Copper polyimide system

Flexible circuit boards consist primarily of polyimide-based carriers. The problem of bonding the copper foil on the polyimide carrier has not yet been solved satisfactorily. Due especially to their low bonding strength at elevated temperatures, the production of such materials is very limited. Nevertheless, adhesives for copper-polyimide systems were developed, where one-component epoxy resins (e.g., epoxy-polyester mixtures) and reactive hot melts (e.g., phenolic resin-nitrile rubbers) reached importance. 

Jenson, R. J.,Cummings, J. T., and Vora, H., "Copper Polyimide Materials System for High Performance Packaging", 1984 IEEE Electronic Components Conference, pp. 73-81. 

In general, conductive adhesive pastes are formulated by mixing polymeric resins (such as epoxies, silicones, and polyimides) and highly conductive metallic fillers The most popular combination has been silver (Ag) particles in epoxy resin [2,3,4,5]. The formulas and preparation techniques for conductive adhesives are still imder development. Nickel, with its lower cost than silver, and its comparatively less oxidation and better thermal stability than copper, offers an appropriate alternative to silver for ICA s, while the copper-filled systems may be unstable after exposure to elevated temperatures due to oxide growth on the particle surfaces [6]. 

Printed circuit boards manufacture is aided by the use of KMnO. Alkaline permanganate solution is used to remove resin smeared on the interior hole wall of multilayered printed circuit boards. Additionally the hole wall is etched, resulting in a surface with excellent adhesion characteristics, for electrodeless copper (250). The alkaline permanganate etchback system containing >60 g/L KMnO and 40-80 g/L NaOH at 70—80°C, is effective for difunctional, tetrafiinctional, and polyimide resin substrates, where the level of etchback is direcdy proportional to the immersion time (10—20 min) (251). 

The concept of using decoupled coils has been developed further by Li et al. [19]. In this case, an RF switch was used for time-domain multiplexing of the signals into a single receiver channel. A four-coil system was constructed for operation at 6 T in a wide-bore (89 mm) magnet. Each coil was fabricated by using 17 turns of 50 pm diameter copper wire wrapped around a 180 pm i.d., 355 pm o.d., polyimide-coated fused silica capillary, giving an observe volume... 

STAFSTROMETAL. Model Systems for Polyimide-on-Copper Interface 313... 

This could be proved by solid-state NMR analysis, showing that the T3 content increases with curing time at the expense of the diphenyl T1 content (index means the number of oxygen bridges to other silicon atoms of the considered silicon atom). It was of interest to investigate how far the stress dissipation ability of the seal can be improved without reducing the sealing properties. Moreover, it was of further interest for practical reasons how far these systems are able to be adapted to polyimide surfaces in connection with copper as a counterpart. 

Preliminary tests of using this system as a seal between copper and polyimide showed that a seal strength up to 4.2 N cm could be obtained with systems containing 70 mol% phenyl silane, 27.5 mol% methyl vinyl silane and 2.5 mol% EOS. Fracture investigations show that a cohesion rupture is also dominating in this case, confirming the above mentioned result that the brittleness of the seal is too high. 

An additional consideration for the selection of an adhesive system is that robust bonds must be formed to all surfaces involved in the interconnection. Materials commonly found include metallizations on the substrate and components (e.g., gold, solder, copper, aluminum, and indium tin oxide), polymer substrates and coatings (e.g., polyimide, polyester, epoxy, and acrylic adhesives), and chip passivation layers (e.g., Si02 and SI3N4). Adhesion promoters may be required. 

Fig. 8, films have resulted which cggtain copper oxide or metallic copper surface strata, respectively. The surface resistivities of these films were decreased up to seven order of magnitude relative to nondoped polyimide films. While this procedure yielded a significant increase in conductivity, the measured resistivities fall significantly short of those theoretically expected for the copper and copper oxide domains encountered. This was believed to be due to the poor inherent continuity that evolves during the production of these multilayered systems. Migration efficiency was also rather poor in these systems, because the surface metal-containing deposit accounted for only 20% of the dopant used. 

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