Conductive Polymers

Chapter C: Conductive Polymers
LEVEL 1: INTRODUCTION
1.1 INTRODUCTION TO POLYMER BONDING

Electrically conductive adhesives have been used for many years for i.c. die mounting and terminal bonding of components in some types of hybrid circuits. There applications are not particularly demanding on either the properties of the conductive adhesive or on the mounting technology. The advent ofSurface Mount Technology (SMT) as applied to large area Printed Circuit Boards (PCB) amounted to a major revolution in Printed Circuit Technology which had seen great progress but few revolutionary developments in a period of over four decades. SMT has certainly included some revolutionary concepts and major changes in the design and construction of components to be assembled. This has mainly relatedto the replacement of leads by solderable pads. This clearly implied the expectation that solder would be used as the bonding agent in the mounting operation. It was perhaps inevitable that in due course conductive adhesives should be considered for surface mounting on PCBs. The incentive to consider this development received strong support from environmental considerations relating to thegrowing awareness of the undesirable aspects of solder technology including the use of cleaning agents for removing the flux and the presence of lead and other heavy metals in the solder itself. However, the apparently simple step of replacing solder with conductive adhesives has proven to be a complex problem where considerable and growing attention has not as yet solved all of the issues. Thechallenges to be faced were the shift to large boards, which were not encapsulated as was the case with i.c. die in their various packages or hybrid circuits, which usually had some form of protection for the added devices. Then there was the variety of components to be bonded, all requiring to be placed with considerable precision and some having a large (and as time went by a rapidly increasing) numberof small and closely positioned terminal pads. Many other relevant parameters have now come under scrutiny such as the flexibility of the joint, which together with such items as expansion coefficient can affect stress levels in the bond and its shear strength. Reliability of the many bonds involved in a surface mounted PCB has become a matter of prime concern. Reworkability is attractingconsiderable attention and the whole question of recovering residues is now regarded as a matter of crucial importance. When we now carry out an information retrieval study on the subject, we find that those early papers that are most relevant to the subject at the present time came from technologists and manufacturing companies engaged in adhesive mounting techniques. The pioneering work of Estes andKulesza And the papers from Emerson and Cuming and from other manufacturers of conductive adhesives are now reinforced with unfortunately only a few number of papers from major companies in the electronics industry describing their experiences and aspirations in the application of conductive adhesives to their main assembly processes. But still there is a lot of secretiveness in the adhesive world.After a decade of encouraging development the subject of conductive adhesives has now reached a critical stage. This is no passing phase or a minor fashion in experimental assembly techniques. If current progress continues this technology will become a major factor in electronics manufacture affection all of the many branches of the electronics industry. If the quality and reliability of theprocess meet the required standard a major reinvestment programme will have to be faced. New manufacturing techniques will involve new test procedures and a new quality and standard structure will be required. Reliability levels at least as high as those that can be achieved today with the best soldering techniques will be established.

The focus is on the use of adhesives as solder alternatives....