Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
2000-08-11
2002-05-14
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S180220
Reexamination Certificate
active
06386435
ABSTRACT:
BACKGROUND OF THE INVENTION
Some circuit boards include component mounting locations formed by circuit board pads or contacts. On such a circuit board, the circuit board pads typically connect to vias (plated through holes) through stringers, which are portions of metallic etch on the surface of the circuit board. A typical stringer is approximately 0.007 of an inch (“7 mils”) wide and can be between 0.000 to 0.007 inches in length.
Mounting a circuit board component (i.e., an integrated circuit or IC, a small circuit board portion, etc.) to a circuit board mounting location typically involves soldering contacts of the component (e.g., leads, balls, pins, pads, etc.) to the circuit board pads. Some soldering approaches involve placing a small portion of solder paste on the top of each circuit board pad prior to soldering the component contacts to the circuit board pads. One such approach involves applying solder paste through a stencil that defines multiple solder paste apertures having circular cross-sectional shapes. This approach deposits small portions of solder paste on the circuit board pads through the solder paste apertures. The solder paste portions melt during the soldering process in order to form solder joints between the contacts of a circuit board component and the circuit board pads.
In general, the aperture placement mirrors the placement of the circuit board pads within the circuit board mounting location such that the portions of solder paste are deposited precisely on the circuit board pads. Additionally, diameters of the circular apertures are at least as large as those of the pads in order to fully cover each circuit board pad with solder paste.
The solder paste generally includes non-solder elements (e.g., flux, binders, etc.) which dissipate during the soldering process (e.g., vaporizes, separates from the solder and subsequently washes away, etc.) thus reducing the volume of material forming solder joints. To compensate for this volume reduction, some circuit board manufactures implement a technique known as “overprinting” which involves the use of stencils having apertures with diameters that are slightly larger than the diameters of the circuit board pads. Such stencils deposit portions of solder paste that are slightly wider than the circuit board pads with the expectation that, as the portions shrink during the soldering process, (i) the solder not covering the pads will pull back over the pads due to surface tension, and (ii) adequate amounts of solder will be left between the contacts of the component and the circuit board pads in order to form reliable solder joints.
There are some footprint designs (i.e., layouts of pads, vias, stringers and other connecting etches on the surface of the circuit board) that maximize particular circuit board structure dimensions (e.g., pad areas, drill sizes for vias, etc.) in an attempt to maximize circuit board reliability. When these more robust footprint features are repeated within a mounting location (e.g., side-by-side), the direct radial clearance (DRC) between adjacent features can be very small (e.g., 6.8 to 7.0 mils). Accordingly, overprinting on such footprints can be difficult or impossible to implement without substantially increasing the likelihood of forming solder shorts between footprint features intended to be electrically isolated (i.e., across DRC points of tangency).
SUMMARY OF THE INVENTION
Unfortunately, there are deficiencies to the above-described conventional soldering approaches. In particular, such approaches often suffer from unintended solder migration during the soldering process. For example, on some circuit boards, the solder mask (i.e., a protective laminate coating provided by circuit board manufacturers to cover conductive and non-conductive surfaces of the circuit board which are not intended for soldering) may be positioned such that a portion of a stringer is left uncovered by the solder mask (e.g., due to minor inaccuracies in aligning the solder mask with the circuit board but which are within tolerance). This exposed stringer portion can wet and draw solder from the portion of solder paste placed on the adjoining pad. Accordingly, the solder can migrate away from the pad, and along the stringer toward a via that connects to the other end of the stringer. In some situations (e.g., a short stringer, a porous solder mask, etc.), the solder can migrate almost completely from the pad and into the via cavity. The end result is often a poor solder joint between the pad and the corresponding component contact which provides an intermittent electrical connection, or in some cases a bad solder joint that provides no electrical connection.
In some situations, overprinting tends to promote solder migration since overprinting provides larger amounts of solder paste. As such, during the soldering process, an exposed stringer leading from a circuit board pad to a via may draw solder away from its intended location between the circuit board pad and a component contact, and toward an unintended location that causes a short (e.g., a pad-to-pad short, a via-to-via short, etc.).
Weak solder joints, bad solder joints and shorts lower manufacturing yields and are typically costly to debug and repair. By way of example, a ball grid array (BGA) component, which has an array of contacts (e.g., balls) that solder to a corresponding array of circuit board pads, typically limits visual access to the solder joints between the BGA component and the circuit board. Accordingly, visual detection of a poor solder joint or a solder short underneath the BGA component typically involves scanning (e.g., X-raying) the hidden solder joints using sophisticated scanning equipment, an expensive and time consuming process.
Furthermore, for stencils used on circuit board mounting locations having small pitches (e.g., stencils having aperture diameters of less than 1.27 mm) and that are not oversized to provide overprinting, the apertures have a tendency to clog with solder paste over time. Moreover, solder paste has a usable life after which it becomes unusable and a clogging contaminant (e.g., a skin forms over the paste, solvents evaporate, etc.). Such clogging leads to subsequent depositing of inadequate amounts of solder paste on the circuit board pads resulting in the formation of poor solder joints and low manufacturing yields.
In contrast to the above-described conventional soldering approaches which use a stencil having apertures with circular cross-sections to apply portions of solder paste over circuit board pads, the invention is directed to techniques for distributing solder paste using a tool that defines a solder paste aperture having a non-circular cross-sectional shape. When the non-circular shape coincides with a circuit board pad and at least a portion of a stringer leading to the pad, solder paste is distributed over the pad and the stringer portion through the solder paste aperture. Since the solder paste now resides on the stringer portion, solder is not drawn from the pad toward the stringer portion during the soldering process. Rather, solder that resides on the stringer portion tends to adhere to the stringer portion, while some of the solder volume over the stringer pulls back to join the solder over the pad due to surface tension of the solder. The end result is a robust solder joint between the pad and corresponding component contact. Furthermore, the non-circular shape of the aperture allows for apertures that are larger in size than apertures for stencils that do not implement conventional overprinting approaches thus reducing the likelihood of clogging.
One arrangement of the invention is directed to a solder paste distribution system having a base, a tool holder coupled to the base, and a solder paste distribution tool coupled to the tool holder. The solder paste distribution tool includes a support member that couples to the tool holder, a distribution member that defines a solder paste aperture having a non-circular cross-sectional shape, and a fastener that secures the distribution
Chapin & Huang , L.L.C.
Dunn Tom
EMC Corporation
Huang David E.
Johnson Jonathan
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