Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1999-09-16
2001-06-26
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S257000, C361S769000, C361S771000, C361S774000, C361S776000, C439S876000, C439S886000, C439S887000
Reexamination Certificate
active
06252175
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to electronic assemblies and testing thereof. In particular, the invention relates to a method of manufacture of protruding, controlled aspect ratio and shape contacts for uses in interconnections of assemblies and testing thereof.
Interconnections which involve protruding electrical contacts are used extensively in packaging of electronics. Pin grid array packages, both plastic and ceramic, housing a variety of semiconductors, use area arrays of pins as interconnect contacts for connection to circuit boards. Pins can be attached to their receiving package conductors by use of a variety of methods. For ceramic packages, pins are inserted into non-reacting brazing fixtures and are then gang-brazed to corresponding conductive terminals on the package. This approach is characterized by significant non-recurring engineering costs and lead times involved in production of the brazing fixture. Plastic pin grid array packages most commonly use pins which are inserted into metallized through holes in a circuit board, while the dimensions of pins and the holes normally chosen to facilitate good contact between the walls of the pins and the coating of the holes. This approach has a disadvantage in that the coated holes and the pins block some circuit routing channels within the circuit board, thus forcing either use of narrow circuit traces, or increase in circuit board area, either of which results in increased costs.
Permanent connection of the pin grid array packages to circuit boards often is accomplished by inserting pins through corresponding holes in a circuit board, the pins protruding to a predetermined length beyond the circuit board. A resulting assembly then is passed through a wave soldering machine, and the pin grid array thus is soldered to the circuit board. Alternatively, a pin grid array can be inserted into a low insertion force or zero insertion force socket for a demountable assembly. Such a socket, in its turn, normally is connected permanently to a board.
A current trend in interconnections is toward face-to-face surface mounting of components to boards and semiconductor chips to substrates. This approach is best accomplished with protruding contact structures on top of (or otherwise protruding from) contact carrying conductive terminals or traces. Conductive terminal arrangements on facing components and substrates are increasingly being made of the area array type, as this allows for larger contact-to-contact separation as compared with components characterized by peripheral arrangement of interconnection contacts.
Pins attached to either ceramic or plastic packages according to the traditional methods are, in general, not appropriate for mounting to patterns of surface contacts on circuit boards, due to pin length variation. For surface mounting, the pins would have to be planarized, which represents an additional expensive step subsequent to pin assembly. In addition, there is a significant cost penalty associated with production of pin-carrying packages with pin-to-pin separations of 50 mils, or lower.
There is currently an increasing need for a low cost method of attaching protruding contacts from conductive terminals, arising from proliferation of surface mountable area array contact packages. Stand-off height of protruding contacts is particularly important when coefficients of thermal expansion of components and of circuit board materials differ significantly. The same is true for attachment of un-packaged semiconductor chips to interconnection substrates. These expansive concerns call for a low cost, high volume method of manufacturing protruding, controlled aspect ratio or shape electrical contacts on top of (or otherwise protruding from) contact carrying conductive terminals, on top of any device or circuit bearing substrate, board material or component, and its applications to surface mount interconnections of devices, components and substrates.
THE PRIOR ART
U.S. Pat. Nos. 5,189,507, 5,095,187 and 4,955,523 disclose a method of manufacturing of controlled height protruding contacts in a shape of wires for direct soldering to a mating substrate. The wires are bonded to terminals without use of any material other than that of wire and the terminals, using ultrasonic wirebonding methods and equipment, which comprises a standard industry technique for interconnecting semiconductor chips to packages. The patents also describe a bonding head which incorporates a wire weakening means for controlling length of free standing severed wires. Vertically free standing wires present a handling problem during assembly, which is addressed in the patents by providing for polymer encapsulation of bonds between the wires and terminals. The polymer coating, which is optional, also compensates for another disadvantage of the approach, namely weak points along the wire, typically the point of contact between the wire and terminal metallization, and in case of ball bonding, in a heat effected zone of the wire just above impression of a bonding capillary. While these patents provide for controlled height contacts, and discuss 2 d to 20 d aspect ratios, in practice they do not assume controlled aspect ratios for all kinds of protruding contacts which are required in various applications. For instance, standard, high speed wirebonding equipment could not handle a 30 mil diameter wire. Therefore, according to these inventions, a 30 mil diameter, 100 mil high contact could only be produced on lower throughput specialized equipment, at higher cost. In addition, a gold wire as described in a preferred embodiment, would have a problem of dissolving in solder during a soldering cycle, which causes long term reliability problems with solder joints. Similarly, direct soldering of copper contacts would in many cases result in undesirable reaction between copper and solder at elevated temperatures. While nickel metal is the material of choice for solder joint reliability, nickel wire can not be used for ultrasonic wirebonding to metal terminals due to its high mechanical strength and passivating, oxide forming properties. Chemical, physical and mechanical properties, as well as permissible dimensions and shapes of the protuberant contacts produced according to this invention are limited to the capabilities and materials choices compatible with known wire bonding techniques.
U.S. Pat. No. 3,373,481 describes a method of interconnecting semiconductor components on substrates by means of dissolving protruding gold projections on the components in solder masses formed on the substrate terminals. The gold projections are formed by compression and extrusion of gold balls against the terminals. This approach is incapable of producing high aspect ratio protruding contacts because of limitations of the extrusion method. In addition, dissolution of gold in solder, as taught by this approach creates a problem due to reliability concerns. The method also limits selection of contact material to easily extrudable metals, like gold.
There are several methods in the prior art for controlled elongation of solder masses between a component and a substrate. The goal is to create a column-like solder shape, preferably an hourglass shape, in order to achieve increased resistance to thermal cycling. To that end, U.S. Pat. No. 5,148,968 discloses a web-like device which upon heating during solder reflow stretches the solder connections, forming such hourglass shaped solder columns. Aspect ratios of the columns are determined by the mass of solder in the joint, dimensions of the solder wettable terminals on the substrate and the component, and by the characteristics of the stretch device. This method is only limited to contact materials which are reflowed during the assembly, and requires external hardware for forming the contact shape, which adds cost of the hardware and increases the process complexity.
U.S. Pat. No. 4,878,611 teaches formation of controlled geometry solder joints by applying controlled volumes of solder to a semiconductor packag
Gaffin Jeffrey
Vigushin John B.
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