Judith Marie Roldan

Evaluation of contact resistance for isotropic electrically conductive adhesives

Electrically conductive adhesives are discussed and studied with ever-increasing interest as an alternative to solder interconnection in microelectronics circuit packaging. A similar level of scrutiny that is used to evaluate contact resistance performance for interconnections made with solder and separable connectors is necessary for electrically conductive adhesives. Experience with solder interconnection and separable connectors shows low initial contact resistance of less than 10 m/spl Omega/ when bulk conductor material is minimized in the measurement scheme. Stability is typically determined to be less than a 5-10 m/spl Omega/ change as a function of stress. The main intent of this study is to characterize the electrical contact resistance performance of joints made with isotropic electrically conductive adhesives. A copper comb pattern test vehicle was designed and fabricated using 0.25-mm thick lead frame material. The plating finishes that were applied to the copper substrate included a palladium alloy, gold, tin, and nickel. Test samples were made with several electrically conductive adhesives. Samples consisted of two comb patterns bonded to each other making a gang of 40 lap joints. Variables from circuit packaging such as coefficient of thermal expansion mismatches are purposely avoided in this study. Contact resistance measurements were made initially and as a function of time during environmental tests. Stresses included thermal cycling, thermal aging, and temperature and humidity conditioning. The stability of electrical contact resistance is shown to be influenced by both plating metallurgy and the conductive adhesive itself. Contact resistance equivalent to solder is possible with some electrically conductive adhesives on appropriate metallurgical finishes. Mechanically, adhesive joints are less robust than solder joints, and therefore care must be taken to eliminate or minimize the effects of mechanical loading. >

Laser‐Assisted Seeding for Electroless Plating on Polyimide Surfaces

Excimer laser pulses with wavelengths of 248 and 308 nm were used to selectively seed Pd on polyimide (PI) surfaces, making them suitable for electroless plating. This novel seeding process for insulating materials is accomplished with the sample immersed in the seeding solution, occurs only on the areas of the substrate that are illuminated (through the liquid) by the laser light, and does not require prior treatment of the surface. The seeding solution is transparent to the laser light and the metal deposition occurs as a consequence of the photoabsorption in the solid. This leads to electron transfer from the solid film into the solution and reduction of the Pd ions in contact with the surface. The Pd content of the seeded samples increased with the number of pulses, but was independent of repetition rate. The deposition rate of Pd did not exhibit a significant dependence on wavelength, in agreement with UV absorption spectra of PI and a single photon absorption process for electron excitation to allowed unoccupied states. As for the PD distribution, the deposits consisted of islands with distributions that depended on surface properties as well as on laser-material interactions. Sufficient PD seeds for uniform electroless plating of Cu and Co were attained after 3000 pulses at fluences ≃30 mJ/cm 2 . Although these fluences are much lower than those used for ablation of PI under water, distinct kinds of surface roughness were observed depending on the laser light and on the different types of PI

Mechanisms of electroless metal plating. III: Mixed potential theory and the interdependence of partial reactions

Electroless plating reactions are classified according to four overall reaction schemes in which each partial reaction is either under diffusion control or electrochemical control. The theory of a technique based on the observation of the mixed potential as a function of agitation, concentration of the reducing agent and concentration of metal ions is presented. Using this technique it is shown that in electroless copper plating the copper deposition reaction is diffusion-controlled while the formaldehyde decomposition reaction is activation-controlled. Values of the kinetic and mechanistic parameters for the partial reactions obtained by this method and by other electrochemical methods indicate that the two partial reactions are not independent of each other.

Corrosion and Protection of a Conductive Silver Paste

One of the possible uses for a conductive paste is as an adhesive in interconnect technology that could replace PbSn solder. The interconnections are expected to perform under a variety of environmental conditions, and with an applied voltage. Thus knowledge of their corrosion and dissolution resistance is of utmost importance. This is a study of the dissolution and protection of polymer/metal composite films, prepared with a high loading of silver or gold particles. Electrochemical tests were conducted in a droplet of triple-distilled water with or without benzotriazole (BTA) and BTA derivatives. Results indicate that, in spite of some protection obtained by the polymer, silver paste dissolution at high anodic potentials is rapid, reaching values of 10 -1 A/cm 2 , which corresponds to a catastrophic silver removal rate of at least 35.6 nm/s. With a reservoir of azole in the corrosive environment, this rate can be reduced by up to five orders of magnitude. This azole effect greatly reduces the probability of electrolytic silver migration, but the Ag dissolution rate is still higher than the anodic activity shown by Au paste under the same conditions.

Tailoring the surface morphology of polyimide for improved adhesion

Semiflexible polyimide structures are not amenable to good adhesion because of their a) spontaneous orientation of the polymer chains parallel to the film substrate during curing, b) formation of an ordered skin, and c) smooth surface topography. We briefly discuss these structural features with regard to metal-on-polyimide (metal/Pl) adhesion. A method is proposed to improve adhesion by tailoring the surface and bulk morphology of the Pl to circumvent these properties. In this method, different precursors of the same polyimide (PMDA-ODA) are blended. Phase separation includes spontaneous roughening of the Pl surface. This novel technique reduces the extent of chain orientation, gives rise to topographical and morphological surface heterogeneities, and produces a discontinuous ordered skin. A variety of topographical features with nanoscale dimensions are produced that range from “mounds” to “dimples.” The process does not alter the overall chemical composition of the Pl, occurs spontaneously, and is extendable to other polyimides or polymer systems. Threefold and ninefold enhancements of adhesion over that of a conventionally cured PMDA-ODA film are obtainable for electroless and vapor-deposited Cu on Pl, respectively.

Polymer/metal composite for interconnection technology

In this paper, we report on the the rheological, electrical and mechanical properties of paste which is composed of a thermoplastic polymer, solvent and silver particles, and the resulting polymer/metal composite (PMC) which forms after the solvent from the paste has dried. We will refer to the two states as paste and PMC. The paste properties indicate the applicability of the material for fine feature size interconnects such as required for flip-chip-attach. The adhesive and electrical properties of PMC demonstrates that this material will be suitable as an interconnect media in high performance applications. Furthermore, the system chosen is reworkable and the bonding process is volatile-free (i.e. the solvent may be completely removed prior to the bonding step).

New high conductivity lead (Pb)-free conducting adhesives

Solder interconnection technology is currently in need of alternatives to address environmental issues associated with lead (Pb) abatement, and elimination of fluxes and flux cleaning solvents, and technical challenges related to extending to fine pitch assembly. Electrically conducting adhesive technology is one of the alternatives being actively considered in this context. The most common conductive adhesive used today is an epoxy resin filled with fine silver particles. Silver particles provide electrical conduction, while epoxy provides adhesive bonding of the components to a substrate. This material has several limitations such as low electrical conductivity, low joint strength, increase in contact resistance upon thermal cycling, and silver migration. In order to overcome these limitations, a new formulation is proposed based on alternative Pb-free conducting filler powders and tailored polymer resins. This new material provides a metallurgical bonding as well as polymer adhesive bonding leading to an increase in joint strength and in electrical conductivity. Several potential applications of these adhesive materials such as glass-to-board connection in LCD packaging, SMT package assembly to PCB, and direct chip attachment to a high density card are discussed.