Recent advances in isotropic conductive adhesives for electronics packaging applications

Electrically conductive adhesives are of large potential interest for the bonding of surface mount devices, especially in those cases where the use of soldering processes is restricted or impossible. Among the claimed benefits are mild processing conditions, process simplicity, flexibility, high resolution potential, and lead omission. Most of the presently available knowledge on the utility and reliability of conductive adhesives has been gathered with silver filled isotropic conductive epoxy adhesives (ICAs). In addition, the use of gold or nickel filled anisotropic conductive film or paste adhesives has been investigated quite intensively. Even conductive adhesive joints made with unfilled nonconductive adhesives are being considered for flip chip applications. To date, quite some work has been published demonstrating the usefulness and limitations of isotropic adhesives to bond a variety of IC (e.g., QFP) discrete (e.g., SOT) and passive components (R and C) under different climate conditions using various circuit and component-metallizations. Most of the isotropic conductive adhesives require noble metallizations (e.g., An or AgPd) to survive harsh environmental conditions as for instance 85/spl deg/C/85%RH and temperature cycling from -40 to +125/spl deg/C. Most of the adhesives give bad results on SnPb surfaces, a few special types show better results in 85/spl deg/C/85%RH. Deterioration of the electrical properties is due to an increase in the contact resistance. The bulk resistance of the adhesive, although considerably higher than that of solder, usually remains quite constant. On passivated Cu substrates reasonable good results are obtained. With Au finished surfaces large components may give problems in drop tests and temperature cycling due to insufficient bond area, unfavorable bond geometries or failures due to thermal expansion differences. The use of a coating or glob top on the leads may overcome these problems. With Au coated plastic spheres filled anisotropic conductive adhesives [ACA(F)] results opposite to the ICAs were obtained. For QFP80 (Au) joints with ACAF on FR-4 (Au) considerable increase in R-value was found after Temperature Humidity (TH) testing at 85/spl deg/C/85%RH and Temperature Cycling (TC,-20/100/spl deg/C), while with QFP80 (SnPb) R-values did not increase. This was found to be due to the formation of microsolder bridges. Microsolder joints can also be made with commercially available SnBi filled ACA. Some insight has been developed into the possible failure causes with ICAs. Surface oxidation of SnPb and interfacial cracks and delamination have for instance been observed, although the extent of their contributions under various conditions still needs to be resolved. The danger of Ag migration seems to be low as long as no liquid water is present. In summary, present results indicate that reliable connections with conductive adhesives even under harsh climate conditions are possible with the right choice of adhesive and metallizations of components and boards.

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Reliability of electrically conductive adhesive joints for surface mount applications: a summary of the state of the art

Electrically Conductive Adhesives (ICAs: Isotropic Conductive Adhesives; ACAs: An-isotropic Conductive Adhesives; and NCAs: Non-conductive Adhesives) offer promising material solutions for fine pitch interconnects, low cost, low-temperature process and environmentally clean approaches in the electronic packaging technology. ICAs have been developed and used widely for traditional solder replacement, especially in surface mount devices and flip chip application. These also need to be lower cost with higher electrical/mechanical and reliability performances. ACAs have been widely used in flat panel display modules for high resolution, lightweight, thin profile and low power consumption in film forms (Anisotropic Conductive Films: ACFs) for last decades. Multi-layered ACF structures such as double and triple-layered ACFs were developed to meet fine pitch interconnection, low-temperature curing and strong adhesion requirements. Also, ACAs have been attracting much attention for their simple and lead-free proc...

Review Of Recent Advances In Electrically Conductive Adhesive Materials And Technologies In Electronic Packaging

Effects of shrinkage on conductivity of isotropic conductive adhesives

Conductive adhesives have been successfully applied to hybrid circuits and die-attach applications. More recently they have become the exclusive bonding techniques for flex interconnections to LCDs and driver ICs (chip on glass). Additional applications for bonding surface mount devices as well as flip chip components on rigid and flexible printed circuits are being explored. In surface mount applications, surface mount conductive adhesives (SMCAs) are an environmentally friendly alternative to Sn/Pb metal solder. In addition, SMCAs offer many other advantages over Sn/Pb solder in surface mount applications, including high pitch capability, mild processing temperature, and fewer processing steps. However, one critical drawback of the current SMCA technology is its unstable contact resistance with non-noble metal finished components. The contact resistance of the assembly usually shows substantial increase over time particularly under high temperature (85/spl deg/C) and high humidity (85%RH) conditions (so called 85/85 conditions). Several possible mechanisms including metal oxidation and electrochemical corrosion have been suggested in the literature. However, prior work has not confirmed the exact mechanism responsible for unstable contact resistance. The focus of this study was to identify the underlying mechanism that results in the unstable contact resistance phenomenon of the current SMCAs. A contact resistance test device consisting of metal wire segments and conductive adhesive dots was developed for this study. Based on the results of this systematic study, electrochemical corrosion has been identified as the underlying mechanism responsible for the observed unstable contact resistance phenomenon of conductive adhesives.

Mechanisms underlying the unstable contact resistance of conductive adhesives

Electrically conductive adhesives offer a lead free, relatively low temperature process for attachment of surface mount components to printed circuit boards (PCB's). The National Center for Manufacturing Sciences (NCMS) Conductive Adhesives project has defined a requirement for an isotropically conductive adhesive (ICA). In this study, 25 commercially available silver-filled epoxies were evaluated for joint resistance stability on copper and tin/lead surface finishes after 500 h of 85/spl deg/C/85% RH aging. Mechanical properties were evaluated through lap shear and impact testing. This testing led to a set of requirements for a surface mount adhesive including a volume resistivity of 1 m/spl Omega/-cm, less than a 20% shift in resistance after aging, and the ability to pass six drops with a plastic leaded chip carrier (PLCC) 44 package from a height of 1.52 m (60 in). Promising candidate adhesives identified in the screening study were evaluated on a circuit board test vehicle with a variety of components. Assembly revealed a narrow processing window when compared to solder. Impact testing demonstrated that current materials have inadequate impact strength for many components and accelerated T/H aging reveals unstable electrical resistance with Sn/Pb finished components. Preliminary data is included on a new adhesive formulation that has been developed to address the shortcomings of currently available ICAs.

T. Hanlon

Papers

Electrically conductive adhesives for surface mount solder replacement