Wire bond method for angularly disposed conductive pads and a device made from the method

A method and device relating the electrical interconnection of angularly disposed conductive is disclosed. Conventional wire bonding equipment is used to apply a wire ball on a first conductive surface in an electronic assembly. A conductive wire is drawn up vertically and terminated such that the central portion of the wire is proximal the second conductive surface. The electronic assembly is reoriented with respect to the travel of the capillary whereby a stitch bond is defined upon the second conductive surface to define an interconnect wire and a terminal wire portion, which terminal wire portion is removed.

BACKGROUND OF THE INVENTION

The invention generally relates to a method for the electrical connection of angularly disposed surfaces, each comprising one or more electrically conductive pads. The invention further relates to an electronic assembly fabricated from the method. The invention provides a simple and cost-effective method for interconnecting such angularly disposed, electrically conductive surfaces as is required for instance, in the interconnection of a three-dimensional, multi-layer electronic module to a printed circuit board.

There is a continuing demand for higher circuit density in a wide variety of electronic applications such as consumer electronics, space hardware, and military weaponry and hardware. To avoid the two-dimensional limitations of conventional printed circuit boards, industry has begun using high density, three-dimensional, multilayer modules comprised of individual interconnected layers of microelectronic circuitry which occupy very little surface area but provide greatly increased circuit density per given area. The individual layers in the respective modules may comprise bare integrated circuit die, encapsulated integrated circuit die such as NEO-LAYERS, modified prepackaged parts, or stacked commercial off the shelf packaged parts such as ball grid array packages.

Another example of using the “Z-axis” to increase circuit density is the use of a daughterboard/motherboard configuration where one or more printed circuit boards are mounted to a motherboard at an approximately perpendicular angle

Required input/output and power/ground interfaces between a multi-layer module or daughterboard and a printed circuit board are presently provided in different formats including ball grid array or wire bond interfaces. Each of these methods has a measure of inefficiency due to the difficulty, time and expense associated with the design and assembly of such interfaces as well as problems with signal path length and related parasitic inductance.

Providing conductive contact pads on a lateral surface of the multilayer module or daughterboard for interconnection with the printed circuit board is desirable due to ease of assembly, reworkability and shorter lead length. There are very few simple and efficient methods for the interconnection of electronic assemblies with interconnection conductive patterns that lie in a perpendicular plane to each other.

The instant invention provides a simple, cost-effective and reliable method for the electrical interconnection of perpendicular or angularly disposed conductive surfaces using industry standard wire bonding techniques and equipment.

BRIEF SUMMARY OF THE INVENTION

The present invention discloses a method for the electrical interconnection of adjacent, angularly disposed conductive patterns such as may exist where a multilayer module with a conductive pad on a lateral surface is to be interconnected to a conductive pad on, for instance, a printed circuit board to form an electronic assembly.

The method and related assembly have particular application where the materials upon which the angularly disposed conductive surfaces are fabricated have different coefficients of thermal expansion (CTE) or different mechanical or physical properties that could cause failure in the form of an electronic “open” in the event of relative motion as the result of vibration, shock or CTE mismatch between the assembled devices (e.g., a multi-layer stack of integrated circuit die mounted on a FR-4 printed circuit board). The disclosed method creates the necessary electronic connection in the form of a stress relieved conductive path.

The method is briefly described as follows: An electronic assembly is provided having a set of angularly disposed conductive surfaces such as is found where a multi-layer electronic module is bonded to a printed circuit board substrate. A ball bond is attached to a first conductive surface such as found on the printed circuit board using an industry-standard wire bond machine having a threaded capillary and a conductive wire. The wire is drawn and cut to a predetermined length and position proximal to a second, angularly disposed conductive surface.

The second conductive surface such as is found on the lateral surface of a multi-layer electronic module, is then oriented with respect to the capillary of the wire bond machine whereby a central portion of the wire strand is “stitched”, for instance by means of a stitch bond (also referred to as a wedge bond or tail bond) to the second conductive surface. The remaining electrically “open” portion of the wire or “tail” is manipulated so that it can be removed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the figures wherein like numerals identify like elements among the several views,FIG. 1is a preferred embodiment showing an electronic assembly10comprising a substrate such as a printed circuit board15with a first conductive surface20. First conductive surface20is preferably a conductive metal pad or trace suitable for receiving a wire ball from a wire bonding machine.

Portions of first conductive surface20may optionally be insulated by use of a dielectric or passivation layer25

FIG. 1also illustrates a three-dimensional electronic module30mounted on printed circuit board15, with a second conductive surface40, preferably a conductive metal pad or trace suitable for receiving a stitch bond or wire ball bond from a wire bonding machine.

While the representative embodiment shown inFIG. 1is intended to illustrate a three-dimensional electronic module30and a printed circuit board15, it is expressly understood that the claimed invention is not limited to such structures and that the invention may be incorporated into any combination of electronic assemblies including combinations of individual electronic components, integrated circuit packages, multichip modules, bare integrated circuit die, monolithic components or assemblies such as multilayer module-to-multilayer module, printed circuit board to flex cable and the like.

Further, while the representative embodiment shown inFIG. 1illustrates a perpendicular orientation of first conductive surface20with respect to second conductive surface40, the invention is not limited to that particular angular orientation.

As shown inFIG. 2, in the preferred embodiment, electronic assembly10is mounted on an adjustable wire bond fixture50whereby electronic assembly10is disposed at an angle of about 10 degrees to allow more accurate placement of the ball bond in small feature applications.

FIGS. 3 and 4illustrate the method steps of orientation of electronic assembly10with respect to a threaded capillary tube60of a conventional wire bond machine capable of applying a first wire end70of a conductive wire in the form of conductive ball from a small diameter wire such as is commonly used in conventional wire bond equipment, so as to allow the attachment of first wire end70on first conductive surface20.

In the preferred embodiment, a capillary with modified side relief (as shown) is used to allow easier placement of the ball on conductive surfaces that are located very close to adjacent vertical surfaces such as the vertical surface of three dimensional electronic module30.

The wire used to form ball on first wire end70is preferably a gold alloy material such as gold or gold-palladium, owing to its metallurgical and physical properties, but wire materials comprising aluminum, copper or tungsten may be used where suitable.

Turning toFIGS. 5-7, it can be seen that capillary tube60and wire75are drawn vertically along a lateral surface of three-dimensional electronic module30so as to dispose wire75proximal to second conductive surface40.

Capillary tube60is then preferably drawn up over the surface of three-dimensional module30to a non-conductive location78and terminated to define second wire end80and central wire portion90.

In the preferred embodiment of the process of the invention, the capillary function is then set to the “stitch” or wedge bond mode for the next wire bond step.

As reflected inFIGS. 8, and9, it can be seen that electronic assembly10is next rotated approximately ninety degrees from the earlier steps so as to orient second conductive surface40approximately normal to capillary tube60.

FIGS. 10 and 11illustrate capillary tube60defining a stitch bond100at a predefined location along central wire portion90and in electrical connection with second conductive surface40. The stitch bond defines an interconnect wire and105and further defines an unconnected, electrically open terminal wire portion110or “tail”.

FIGS. 12 and 13illustrate a preferred method of removing terminal wire portion110by means of manipulation by tweezers.

The final step of removal of terminal wire portion110results in an electrically connected set of angularly disposed conductive surfaces as shown inFIG. 14. The resulting assembly comprises a robust and rugged electronic device with reliable electrical connections capable of operating where there are high material property mismatches such as CTE.

Additionally, the claims below are expressly intended to encompass structures where conductive patterns exist on more than one plane such as where conductive patterns exist on multiple surfaces of a multilayer module or on two sides of a daughterboard.

Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purpose of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed even when not initially claimed in such combinations.

The definitions of the words or elements of the following claims are therefore defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim.

Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can, in some cases be excised from the combination and that the claimed combination may be directed to a sub combination or variation of a sub combination.

The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the fundamental idea of the invention.