Conductive Adhesives as Solder Replacements

Electrically conductive adhesives, primarily silver-filled epoxies, are finding uses as replacements for solder in surface-mounting components on printed-circuit boards and in flip-chip attachments. There are several driving forces for this application, amajor one being the trend to eliminate lead and tin-lead solders because they may be health hazards. Also associated with the use of solder, is the need to eliminate ozone-depleting solvents presently used to clean and remove flux residues. Electrically conductive polymer 

Solder Replacement/ Supplier Compatability with Surface Finish Printing Resolution  

Product Name/ Supplier Type Viscosity cps Shelf Life (-40°C)/ Pot Life (25°C) Cure Schedule % Weight Loss 300°C by TGA Bond Strength psi Modulus of Elasticity MPa psi CTE ppm/°C °C Volume Resistivity ohm-cm Thermal Conduc tivity W/mK Ionic Impurities ppm 

ND — None Detected. Source Com[dled from various suppliers data sheets. 

---

Chung K, Ozguz V. Flexible conductive adhesive as solder replacement in flip chip interconnection. In Proc. Semicon. West. 1998 127-131. 

Chung, K., and Ozguz, V., Flexible Conductive Adhesive as Solder Replacement in Flip Chip Interconnection, Proc. Semicon. West 1998, pp. 127-131(1998)... 

Adhesives used for screen or stencil printing in surface-mount applications are generally electrically insulative types whose functions are mechanical attachment and thermal dissipation. However, electrically conductive, silver-filled epoxies have been used for many years as ohmic contact adhesives to interconnect bare-chip devices in hybrid microcircuits and are used as solder replacements for surface mounting of components on printed-circuit boards. Regardless of their... 

A new area of concern for electrical stability arises because of the increasing use of conductive adhesives as replacements for solder. Some conductive adhesives show unstable electrical-contact resistance when used on non-noble metal surfaces such as copper or tin-lead solder. Although stable on gold, palladium, platinum, and silver surfaces, the same adhesives were found to be unstable on tin, tin-lead, copper, and nickel surfaces.The unstable resistance and increase in resistance in temperature-humidity exposures have been attributed to the growth of an oxide layer separating the filler particles from the substrate at the interface, a mechanism similar to that for the loss of backside contact in die-attach materials. 

Electrically conductive adhesives that are used as solder replacements are evaluated for material characteristics and contact resistance on solder surfaces. In one study, the requirement for volume resistivity is given as 0.001 Q-cm or less and for contact-resistance change as 20% or less after 500 hours of 85/85-exposure. ... 

Contact resistance between an ICA (generally a Ag-flake-filled epoxy) and nonnoble metal finished components increases dramatically during an elevated temperature and humidity aging, especially at 85°C/85% RH. The National Center of Manufacturing and Science (NCMS) defined the stability criterion for solder replacement conductive adhesives as a contact resistance shift of less than 20% after aging at 85°C/85% RH conditions for 500 h (67). 

The use of both nonconductive epoxy adhesives and anisotropic conductive adhesives as a solder replacement for SMT was studied [121]. Quad flat pack components were connected to FR4 substrates and then subjected to temperature cycling. None of the tested adhesives passed the temperature cycling test in accordance with the military standard 883C [121]. Failure analysis indicated that impressions made in the contact area depend on the contact force and surface preparation. At very low contact forces, the presence of insulating oxide layers on the metal surfaces increased the contact resistance dramatically. However, as the contact force was increased, the contact resistance dropped rapidly [122]. 

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

AniSOtropically Conductive Adhesives. Significant research has been conducted on ACAs as a potential solder replacement for some electronics packaging appUcations. However, many aspects of this technology must be better... 

Isotropically Conductive Adhesives. There have also been a significant number of studies to improve ICA technology. However, several critical issues associated with this technology must also be addressed before it can be used as a replacement for soldering technology ... 

Problems with electrically conducting adhesives provide another possible opportunity for conducting polymer-dopant compositions. Today s adhesives, such as used to replace soldered connections, conduct because of metal or carbon fillers. While high loadings are necessary to obtain high conductivities, these loadings dilute the mechanical properties of the adhesive and the conductive particles serve as possible nucleation sites for crack initiation. 

Electrically conductive adhesives are used for such applications as bonding lead wire to electrodes, solder paste replacement in the assembly of SMD, die attachment, and repair of conducting paths. Adhesives such as epoxies or acrylics are highly filled (typically 80%) with conductive metal particles. Metals such as nickel and copper cannot be used because of their susceptibility to oxidation. Precious... 

---