Metal fusion bonding – Process – With condition responsive – program – or timing control
Reexamination Certificate
2002-05-06
2003-10-28
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process
With condition responsive, program, or timing control
C228S248100, C228S023000, C228S035000, C228S041000, C228S043000
Reexamination Certificate
active
06637641
ABSTRACT:
One conventional circuit board includes pads which are formed using nickel and gold immersion technologies. These circuit boards typically include standard copper traces supported by layers of fiberglass (e.g., FR4 fiberglass) which are glued together. Some of the copper traces terminate at nickel/gold surface mount pads, i.e., nickel pads plated with a gold finish, which are configured to solder to surface mount devices such as Ball Grid Array (BGA) devices.
To manufacture a circuit board having nickel/gold immersion pads, a manufacturer typically forms individual circuit board layers, i.e., individual layers of fiberglass having etched copper traces thereon, using an acid etch process. Next, the manufacturer typically glues the individual layers together, as well as cuts and drills edges, grooves and holes to form the section of circuit board material. Then, the manufacturer typically forms nickel pads and nickel plated-through holes by exposing the section of circuit board material to a nickel immersion process, e.g., by placing the circuit board section in a bath containing nickel and other additives that facilitate reliable and consistent pad growth such as phosphates, sulfides, etc. Next, the manufacturer typically provides a protective coating of gold to inhibit nickel oxidation. It should be understood that the manufacturer typically performs additional cleaning steps (e.g., rinsing steps) along the way (e.g., interspersed within the above-described series of manufacturing steps) to clean the section of circuit board material of residues, byproducts and contaminants.
The end result circuit board includes a section of circuit board material which supports a set of virgin surface mount nickel/gold pads (surface mount pads which previously have not been soldered to a circuit board component but which are ready for soldering to a circuit board component) and a set of virgin nickel/gold plated-through holes. A typical virgin surface mount nickel/gold pad includes a layer of nickel which is approximately 200 to 250 micro-inches in thickness, and a top layer of gold which is approximately 6 micro-inches in thickness (e.g., 3 to 8 micro-inches). When circuit board components mount to the virgin pads, molten solder forms solderjoints between the pads and the leads (i.e., contacts) of the circuit board components. During the soldering process, the protective gold finish typically melts away (e.g., mixes with the solder) leaving solder to form intermetallic boundaries between the surface mount pads of the circuit board and the component leads.
Populated circuit boards, which include nickel/gold surface mount pads, occasionally suffer from “Black Pad” defects. A “Black Pad” defect is a flaw (e.g., a fracture) in an intermetallic boundary between a nickel surface mount pad and a lead of a circuit board component that results in an electrically unreliable connection between that nickel pad and that component lead. Such a defect often appears as a pressure-sensitive intermittent electrical connection between the device and the circuit board, i.e., between the nickel pad and the component lead. Studies have shown that “Black Pad” defects can be caused by excessive pad corrosion (i.e., oxidation of the nickel layer) prior to soldering. Such corrosion results in low solder-wettability (i.e., a low affinity for solder) thus providing a weak and unreliable solder joint after the soldering process.
Circuit board manufacturers can take a variety of approaches to handling “Black Pad” defects. One conventional approach involves the manufacturer inspecting each populated circuit board for “Black Pad” defects, and simply throwing away any circuit board having a “Black Pad” defect. Another conventional approach involves the manufacturer inspecting each populated circuit board for “Black Pad” defects, and reworking any circuit board having a “Black Pad” defect, i.e., unsoldering a circuit board component exhibiting symptoms of having a solder joint to a “Black Pad”, cleaning the exposed surface mount pads, and soldering on a new circuit board component. Yet another conventional approach involves the fabrication manufacturer redesigning the board fabrication process to avoid using surface mount pads formed by nickel/gold immersion (e.g., redesigning the circuit board manufacturing process to use bare copper pads, silver pads, palladium pads, etc.).
SUMMARY OF THE INVENTION
Unfortunately, there are deficiencies to the above-described conventional approaches to avoiding “Black Pad” defects. For example, in the above-described conventional approach which involves throwing away populated circuit boards, a significant amount of added value is lost. In particular, some circuit boards may cost several thousands of dollars to make and it may be a significant drawback for a company to bear the burden of regularly writing-off such a cost.
Additionally, in the above-described conventional approach which involves reworking a populated circuit board having a “Black Pad” defect, the rework process does not consistently and effectively repair the intermittent connection caused by the “Black Pad” defect. That is, the intermittent connection is often formed by flaws in the intermetallic boundaries of the nickel layers of the metallic pads and, as such, is not fixed by simply replacing a circuit board component. To the contrary, a metallic pad suffering from a “Black Pad” defect typically has corrosion which extends below the pad surface (e.g., 20 micro-inches below the pad surface) as well as low solder-wetting ability (i.e., low affinity for solder) which does not improve when a new component lead is soldered to the pad. Accordingly, any new solder joint formed on the metallic pad is also likely to be unreliable and prone to failure.
Furthermore, in the above-described conventional approach which involves modifying the circuit board manufacturing process to use other types of pads (e.g., bare copper pads, silver pads, palladium pads), the alternative circuit board manufacturing processes can be more susceptible to other deficiencies which are not present in circuit boards using a nickel/gold immersion processes. For example, circuit boards, which use nickel/gold immersion where the nickel overplates the via copper and forms nickel eyelets, are well-suited to slowing down mechanical expansion of the circuit board in the Z-direction (i.e., circuit board expansion which is perpendicular to the circuit board plane) due to the clamping force provided by the nickel thus avoiding other circuit board drawbacks such as warping, fractures in metallic traces, separation of circuit board layers, etc. Eyelets formed of other metals have not inhibited circuit board expansion in the Z-direction as well as nickel eyelets. Accordingly, eliminating the nickel/gold eyelets and using other metallic eyelets (e.g., copper, silver, palladium, etc.) can provide poorer circuit board expansion results and thus promote other circuit board drawbacks.
The invention is directed toward techniques for manufacturing a circuit board having virgin metallic surface mount pads which involve removing a portion of each virgin metallic surface mount pad (e.g., removing several micro-inches from the tops of pads formed by a nickel immersion process). Accordingly, any corrosion or contaminants which collected within these removed portions are no longer available to promote “Black Pad” defects.
For example, phosphate compounds, which typically reside within nickel immersion baths to control nickel deposition rates, can become incorporated into the nickel immersion pads. In particular, such phosphate compounds can collect near the top surfaces of nickel immersion pads. Although these phosphate compounds in theory are supposed to provide metallic properties, these phosphate compounds may actually operate more like organic contaminants that interfere with formation of healthy solder joints (i.e., may lower solder-wettability of the pads). Removal of these contaminated top surfaces prior to the soldering process promotes formation of robust and healthy solde
Downes Stuart D.
Liang Jin
Chapin & Huang , L.L.C.
Elve M. Alexandra
EMC Corporation
Huang, Esq. David E.
McHenry Kevin
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