Compositions: coating or plastic – Coating or plastic compositions – Corrosion inhibiting coating composition
Reexamination Certificate
2000-05-31
2002-04-16
Brunsman, David (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Corrosion inhibiting coating composition
C106S287200, C106S287210, C252S079100, C252S079400, C427S096400, C427S098300, C427S327000, C148S269000, C148S274000, C216S013000, C216S106000, C216S108000
Reexamination Certificate
active
06372027
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process and compositions for promoting adhesion between an inorganic substrate and an organic polymer. The invention mainly finds application in the manufacture of printed circuits for electronic use. More particularly, the invention relates to a process for promoting adhesion between the copper surfaces of printed circuits and a polymer resin.
Printed circuits have been used for a long time in the manufacture of electronic equipment. Printed circuits constitute the background to electronic components and ensure the necessary interconnections between components.
Printed circuits comprise a polymer substrate, generally formed from a glass fibre reinforced epoxy resin, or a phenol, polyamide, polyester or teflon resin or other resins, covered with thin sheets of copper on one or both outer surfaces.
The pattern of the circuit is constructed by protecting some areas of the copper sheet with a layer of organic polymer (known as a photoresist) and removing copper from those areas which are not protected using an etching treatment.
With this process it is possible to obtain single-sided and double-sided circuits which offer a rather limited density of interconnections. In order to overcome this limitation multilayer printed circuits are currently widely in use.
In the manufacture of multilayer printed circuits, circuits with two thin surfaces (known as internal layers) are prepared, and are then stacked, separating them with layers of a dielectric consisting of an only partly polymerized resin (known as a prepreg), and then laminated under the combined action of pressure and heat. Multilayer printed circuits obtained in this way have various circuit planes contained within them, and are therefore able to provide a high density of interconnections. The conductive elements of the various circuit planes are connected together by making a suitable grid of interconnection holes which may be through-holes, blind holes or embedded holes; the latter only connect internal layers with each other, but are always constructed before the stacked layers are laminated. The holes are subjected to a chemical cleaning treatment (known as desmearing) to remove the drilling residues present on the hole walls and in particular on the internal layers of copper. Subsequently the walls of the holes are coated with metal, thus making the electrical connections between the different planes in the multilayer circuit.
A typical problem with multilayer circuits is that of possible delamination between the copper surfaces in the conducting patterns of the internal layers and the layers of resin in the dielectric. In order to overcome this problem by promoting adhesion between copper and the resin, copper oxide has been used as a promoter of adhesion for many years. Processes which make it possible to obtain “black oxide” and “brown oxide” are commercially available. These processes use solutions based on sodium chlorite and sodium hydroxide, at high temperatures. With these processes a very porous layer of copper oxide is obtained, characterized by a high surface area. During the stage of laminating a multilayer circuit, the layer of oxide interacts with the resin of the dielectric ensuring high adhesion and thus preventing subsequent delamination problems. Depending on the type of process used it is possible to obtain delamination resistance values (known as peel strength) of between 0.5 and 1 kg/cm. In particular brown oxide makes it possible for greater adhesion to be obtained and is therefore the more widely used.
The first patents describing a process for obtaining black oxide, for decorative purposes, are U.S. Pat. Nos. 2,364,993, 2,460,896 and 2,481,854. The application of copper oxide in the field of printed circuits has been described in various patents, e.g. in U.S. Pat. Nos. 2,955,974 and 3,374,129.
Although copper oxide is able to overcome the problems of delamination, its use in the manufacture of multilayer circuits is not however problem-free, the most important of these problems being known as “pink ring”. Multilayer printed circuits are subjected to high mechanical stresses during drilling and this causes microdelamination in the areas surrounding the holes, at the interface between the oxidized copper surface and the layer of dielectric. During subsequent treatments for cleaning the hole walls (desmearing) and depositing copper using chemical and electrolytic means, the hole walls come into contact with acid solutions which seep by capillarity into those areas where microdelamination has occurred and cause the layer of oxide, which is soluble in acids, to be dissolved. Dissolution of the oxide layer in the areas surrounding the holes exposes the surface of the underlying copper, which is of a pink colour, hence the name “pink ring” given to this phenomenon.
Pink ring is often only an aesthetic problem, but sometimes it also constitutes a functional problem. In any case it is a very serious problem, as multilayer circuits showing evidence of this problem are commercially regarded as scrap. The pink ring problem is very difficult to control; by operating on various parameters in the process for the production of multilayer circuits the problem can sometimes be minimized, but it cannot be eliminated. Also, occasionally and without any possibility of control, pink ring can occur to an extent which is unacceptable.
Various processes which in general provide for the use of reducing and alkaline solutions which are capable of reducing copper oxide to cuprous oxide and/or copper metal, which are insoluble in acids, have been proposed in order to overcome the pink ring problem. In particular, a process described in U.S. Pat. No. 4,642,161, which provides for the use of aqueous aminoborane solutions, in particular dimethyl aminoborane, which make it possible to reduce copper oxide to metallic copper, is widely used. This process is effectively able to eliminate the pink ring problem and greatly increase adhesion between the copper surfaces and the polymer resin layer. In the case of epoxy resin it is possible with this procedure to obtain peel strength values of between 1 and 1.5 kg/cm. This process does however have some disadvantages. The first disadvantage lies in the fact that it is quite a complex process, which requires many treatments. The second disadvantage lies in the difficulty of controlling the process, given that the copper oxide reduction reaction involves very variable times depending upon the design of the circuit. The final and most serious disadvantage lies in the very high cost of this process.
Processes which provide for the use of alternative less costly reducing agents are described in European patent applications nos. 0,321,067 and 0,402,966 and in U.S. Pat. Nos. 4,997,516, 5,492,595 and 5,753,309, but for various reasons these processes have not found industrial application.
Recently, alternative processes have been proposed with a view to promoting adhesion, and these are essentially microetching processes capable of ensuring high surface microroughness on the copper surfaces. These processes make it possible to obtain appreciable adhesion and above all to eliminate the pink ring problem. The adhesion thus obtained is not very great, but for not particularly critical applications it is sufficient to avoid problems of delamination between the copper surfaces and the layer of polymer resin. In addition to their low cost, these processes have the great advantage that they are quite simple and can be used in a conveyor system, making it possible to automate the treatment and ensure high levels of productivity.
The simplicity of these processes has also made it possible to use them for new applications, e.g. to promote adhesion between the copper surfaces of the outer faces of printed circuits (both double-sided and multilayer circuits) and a sheet of polymer resin used to protect the surface of the circuit during the welding stage (the so-called solder mask).
One of these processes is described in European patent application no
Ottria Riccardo
Tomaiuolo Francesco
Alfachimici S.p.A.
Brunsman David
Young & Thompson
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