Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-02-27
2001-05-29
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C439S063000
Reexamination Certificate
active
06239385
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to electronic packaging and assembly. More particularly, this invention relates to electronic interconnect technology, specifically for high-frequency and microwave applications.
2. Description of the Related Art
As the operating frequencies of electronic circuits move into the gigahertz range and beyond, the shapes of electronic interconnects become critical to their function. At relatively low frequencies (a “low” frequency is a frequency at which the wavelength of the signal is much longer than the dimensions of the interconnects), simple conductive paths are sufficient and the shapes of those paths have no significant electrical effect. Examples of simple conductive interconnects include: pin-through-hole, wirebonding, tape automated bonding (TAB), leaded surface mount, ball grid array, and flip-chip interconnects.
But at relatively high frequencies, the wavelength is of the same order as, or shorter, than, the dimensions of the interconnects. Simple conductive paths are no longer adequate because of electromagnetic losses and reflections that result from characteristic impedance mismatches between components. This true for both first level (integrated circuit (IC) to package) connections as well as second level (electronic component package to substrate) connections.
One method of connecting components to substrates has been to use surface mount solder interconnects. In general, this process involves placing the electrical contact of an electronic component or substrate, a small amount of solder or solder paste, and a solder wettable pad on a printed circuit board in close proximity. They are then heated until the solder reflows, forming an electrical connection between the solder wettable pad and the electrical contact of the electronic component. Once the solder has cooled, it forms both an electrical and a mechanical connection between the electronic component and the printed circuit board. This process has numerous advantages over other methods of interconnection. First, a large number of components can be interconnected simultaneously. Second, the process is highly repeatable and relatively low cost and is easily adapted for mass production. These interconnections, however, are not generally suited to high-frequency circuits because of electromagnetic leakage and unwanted signal reflections caused by impedance mismatches.
It is known in the art that the high-frequency electrical performance of these interconnects may be improved by surrounding signal interconnects with ground interconnects. For example, a ball grid array signal ball may be surrounded on all sides with ground balls. While this approach improves high-frequency electrical performance, there is still be a considerable amount of electromagnetic leakage between the ground balls. In addition, the characteristic impedance of the interconnect is generally not controlled, potentially causing unwanted signal reflections. The magnitude of both electromagnetic leakage and unwanted signal reflections increase as the operating frequency increases.
In high-frequency applications such as microwave technology in which the electromagnetic leakage and signal reflections must be tightly controlled, interconnects have been made with individually machined coaxial connectors, glass feeds, and semi-rigid coaxial cable. See, for example, U.S. Pat. No. 5,618,205 (Wideband solderless right-angle RF interconnect); U.S. Pat. No. 5,580,276 (Coaxial plug connector component for connection to printed circuit board); U.S. Pat. No. 4,964,805 (Microcoaxial connector having bipartite outer shell); U.S. Pat. No. 4,631,505 (Right angle microwave stripline circuit connector); U.S. Pat. No. 4,577,923 (Microwave integrated circuit and mounting device therefor); and U.S. Pat. No. 4,466,160 (Surface mount type receptacle of coaxial connector and mounting arrangement for mounting receptacle of coaxial connector on substrate). While these types of interconnects have excellent high-frequency electrical characteristics, they consist of many precision made small parts. As a result, they are both expensive to manufacture and not well suited for mass production. Additionally, their relatively large size often prohibits using these interconnects in applications where space is limited.
A first level coaxial interconnect deposited by electroplating and connected through a welding process is also known. See, for example, U.S. Pat. No. 5,347,086 (Coaxial die and substrate bumps). While this type of interconnect has good high-frequency electrical characteristics, it does not meet the need for a low-cost, passively self-aligning electronic interconnect that is easily mass produced.
Accordingly, it is apparent that there is a need for a small, low-cost, passively self-aligning, electronic interconnect that can be mass-produced and assembled with the ease of surface mount solder interconnects while providing the high-frequency electrical performance of individually machined coaxial connectors.
SUMMARY OF THE INVENTION
Generally the invention provides a small, low-cost, passively self-aligning, electronic interconnect that is readily adaptable to mass production. More specifically, the invention provides a surface-mountable coaxial solder interconnect including a substrate, a signal conductor, and an annular conductor. The substrate incorporates an annular pad and a signal pad substantially centered within the annular pad. The signal conductor and the annular conductor include reflowed solder and are wetted to the signal pad and annular pad, respectively.
The invention may also provide a second substrate substantially parallel to the first substrate that includes a second annular pad and a second signal pad substantially centered within the second annular pad. In such a case, the signal conductor is also wetted to the second signal pad, and, similarly, the annular conductor is also wetted to the second annular pad.
Further, the substrate according to the invention may include a primary shielding layer, a secondary shielding layer, and a signal trace arranged in a stripline configuration. In such a case, the primary shielding layer is electrically connected to the annular pad and the secondary shielding layer is substantially parallel to the primary shielding layer and electrically connected to the primary shielding layer. Additionally, the signal trace is electrically connected to the signal pad and located between and substantially parallel to the primary shielding layer and the secondary shielding layer.
The invention additionally provides a method for forming a surface mountable coaxial interconnect. The method according to the invention includes obtaining a mask and a substrate. The mask obtained should define a center cavity and an annular cavity surrounding the center cavity, and may also define an outer cavity in fluidic communication with the annular cavity. The substrate obtained includes a solder-wettable signal pad, and a solder-wettable annular pad surrounding the signal pad. The mask is filled with solder paste and aligned with the substrate such that the center cavity of the mask is in registry with the signal pad. The solder paste is then reflowed, generating solder features that wet both the signal pad and the annular pad.
The method according to the invention may additionally include obtaining a second substrate. The second substrate includes a second solder-wettable signal pad and a second solder-wettable annular pad surrounding the signal pad. The second substrate is aligned with the first substrate such that the solder is in contact with one of the second signal pad and the second annular pad. The solder features are then reflowed forming an interconnect between the substrates.
While the solder is molten, the second substrate may be allowed to passively self-align and gasses generated by the reflow process are allowed to escape from between the annular pad and the signal pad via a provided path.
Other features and advantages of the present invention will
Barnett Ron
Chew Geary L.
Gleason Gerald J.
Nicholson Dean B.
Schwiebert Matthew K.
Agilent Technologie,s Inc.
Gaffin Jeffrey
Mayer Marc R.
Morris Jeremy
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