Isotropically Conductive Adhesives

DoD Participants:  U.S. Navy – Fleet Readiness Center East; U.S. Air Force – Warner Robins Air Logistics Center; U.S. Army – Tobyhanna Army Depot

This report describes the Isotropically Conductive Adhesive (ICA) project team’s effort to develop a novel replacement for lead- (Pb) based solders. The focus of the material for development was a copper- (Cu) based ICA for use in the Electronics Industry, since there is a resolute market drive to eliminate lead-containing solders from all commercial electronics.

The push to eliminate lead-containing solders is coming mainly from foreign electronics manufacturers, particularly in Japan and Europe. The Japanese Electronic Industry Development Association (JEIDA) released a Roadmap for lead-free soldering in cooperation with the Lead-Free Soldering Committee of the Japanese Institute of Electronic Packaging Association (JIEPA). This set a deadline of 2005 for lead solder use only in exceptional circumstances. Europe has followed suit, with an initiative aimed at eliminating lead from all products by 2006.

Risks to defense contractors and weapon system managers of continued use of tin-lead (Sn-Pb) solder include:  compliance with current environmental regulations, concerns about potential environmental legislation banning lead-containing products, risk of trade barriers and lost sales, reduced mission readiness, component obsolescence with lead surface finishes, compatibility of current lead-containing assemblies with lead-free components, and availability of lead containing solders. Because no single lead-free solder is likely to qualify for all defense applications, it is important to validate alternatives to lead solders for discrete applications.

The most overwhelming problem associated with lead-free alloys is their higher reflow temperatures and incompatibility with existing electronic assemblies, electronic components, and solderable surfaces that contain lead. These higher temperatures add cost and time to the manufacturing operations as well as potential damage to both assemblies and components. Much more research into electronic materials and processes will be required to overcome the “collateral” damage caused by a shift to lead-free solder alloys. Additionally, lead-free solders are not compatible with lead. Thus elimination of lead from the substrate and components is required to assure the reliability of the solder joint.

Because the long service life of military electronic components will necessitate bonding to components that still contain lead, this reliability concern is a major issue. Commercially available metallic-filled ICAs offer a promising alternative to solders for joining electronic components for printed wire boards.

The purpose of the NCMS Commercial Technologies Maintenance Activities (CTMA) ICA project was to demonstrate and validate an alternative material, ICA, to the conventional tin-lead solders used on circuit card assemblies.

U.S. companies are developing lead-free solders, but many of these alloys have been shown to have degradation, compatibility, reliability and contamination problems. ICAs are a commercially available alternative to solder but most use silver (Ag) as the conductive medium and suffer from such issues as poor mechanical reliability, conductivity degradation, the potential for silver migration and the high cost of silver powder.

To address the needs of the Electronics Industry, the Foster-Miller, Inc (FMI) partners developed a new technology, dubbed ECORAP (electrically-conducting, oxidation-resistant adhesion promoter), to enhance the performance and reduce the cost of ICAs. The key advantage of ECORAP technology is that it enabled the use of copper particles as the conductive filler, thus replacing the costly and deficient silver filler.

To develop ECORAP technology, the Air Force then granted FMI a Phase II SBIR program. Under the SBIR program, FMI developed and tested a polyaniline-based electrically-conductive corrosion-resistant coating that, when applied to the surface of copper powder, preserved conductivity on exposure to extreme environments (test conditions – 500 hours at 80°C and 80% relative humidity (RH).

The major drawback of this coating process was that the resulting powder was highly agglomerated which made it extremely difficult to mix with an adhesive resin and severely degraded electrical performance of the final product.

FMI through the NCMS CTMA program sought to overcome this issue by carrying out the polymerization of aniline onto the surface of the copper powder in a process which could be scaled up for commercialization. The objectives of the efforts of the CTMA ICA project were to:

·        Demonstrate a process to produce agglomerate-free ECORAP-coated copper powder that was scaleable to production level quantities

·        Develop an ICA based on ECORAP-coated copper powder.

During this ICA project, the team investigated two different methods to apply the ECORAP coating without agglomerating the particles using scaleable processes. The two coating methods were based on vapor phase application and solution processing. The successful demonstration of a scaleable vapor process was pursued with a fluidized bed reactor, while the solution coating process was demonstrated using a stirred-tank reactor. Successful criteria of a scaled ECORAP process consisted of two main targets:  1) coated copper particle size below 15 microns, and 2) demonstration of retention of 80% of the initial conductivity of the coated powder after exposure to 85% (RH) at 85ºC. To assist with the second objective, a novel apparatus was developed through this project for measuring the relative conductivity of bulk powders.

During this project, dozens of attempts were made to produce agglomerate-free coatings with high conductivity retention utilizing both methods. Both processes required the copper powder to be totally free of organic matter and oxidation. However, overcoming these issues was no small feat as the copper powder has a native oxide layer and is highly susceptible to further oxidation in air. The supplier prevents total oxidation of the product by supplying it with a waxy organic coating. Both of these layers needed to be removed in order for the ECORAP coating to be effective, and therefore a process consisting of cleaning and de-oxidizing the powder in an oxygen-free environment was developed and employed.

By the end of the project, copper powder with good retention of conductivity was produced using both processes. However, the main issue for both processes was producing these powder samples free of agglomerates. The fluidized bed process proved to be a much more cumbersome process, difficult to control and scale up, so that eventually the project focused on improving the solution coating process. Once the team demonstrated that samples maintained their conductivity and the results were reproducible, this process was scaled up to produce larger powder batch sizes (100 – 300 grams). Ultimately, however, agglomerate-free powder could not be produced in this way.

Samples of the material were sent to Henkel and Loctite for analysis and adhesive formulation. As part of their effort, Henkel and Loctite simultaneously developed a bismaleimide resin system for the final ICA product. Henkel and Loctite attempted to produce ICAs using the agglomerated powder alone and also in conjunction with silver filler. However, the resin systems they produced containing ECORAP-coated copper powder were not useful as conductive adhesive systems. The particle size of the agglomerated coating was much too large to work with for Henkel and Loctite. Furthermore, the CTMA ICA team now believes that the resin system developed by Henkel and Loctite may not be compatible with the ECORAP coating.

In the final analysis, FMI was able to develop a coating for the copper which is electrically conductive and oxidation-resistant with a reproducible process. The coated powder maintains electrical conductivity through 55 days of environmental testing at 85ºC and 85% RH. However, the issues with conductive particle agglomeration and incompatibility of the coating with the Henkel and Loctite resin system prevented the development of an ICA product. While a final product was not developed through this effort, the groundwork has been laid for development of the ECORAP-coating process and the scale up to a working industrial process using a tank reactor.

Although the ICA project did not develop an alternative to lead-free solder at this time, it is important to mention that other efforts are underway in the Industry to address the many issues around the use of lead-free solder in high-reliability applications. The Joint Group Pollution Prevention (JG-PP) is partnering with the DoD’s Joint Council on Aging Aircraft (JCAA) on a project to generate critical reliability data on circuit cards manufactured and reworked with lead-free and eutectic tin-lead solders for military and space applications. Successful completion of this project will provide baseline data for use in the eventual qualification of lead-free solder in high reliability systems. It is anticipated that this project will reduce technical risk in qualifying lead-free solder, decrease the use of leaded solder manufacturing and sustaining maintenance costs, and reduce pollution without degrading solder quality or performance. For more information visit the JG-PP website at www.jgpp.com