Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2002-04-05
2003-04-08
Nguyen, Nam (Department: 1742)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S22400M, C204SDIG007, C118S400000
Reexamination Certificate
active
06544392
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improvements in a process for manufacturing electrical or electronic printed circuit boards (pcb's) used in various electrical apparatus such as, for example, computer equipment including storage and systems. More particularly, the present invention relates to improved methodology and apparatus incorporated into such process employing at least one immersion bath, to reduce the number of pcb failures otherwise derived from failed deposition interfaces.
2. Description of Prior Art
(A) Printed Circuit Board Manufacture:
Printed circuit boards have been in use for many years. They are normally parallel-planed structures having conductive planes alternating with insulating planes. Certain of the internal conductive planes can be used as electrical shields being connected to ground, and other of the internal planes as well as the two external planes on either side of the board are generally used for conducting electrical signals for the purpose of achieving a useful result.
These multilayered boards are manufactured by using sheets of solid FR-4 epoxy laminate with copper foil bonded to the laminate. Conductive patterns are etched in the sheets of copper which are placed between what is called B stage (partially cured) green epoxy sheets. Some fourteen or sixteen sheets are thus sandwiched and are referred to as a “book” which gets laminated in a press with heat, pressure, and vacuum applied. The result is a solid sheet of epoxy laminate with layers of copper patterns trapped inside. Copper is pre-bonded onto the outside of these panels as noted above. Holes known as “vias” or “interstitial interconnects” are then drilled where needed. The cylindrical wall of the hole is thus made from alternating rings of FR-4 and copper. If a connection to a particular inner copper plane is needed, that copper ring is part of the hole wall. If a connection to another copper plane is not needed, then that copper ring is not part of the hole wall because that copper plane at that hole location would have earlier been concentrically pre-etched to a larger diameter during inner layer manufacture (the resultant disconnect being called an “antipad”).
To get copper into the hole properly, the hole has to first be de-smeared, a process which gets rid of the resin in the hole which is a by product of the drilling operation. Friction causes some of the resin to melt and smear over the copper rings. Typically one uses a permanganate solution in the desmearing process which eats away resinous materials. The permanganate is then cleaned out of the hole with hot water rinse. Thereafter the hole is catalysed by seeding the exposed copper rings with negative ions using, for example, palladium-chloride. Next, the board is inserted into a copper immersion bath shown schematically in FIG.
2
. Bath container
200
holds copper solution (not shown). Printed circuit board
202
is coated with negatively catalyzed palladium-chloride
203
which attracts positive ions to its surface as shown. The hole wall (not shown in
FIG. 2
) is thus plated to a copper thickness of approximately 30 microinches thereby conductively connecting certain inner copper planes. In the next step, and, referring to
FIG. 1
, anodic or galvanic plating bath
101
is used with external dc current applied through variable power source
104
, variable resistance
105
and switch
103
(when closed) to drive copper metal out of immersed copper bar
101
into solution, and by electrolysis quickly build a layer of deposited copper onto all exposed copper surfaces of immersed board
102
including the hole wall surface (not shown). This copper deposition is usually allowed to build to three mils thick (diametrically).
Thereafter the external copper planes are etched into the conductive pattern desired using photomasks to protect the required copper circuitry from acid etchants, in a complex procedure familiar to practitioners in this art. Although such procedure will not be discussed in detail, reference is hereby made to copper thieves
305
of FIG.
3
. These electrically-conductive (but otherwise electrically-isolated) pads, or “thieves”, which are not functionally connected to the electrical signal paths of the etched pcb, have been regularly employed to attract copper to them, but only during the anodic or galvanic externally-powered plating process described in FIG.
1
. The purpose of these thieves is to “steal” or attract copper ions away from the functionally conductive features or pads for the purpose of effecting uniform plating, during the galvanic process, of the plateable areas over the pcb in general. Accordingly, the surface areas of each of the functional pads achieves generally well-balanced plating thicknesses in the anodic environment. Finally, the etched pcb with exposed copper conductive pads is protected by depositing either solder or nickel onto the exposed copper, which enhances its shelf life immensely. If nickel is used, the nickel is then immediately preserved by depositing gold thereon. Solutions to problems associated with these solder or nickel/gold deposition procedures are the subject of the preferred embodiment of the present invention discussed hereinbelow.
(B) Problems of Manufacture—Contaminants:
Although pcb's have been in use for some time, the design and manufacture of these boards are still complex and challenging tasks and there still are problems of manufacture to be addressed and overcome. One of the problems to be overcome derives from short “shelf-life” of some of the materials used in manufacture such as copper and nickel, which quickly oxidize if not properly protected. Oxidized surfaces can create manufacturing problems. As an example, consider copper, which is widely used as an electrical conductor. If one observes fresh copper foil coming out of a plating bath it is a very bright, vivid, pink-orange color. Within a short time, perhaps an hour, its color turns toward a chocolate brown as a result of oxidation. Copper takes oxides quickly, i.e., it absorbs moisture because it is porous; thus it tarnishes and forms the oxide layer which is still electrically conductive but is resistant to metallurgical wetting and therefore to the soldering process, and one therefore cannot readily solder to copper-oxide. The longer copper is left untreated, even for a few weeks, the more aggressive an acid flux is needed to remove the oxide in order to accomplish good solderability. There is a similar oxidation problem with the other metal, nickel.
The solution usually applied to this oxidation problem involves rapid coating of the metal with a protective shield to prevent oxidation. If copper has not oxidized, it forms a uniform intermetallic bond or interface with solder (formed from a combination of tin and lead). Thus solder can be a protective shield for copper. Otherwise, the oxide repels the solder so that it beads up and de-wets (as noted above), like water beading up on a waxed surface. Likewise, with nickel, typically one applies “flash gold” to the surface of un-oxidized nickel to prevent the nickel from oxidizing. The gold layer is typically very thin, perhaps in the range of two-ten microinches, and is sacrificed later when solder is applied to the gold-nickel bond. The gold is removed by the soldering process, is dispersed within the solder joint, and the solder is deposited directly against the nickel, again forming an excellent intermetallic bond because of the lack of oxides/impurities on the previously-protected nickel surface.
Other problems of pcb manufacture are derived from operation of the chemical baths in which plating of exposed conductive electrical surfaces on the pcb's is accomplished. As earlier noted, there are at least two kinds of chemical baths used: (1) anodic or galvanic baths—the kind that uses external electrical energy such as a source of dc power to cause a plating result; and, (2) immersion baths—the kind that does not use any external electrical energy to cause a plating resul
EMC Corporation
Gunther John M.
Gupta Krishnendu
Nguyen Nam
Nicolas Wesley A.
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