This invention generally relates to materials used in making multilayer circuits.
As used herein, a circuit subassembly is an article used in the manufacture of circuits and multilayer circuits, and includes circuit laminates, packaging substrate laminates, build-up materials, bond plies, resin coated conductive layers, and cover films. A circuit laminate is a type of circuit subassembly that has a conductive layer, e.g., copper, fixedly attached to a dielectric substrate layer. Double clad laminates have two conductive layers, one on each side of the dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern.
Typically, multilayer circuits are formed by laminating, using heat and/or pressure, two or more materials, at least one of which contains a circuit layer, using bond plies. For example, a bondply can be in contact with a circuit layer of each of two double-clad dielectric substrates, when laminated in proper alignment.
In use, a bondply, or a portion thereof, can flow and completely fill the space and provide adhesion between circuits, between a circuit and a conductive layer, between two conductive layers, or between a circuit and a dielectric layer. The one or more of the polymers in a bondply are designed to soften or flow during manufacture of the multilayer circuit but not in use of the circuit. In multilayer structures, after lamination, known hole-forming and plating technologies may be used to produce useful electrical pathways between conductive layers.
The optimum structural design of a bondply in a laminate would involve a composition that is homogeneous throughout and that provides the same electrical, thermal, and mechanical properties (including low dielectric constant and low dissipation factor) as that of the copper clad laminate. A bondply used in the formation of rigid circuit laminates, multilayer circuits, and subassemblies, can optionally comprise a glass fabric saturated with an uncured or B-staged polymer composition, which cures in the circuit or subassembly lamination process. The glass fabric can provide a hard stop to prevent conductors on opposing layers from coming too close to each other and causing low resistance or other problems.
Bond plies and other circuit subassembly materials can contain synthetic organic materials having high carbon and hydrogen contents, which are potentially combustible. Many applications, however, demand that they meet strict flame retardancy requirements such as those mandated in the building, electrical, transportation, mining and automotive industries. To meet such demands, such materials can include additives intended to interfere in various ways with a chemical exothermic chain of combustion.
In particular, compositions for circuit materials can use halogenated, specifically brominated, flame retardant additives to achieve necessary levels of flame retardancy. Alternatively, ‘halogen-free’ circuit materials that have a UL94 flame retardance rating of V-1 or better, especially without bromine or chlorine, can be used, wherein the specification for ‘halogen-free’ in a circuit material is less than 900 parts per million (ppm) of bromine, chlorine, or a combination thereof.
Among halogen-free flame retardants are organo-phosphorous flame retardants with reactive groups (active hydrogens), such as those derived from 9,10-dihydro-9-oxa-10-phosphaphenantrene-10-oxide (“DOPO”), which flame retardants have been used in epoxy resin formulations and laminates, for example, as disclosed in US 2010/0234495. More recently, DOPO-derived flame retardants that do not have active hydrogen groups have been disclosed for use in various formulations, for example, as disclosed in WO 2011/123389 A1 and WO 2010/135398 A1.
Flame retardants in a bondply, however, can impair the desired physical properties or electrical properties of a bondply or other circuit material. It is desirable, therefore, to obtain an improved bondply that has both the desired flame retardance and the desired flow characteristics during lamination of a circuit laminate. In particular, there is a need for a new class of bondply materials having improved fill and flow properties, while having a non-tacky surface before lamination, compared to standard bondply offerings in the industry today.