. If the adhesive should be required to have a low electrical resistance, then a high mechanical contact pressure is required.
While the adhesive can be used to provide electrical interconnects, the majority of its applications is to provide electric insulation or sealing the electronic components where they are used to prevent short circuiting of the electrical components. The adhesives are also used as a low-cost option for flip-chip assemblies. It offers a number of advantages because of its cost effectiveness and the adhesive’s compatibility with a wide range of contact materials [YIL10].
The performance of the adhesive joints however degrade with exposure to humid environments. It is observed that cyclic exposure to these environments is more harmful for the joints as compared to a constant exposure to humid environments [ALA11]. This failure is caused due to stress relaxation and hygroscopic swelling that has been induced by moisture. This affects the compressive force that maintains the physical or mechanical contact, which is required for the conductivity to be present. Common failure modes observed in NCA joints are cracking, interfacial delamination and