Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – With programmed – cyclic – or time responsive control
Reexamination Certificate
1999-07-01
2001-05-08
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
With programmed, cyclic, or time responsive control
C205S665000, C205S671000, C205S705000, C204S229500, C204S230600, C204S242000
Reexamination Certificate
active
06228246
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method of removing a metal skin from a through-hole surface of a copper-Invar-copper (CIC) laminate without causing differential etchback of the laminate.
2. Related Art
A copper-Invar-copper (CIC) laminate is commonly included in electronic packages. For example, a CIC laminate may be present within a dielectric substrate. Exterior surfaces of the substrate may be circuitized such that a plated through hole (PTH) passes through the CIC laminate and electrically couples the surface circuitizations. During formation of the preceding electrical structure, laser or mechanical drilling of a through hole that passes through the CIC laminate generates debris along the drilling path. A portion of the debris forms a porous “metal skin” over a CIC surface within the through hole. The predominant constituents of the metal skin are metallic debris derived from the CIC laminate, such as copper, iron, and nickel. The metal skin may also include a relatively small amount of dielectric debris derived from the dielectric substrate.
The metal skin is problematic because its porosity allows processing chemicals, such as acid solutions, to penetrate the pores of the metal skin and become trapped between the metal skin and the CIC surface covered by the metal skin. Subsequent copper plating of the through hole to form the PTH may seal the acid solutions or other chemicals between the metal skin and the CIC surface covered by the metal skin. The sealing of acid solution is a reliability concern. For example, a pocket void may be generated within the Invar layer of the CIC laminate due to formation of a galvanic cell comprising a CIC copper layer, the CIC Invar, and the acid solution. Noting that Invar is an alloy that includes iron (Fe) and nickel (Fe), the galvanic cell causes oxidiation of the Invar and consequent formation of iron and/or nickel ions. Such ions dissolve in the acid solution and are thus permanently removed from the Invar, thereby creating a pocket void in the Invar layer. Such pocket voids impair the structural integrity of the overlaying copper plating and may subject the copper plating to mechanical and thermal stresses.
Another adverse aspect of the metal skin is that the metal skin is an unnecessary interface between the copper plating and the CIC surface covered by the copper plating. This extra interface could fail, mechanically or thermally, even in the absence of pocket voids.
Fabricating an electronic device having a CIC laminate with a drilled through hole may include removing the metal skin that covers the through hole and etch-cleaning the CIC surface beneath the metal skin. Thus, a method is needed for removing the metal skin and etching the underlying CIC surface such that the depth of CIC material etched away is about the same for the copper and Invar layers of the CIC laminate.
SUMMARY OF THE INVENTION
The present invention provides a method for removing a metal skin from a copper-Invar-copper (CIC) laminate, comprising the steps of:
providing a pulse power supply, a conductive cathode, an acid solution, and a dielectric substrate including:
the CIC laminate having an Invar layer sandwiched between a first copper layer and a second copper layer;
a through hole passing through the CIC laminate; and
the metal skin on a through-hole surface of the CIC laminate;
forming an electrochemical structure, including:
immersing the transducer in the acid solution;
electrically coupling the transducer to the power supply;
immersing the conductive cathode in the acid solution;
electrically coupling the conductive cathode to a negative terminal of the pulse power supply;
immersing the CIC laminate in the acid solution;
electrically coupling the CIC laminate to a positive terminal of the pulse power supply;
turning on the pulse power supply and generating a periodic voltage, wherein a period of the periodic voltage includes a pulse voltage for a first time interval followed by a zero voltage for a second time interval; and
dissolving away the metal skin.
The present invention has the advantage of removing the metal skin and etching the underlying CIC surface such that the depth of CIC material etched away is about the same for the copper and Invar layers of the CIC laminate.
The method of the present invention for removing the metal skin is insensitive to the diameter or aspect ratio of the through hole. Thus the method of the present invention has the advantage of being applicable to through holes having a small diameter such as 2 mils or less.
The present invention has the advantage of etching an exposed surface of a CIC laminate in a manner that controls the relative etch rates of exposed copper and Invar surfaces of the CIC laminate.
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Datta Madhav
Galasco Raymond T.
Lehman Lawrence P.
Magnuson Roy H.
Susko Robin A.
Fraley Lawrence R.
International Business Machines - Corporation
Phasge Arun S,.
Schmeiser Olsen & Watts
LandOfFree
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