Etching a substrate: processes – Forming or treating fibrous article or fiber reinforced...
Reexamination Certificate
2000-05-22
2002-09-24
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Forming or treating fibrous article or fiber reinforced...
C216S013000, C216S017000, C427S097100, C427S098300, C427S304000, C427S534000
Reexamination Certificate
active
06454954
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to desmear and etchant baths. In particular, this invention relates to new permanganate baths having improved desmear and surface roughening capabilities.
Hole forming operations in resin containing materials often result in the smearing of resin over the interior wall or barrel of the hole. This resin smear is primarily attributable to the generation or utilization of temperatures exceeding the melting point of a resinous component of the material during the hole forming process.
Where holes are drilled in epoxy impregnated fiber glass laminate materials, such as those employed to make printed circuit boards, friction of the drill bit against the material raises the temperature of the bit. Often, drill bit temperatures are generated which exceed the melting temperature of many resin systems. The drill bit thus picks up melted resin on its course through the material being drilled, and this melted accretion is smeared in the barrel of the hole. In laser drilling operations to contact interior conductors in organic insulating substrates, a similar resin accretion or smear can develop on the exposed conductor surface.
While the problem of resin smear on the hole walls may be ignored in some applications, it is at times imperative that it be removed such as in the manufacture of multilayer printed circuit boards. Multilayer printed circuit boards are used for a variety of electrical applications and provide the advantage of conservation of weight and space. A multilayer board is comprised of two or more circuit layers, each circuit layer separated from another by one or more layers of dielectric material. Circuit layers are formed by applying a copper layer onto a polymeric substrate. Printed circuits are then formed on the copper layers by techniques well known to the art, for example print and etch to define and produce the circuit traces—i.e., discrete circuit lines in a desired circuit pattern. Once the circuit patterns are formed, a stack is formed comprising multiple circuit layers separated from each other by a dielectric layer, typically a resin-containing material such as epoxy, epoxy/glass or polyimide. Once the stack is formed, it is subjected to heat and pressure to form the laminated multilayer circuit board. When such a multilayer circuit board is made, holes are formed in the resin-containing material which includes a plurality of parallel planar metallic conductors, with the hole perpendicular to, and communicating with, two or more parallel metallic conductors. It is often desired to metallize the hole walls in order to form a conductive path between two or more of the metallic conductors. In such instances, the resin smear must be removed from the edges of the hole through the metallic conductors if conductive contact between the metallized hole wall and the metallic conductors is to be achieved. Thus, when circuit board holes are drilled through a copper clad base plastic laminate or through a plastic laminate containing internal conductor planes such as in a multilayer circuit board, resin smear on the metallic surfaces exposed to the walls of the holes must be removed to achieve proper functioning of the metallized, or plated, through-holes.
Plated through-holes as described above are useful as electrical connections between printed circuits having metallic conductors on both sides of the plastic laminate or between two or more of the various planes and surface conductor layers in multilayer boards. The electrical and mechanical integrity required for this function can only be attained by insuring complete removal of resinous materials from the entire inner circumference of the portion of the metallic conductor exposed by the hole.
Numerous methods are known for removing resin smear. One approach is a mechanical one and involves channeling a dry or wet stream of abrasive particles through such holes. A similar method is the use of hydraulic pressure to force a thick slurry of abrasive material through the holes. However, these mechanical methods are generally slow and difficult to control and complete removal of smear in all holes in a given circuit board is difficult to achieve.
Typically, chemical methods are used to desmear holes formed during printed circuit board manufacture. For example, acids such as concentrated sulfuric acid (down to about 90 percent concentration) and chromic acid, have been used to remove smeared epoxy resin. The high acid concentration required is very hazardous and requires extraordinary precautions by operators. Such acid desmear also provides undesirably rough holes. In addition, the concentrated sulfuric acid rapidly absorbs water, which limits its useful life span and can cause variations in the immersion times required to desmear the holes. Chromic acid also presents toxicity and waste disposal problems.
The most common chemical resin desmear method uses permanganate, such as potassium or sodium permanganate. For example, U.S. Pat. No. 4,601,784 (Krulik) discloses desmear solutions containing an alkali metal hydroxide, sodium permanganate and from 0.1 to 3.0 moles per mole of permanganate ion of a co-cation selected from potassium, cesium, rubidium and mixtures thereof. The concentrations of sodium permanganate used require the presence of the co-cation. The amount of sodium permanganate used in the baths according to this patent is at least 70 grams per liter of solution.
U.S. Pat. No. 5,019,229 (Grapentin et al.) discloses a method of etching epoxy resin having a high backetching rate with a stable, strong basic alkaline permanganate etching,solution which is stabilized by electrochemical anodal oxidation.
Conventional desmear baths typically require a ratio of active permanganate ion concentration to total manganese concentration (as both manganate and permanganate) of 0.6 or greater. When the active:total ratio falls below 0.6, delamination of the plated metal to the substrate may occur. Such failure manifests itself as loss of metal adhesion or blistering of the base dielectric material. When such baths are regenerated, i.e. when the active:total ratio is adjusted to 0.6 or greater, the bath still results in substrates showing delamination with some of the new, advanced dielectric materials. This situation does not hold true for most conventional laminates, i.e., FR4, FR5 and high Tg reinforced laminates.
Typically, conventional permanganate based desmear baths result in substrates having a highly irregular topology and induced stresses after plating. Thus, it is desirable to provide desmear and etching baths that are effective at removing resinous accretions without providing substrates having highly irregular topology or induced stresses after plating. It is further desirable to provide permanganate etching solutions that are stable and do not require electrochemical anodal oxidation. It is still further desired to provide permanganate based desmear baths that can be regenerated without showing delamination.
SUMMARY OF THE INVENTION
It has been surprisingly found that the alkaline permanganate baths of the present invention are very effective at removing resinous accretion, i.e. desmear, and providing surface roughening. It has been also surprisingly found that printed circuit boards that have been subjected to the alkaline permanganate baths of the present invention have higher peel strengths as compared to those subjected to conventional permanganate baths.
In one aspect, the present invention provides a composition including one or more permanganate ion sources, one or more hydroxide ion sources and water, wherein the hydroxide ion is present in an amount of from about 25 to about 85 g/L and wherein the composition has a total manganese ion concentration of from about 15 to about 70 g/L.
In a second aspect, the present invention provides a method for preparing a resinous substrate for subsequent metallization including the step of contacting the substrate with a composition including one or more permanganate ion sources, one or more hydroxide ion sources
Brewster Wanda Darlene
Ho Tuan Hoang
Cairns S. Matthew
Gulakowski Randy
Piskorski John J.
Shipley Company L.L.C.
Winter Gentle E
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