Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1998-12-22
2002-10-08
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
Reexamination Certificate
active
06462107
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to compositions useful for producing polymer layers on printed wiring boards and other electronic and electrical interconnect devices. In many embodiments, the invention relates to photosensitive, positive-acting, aqueous-processable dry films made from those compositions. The films are particularly useful for forming a permanent dielectric on multilevel printed wiring boards.
BACKGROUND OF THE INVENTION
Consumer demand for smaller, lighter electronic products has caused product manufacturers to require smaller semiconductor packages with increased levels of functional integration. These smaller, highly integrated packages require printed wiring boards having a large number of interconnection paths per unit board area. Frequently, multilayer printed wiring boards are required to achieve the “high density” of interconnection required to support these highly integrated packages.
Multilayer, high density printed wiring boards consist of alternating layers of electrically insulating polymeric material and metallic conductors which have been deposited on a copper-clad glass-epoxy substrate. Connections between metallic conductor layers are made through a plurality of tiny holes or “vias” in the polymer layer located between two conductor layers.
A two layer printed wiring board, for example, can be produced by first etching a circuit board pattern on the upper surface of a copper-clad glass epoxy substrate. An electrically insulating polymer layer is then deposited on the patterned surface, and holes are made through the polymer layer at desired points of conductor interconnection. An electrically conductive layer is then deposited on top of the polymeric layer, with the deposited conductive layer penetrating the vias to form electrical connections between the deposited layer and the circuit board traces on the glass epoxy substrate. A patterned conductive layer is then formed from the deposited conductive layer via a multistep photoetching process, resulting in two levels of circuit traces interconnected by the copper that was deposited within the vias.
Increased board interconnect density requires vias in a circuit board's dielectric polymer layer that are smaller than can be economically achieved by conventional mechanical through-hole drilling. Vias smaller than what can be achieved by conventional mechanical drilling, typically 6 mils or less, are referred to as “microvias”. High density boards currently in production require microvias having diameters of about 3 to 5 mils.
Board manufacturers produce microvias by laser ablation, plasma etching, or photoimaging techniques.
Laser ablation is a sequential via formation technique. Vias are formed one at a time as a laser pulse is directed at a specific area of the circuit board. The etching process is anisotropic, which inherently limits resolution of the produced microvias. Because resolution is limited, and because sequential production processes are relatively slow compared to other processes, laser ablation is not preferred by board manufacturers.
Unlike laser ablation, photoimaging and plasma etching mass produce all of a given board level's microvias simultaneously. Photoimaging techniques are more cost effective for mass producing high density wiring boards than plasma etching, and therefore are preferred by printed wiring board manufacturers for boards having high microvia densities.
In a photoimaging process, microvias are produced by positioning a pattern or “mask” over a photosensitive polymer surface, exposing the masked polymer surface to actinic radiation, and then developing the surface to remove polymer from the board.
If the photosensitive polymer is made insoluble by exposure to actinic light, the coating is said to be “negative acting.” In this case, the developer dissolves the unexposed polymer surface, leaving an image of polymer on the board that is a negative of the image on the mask. On the other hand, if the polymer is rendered soluble by exposure to actinic light, the polymer coating is said to be “positive acting.” In a positive acting system, the developer removes the exposed polymer, leaving a positive image of the mask.
Positive acting polymeric materials are preferred in photopolymer applications because they provide improved resolution and yields.
Improved via resolution occurs because, unlike negative acting materials, positive acting materials do not swell during development.
If used in printed wiring board applications, board yields can be higher using positive acting systems for two reasons. First, if a dust particle is present during exposure in an unmasked area of a board bearing a negative acting polymer, the end result will be that an undesired via will be formed. As the undesired via will provide an undesired electrical connection between adjacent conductive layers of the board, the board is unusable. On the other hand, if the dust particle is present when a positive acting photosensitive polymer is exposed, the end result will be a spot of insulating dielectric material in an undesired location. Often, the presence of a small spot of additional dielectric will not spoil the board. Second, if an undesired spot will spoil a board manufactured using a positive acting material, the coating may be stripped from the board simply by developing under more severe conditions, and the layer can be remanufactured. Alternatively, the spot can be removed by reexposure to light followed by redevelopment of the board. Similar reworking is not practical with negative acting coatings because the coatings are rendered insoluble (and unworkable) by the exposure process. The use of positive acting materials can, therefore, greatly increase board yield. Consequently, board manufacturers prefer to work with a positive acting polymer system.
Another important factor in selecting a polymer system for printed wiring board manufacture is the type of material used to develop photoimaged boards. While many polymers will be soluble in a wide range of organic developers, the use of organic developers may present occupational hazards to workers. Furthermore, disposing of waste organic materials can be expensive. For these reasons it is preferred that polymer systems used in printed wiring board manufacture be developable in aqueous solutions.
Yet another requirement for polymer systems used to form a permanent dielectric layer on printed wiring boards is that the polymer system exhibit good mechanical properties at sufficiently high temperature to withstand subsequent board manufacture and soldering steps. One method to ensure good mechanical properties at high temperature is for the permanent dielectric layer to have a high glass transition temperature. Typically, the circuit boards will be required to withstand temperatures of at least 125 degrees Centigrade, and in some applications, as high as 200 degrees Centigrade. Polymer systems selected for board use must therefore have a relatively high glass transition temperature.
Polymer system selection also depends on the desired method of application of the polymer to the circuit board. Photosensitive polymer systems typically are applied to printed wiring boards as a liquid or as a dry film. In either case, an important requirement is that each cured polymer surface be essentially flat to enable the application of subsequent board layers. Such a flat surface is said to “planar” or possess a high “degree of planarity.”
Liquid polymer coatings are difficult to use in printed wiring board manufacture for at least two reasons. First, liquid coatings are difficult to apply and dry to the uniform thicknesses and degree of planarity desired by printed wiring board manufacturers. This problem is especially pronounced when manufacturing built up multilayer printed wiring boards where alternating layers of conductive and insulating materials are sequentially applied to the printed wiring board substrate. Additionally, liquid coatings can trap foreign objects such as dust particles and continue to trap such objects until the
Fjare Douglas E.
Hoback John T.
Liu Wen-Feng
Rubis Donald E.
Sanchez Paul A.
Baker & Botts L.L.P.
Cain Edward J.
The Texas A&M University System
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