Patent Application
20130087371
This disclosure relates to apparatus and methods for electronic packaging for integrated circuits (IC) and, more particularly, to apparatus and methods for surface-mount packaging connectors.
ICs play an increasingly important role in modern life. Products which include ICs can be found in many modern products, and as such there is a continuing need to provide more capable products at decreased costs. The famous so called “Moore's law”, for instance, has predicted a trend for the last half a century, namely that the number of transistors placed in an IC doubles approximately every two years.
In conjunction with developments in IC technology, IC packaging design has trended towards smaller, thinner, lighter, and more robust packaging. A critical part of IC packaging is the package connectors to connect the IC package with external circuitry such as, for example, a printed circuit board (PCB). Given the trend of smaller, thinner, lighter IC packaging, reliable package connectors are difficult to develop and have thus lead to intensive research and development.
Electronic package 16 may include several layers as depicted. One of the layers of electronic package 16 is an outer layer, and in particular comes in contact with electrically conductive contact ball 12 at stress points 18. Electrically conductive contact ball 12 is constructed of a chosen material that may have a coefficient of thermal expansion (CTE) that is different than the material and CTE of the outer layer of electronic package 16 at stress points 18. As a result of the CTE mismatch between the two materials, thermal cycling of surface mount packaging connector 10 results in permanent ongoing stress of the solder at stress points 18. That is, because of asymmetrical thermal expansion of electronic package 16 and further external electronic assemblies, such as a connectable PCB board, the solder may start to crack and may eventually break the electrical connection between electrically conductive contact ball 12 and interconnect pad 14. After board level assembly, such a break in the electrical connection between electrically conductive contact ball 12 and interconnect pad 14 may render the product inoperable since surface mount packaging connector 10 serves to electrically connect electronic package 16 with further external electronic assemblies.
Other external stresses, such as mechanical vibrations or shocks, may add additional or separate stresses to surface mount packaging connector 10. Such external stresses are not uncommon in assemblies which include IC packages. For example, an automobile may include an embedded computer system which employs IC packaging including surface mount packaging connector 10. In addition to internally generated heat, the embedded computer system in the automobile may likely experience external heat sources, climate variation, and mechanical shocks such as vibrations. Owing to these effects, surface mount packaging connector 10 as used in such an automotive embedded computer system may experience solder cracking and eventual product failure. Trends towards smaller, thinner, and lighter packaging add further complexity in producing a robust surface mount packaging connector that can reliably withstand these stresses.
In a first implementation a surface mount packaging connector includes an elastic conductor, an interconnect pad, and a conductive layer. The elastic conductor has a top surface. The interconnect pad is electrically coupled to the elastic conductor. The top surface of the elastic conductor is arranged away from the interconnect pad. The conductive layer is on the top surface of the elastic conductor. The conductive layer provides an increased electrically conductive surface area.
One or more of the following features may be included. The surface mount packaging connector may be employed in a land grid array (LGA). The conductive layer may be a metal dust. The conductive layer may be applied as dirty plasma. The conductive layer may have electrical conductivity within at least 10% of copper electrical conductivity. The conductive layer may be composed of a metal such as copper, copper-alloys, gold, gold-alloys, silver, silver-alloys, nickel, and nickel-alloys. The elastic conductor may have thermal conductivity between 2 and 15 watts per meter Kelvin and volume resistivity between 40 and 120 micro-ohms per centimeter. The surface mount packaging connector may be employed on a surface for package assembly. The surface for package assembly may be a PCB. The elastic conductor may be a glue that has been cured and the conductive layer may be formed integrally during the curing of the glue. The elastic conductor may be composed such that the elastic conductor can elongate relative to the interconnect pad by at least 10% while remaining in electrical contact therewith.
In another implementation a surface mount packaging connector includes an electrically conductive contact ball, an elastic conductor, and an interconnect pad. The electrically conductive contact ball has a surface, and the elastic conductor is affixed and electrically coupled to the surface of the conductive contact ball. The interconnect pad is electrically coupled to the elastic conductor. The electrically conductive contact ball is elastically movable relative to the interconnect pad while remaining in electrical contact with the interconnect pad.
One or more of the following features may be included. The surface mount packaging connector may be employed in a ball grid array (BGA) or an embedded wafer level ball grid array (eWLB). The surface mount packaging connector may also include a solderable layer on the elastic conductor at the affixing point between the electrically conductive contact ball and the elastic conductor. The electrically conductive contact ball may be a solder ball and may thereby be affixed through solder processing onto the solderable layer. The conductive layer may be a metal dust. The conductive layer may be applied as dirty plasma. The solderable layer may have an electrical conductivity within at least 10% of copper electrical conductivity. The conductive layer may be composed of a metal such as copper, copper-alloys, gold, gold-alloys, silver, silver-alloys, nickel, and nickel-alloys. The elastic conductor may be a glue that has been cured and the solderable layer may be formed integrally during the curing of the glue. The electrically conductive contact ball may have an electrical conductivity within at least 10% of copper electrical conductivity. The electrically conductive contact ball may be composed of a metal such as copper, copper-alloys, gold, gold-alloys, silver, silver-alloys, nickel, nickel-alloys, tin, and tin-alloys. The electrically conductive contact ball may have an organic surface protection (OSP) layer affixed to another part of the surface of the electrically conductive contact ball. The electrically conductive contact ball may have a protective metal layer affixed to a second portion of the surface of the electrically conductive contact ball, the protective metal layer capable of preventing oxidation of the electrically conductive contact ball. The elastic conductor may have thermal conductivity of between 2 and 15 watts per meter Kelvin and volume resistivity between 40 and 120 micro-ohms per centimeter. The surface mount packaging connector may be employed on the base of a PCB. The elastic conductor may be composed such that movement of the electrically conductive contact ball relative to the interconnect pad can elongate the elastic conductor by at least 10% while remaining in electrical contact with the interconnect pad.
In a further implementation a method for producing a surface mount packaging connector includes providing an interconnect pad, applying an elastic conductor to the interconnect pad such that the interconnect pad is electrically coupled to the elastic conductor, the elastic conductor having a top surface arranged away from the interconnect pad, and affixing a electrically conductive contact ball to the top surface of the elastic conductor. The electrically conductive contact ball is elastically movable relative to the interconnect pad while remaining in electrical contact therewith.
One or more of the following features may be included. The elastic conductor may be composed such that movement of the electrically conductive contact ball relative to the interconnect pad can elongate the elastic conductor by at least 10% while remaining in electrical contact therewith. The electrically conductive contact ball may have an electrical conductivity within at least 10% of copper electrical conductivity. The elastic conductor may have thermal conductivity between 2 and 15 watts per meter Kelvin and volume resistivity between 40 and 120 micro-ohms per centimeter. The electrically conductive contact ball may be composed of a metal such as copper, copper-alloys, gold, gold-alloys, silver, silver-alloys, nickel, nickel-alloys, tin, and tin-alloys. The electrically conductive contact ball may have an OSP layer affixed to another part of the surface of the electrically conductive contact ball. The electrically conductive contact ball may have a protective metal layer affixed to a second portion of the surface of the electrically conductive contact ball, the protective metal layer capable of preventing oxidation of the electrically conductive contact ball.
In yet a further implementation a method for producing a surface mount packaging connector includes providing an interconnect pad, applying an elastic conductor to the interconnect pad such that the interconnect pad is electrically coupled to the elastic conductor, the elastic conductor having a top surface arranged away from the interconnect pad, and applying a conductive layer on the top surface of the elastic conductor.
One or more of the following features may be included. The elastic conductor may be a glue that has been cured. The surface mount packaging connector may also include a solder ball affixed to the conductive layer, such that the conductive layer is a solderable layer, and the solder ball is thereby affixed through solder processing onto the solderable layer. The elastic conductor may be composed such that movement of the solder ball relative to the interconnect pad can elongate the elastic conductor by at least 10% while remaining in electrical contact therewith. The conductive layer may provide an increased electrically conductive surface with electrical conductivity within at least 10% of copper electrical conductivity. The elastic conductor may have thermal conductivity between 2 and 15 watts per meter Kelvin and volume resistivity between 40 and 120 micro-ohms per centimeter. The conductive layer may be composed of a metal such as copper, copper-alloys, gold, gold-alloys, silver, silver-alloys, nickel, and nickel-alloys.
To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
Reference will now be made to figures wherein like structures will be provided with like reference designations. It is understood that the drawings are diagrammatic and schematic representations of exemplary embodiments of the invention, and are not limiting of the present invention nor are they necessarily drawn to scale.
Several glues are both electrically conductive and elastic, and retain such properties even after curing. Such glues can therefore be used as elastic conductor 22, or in other words, elastic conductor 22 may be a glue that, for example, has been cured. Such glues include, for example, MT815 or DA-6534, made respectively by the Lord Corporation and the Dow Corning Corporation. The Tanaka Kikinzoku International company manufactures TS-333 or TS-368 that can also be used as elastic conductor 22.
More generally the electrical properties of elastic conductor 22 should preferably have thermal conductivity between 2 and 15 watts per meter Kelvin (W/(m·K)) and electrical resistivity between 40 and 120 micro-ohms per centimeter (μΩ/cm). More particularly, while the preference of between 2 and 15 W/(m·K) describes thermal conductivity, it more generally is used to describe the likely electrical conductivity of elastic conductor 22. That is, low thermal conductivity generally directly relates to poor electrical conductivity.
Conductive layer 32 may be a solderable layer. A solderable layer is capable of being soldered to with a solder, while also not allowing the solder to penetrate through the solderable layer during soldering. Thus after the glue is cured conductive layer 32 may be such that solderable particles are firmly but elastically connected to elastic conductor 22. If conductive layer 32 is generated by curing the glue, conductive layer 32 is preferably solderable such that conductive layer 32 does not completely dissolve with solder processing. Moreover, to the extent that conductive layer 32 is soldered to directly, the conductive layer must remain sufficiently distinct to retain its conductive properties after soldering. Likewise, it must not diffuse into or react with elastic conductor 22, such that elastic conductor 22 becomes inelastic due to the soldering process.
Conductive layer 32 may be composed of highly conductive metal such as, for example, copper, copper-alloys, gold, gold-alloys, silver, silver-alloys, nickel, and nickel-alloys or a mixture thereof. For example, conductive layer 32 may have an electrical conductivity within at least 10% of copper electrical conductivity. Conductive layer 32 may provide a solderable surface. Conductive layer 32 may alternatively or additionally provide an increased electrically conductive surface area as a terminus for connection to elastic conductor 22, which may be used to electrically connect electronic package 16 with further external electronic assemblies such as a PCB. When surface mount packaging connector 30 is employed, for example, in a packaging connector for LGA packages, a larger interconnect pad is usually provided, as compared to interconnect pads on BGAs or eWLBs. For example, conductive layer 32 may be on the order of a 3×5 or a 10×8 square millimeter area as employed in LGA packages. As employed in BGA packages, conductive layer 32 may be on the order of a 0.25 to 1.5 millimeter diameter, for example.
Optimal thickness of elastic conductor 22 is based on the CTE mismatch and distance to neutral point (DNP) between interconnect pad 14, conductive layer 32, and further external electronic assemblies, such as a connectable PCB board. More particularly, the thickness of elastic conductor 22 should be chosen such that excessive elongation of the elastic conductor does not occur. While a thick application of elastic conductor 22 may increase its durability, the durability gained by increased thickness should be balanced with other requirements that push towards a thinner application of elastic conductor 22. Cooling of electronic package 16 may be hampered, for example, as a result of a thick application of elastic conductor 22. Overall package weight, size, thickness and cost may also, for example, be negatively affected by a thick application of elastic conductor 22. Generally when elastic conductor 22 is applied to a larger surface area the thickness of elastic conductor 22 may be reduced. Elongation to break should, preferably, be at least 10%. Given a set of product dimensions, calculation can be made of the likely elongation distances that elastic conductor 22 may experience during the life of electronic package 16 as an absolute dimension or as a percentage of another dimension, such as that of the elastic conductor. Elastic conductor 22 may be correspondingly applied such that the likely elongation of elastic conductor 22 is below 10% while still meeting the absolute dimension of expected movement between the attached devices. As an example, a given likely elongation distance may be 20 micro-meters. Thus a given material to form an elastic conductor may be applied in such a manner that elastic conductor 22 can reliably withstand displacement of about 20 micro-meters. It is foreseeable that some applications may necessitate elongation to break beyond 10%. In other applications, it may be desirable to accommodate greater displacement, or a smaller percentage of elongation through thicker application of elastic conductor 22, keeping in mind the tradeoff between elasticity and thickness described above.
Surface mount packaging connector 30 may also be employed on the base of a PCB. That is, surface mount packaging connector 30 is not limited to surface mount packages, but also to other surfaces needing such an electrical connector, for example for connection to surface mount devices.
Conductive layer 32, which may be applied as disclosed in
Electrically conductive contact ball 62 may be added during the production of surface mount packaging connector 60 before elastic conductor 22 is cured. Thus electrically conductive contact ball 62 is affixed to elastic conductor 22 as elastic conductor 22 is cured. In this manner, a bond between electrically conductive contact ball 62 and elastic conductor 22 can be accomplished without the use of an intermediate conductive layer 32 shown in previous figures.
Electrically conductive contact ball 62 is thus 3-dimensionally flexibly attached to electronic package 16 by way of interconnect pad 14, and elastic conductor 22. A CTE mismatch between electronic package 16, electrically conductive contact ball 62, and further external electronic assemblies, such as a connectable PCB board is therefore less likely to break the electrical connection between electrically conductive contact ball 62 and interconnect pad 14 during thermal cycling, and the product is more likely to continue proper operation despite thermal cycling and mechanical stresses.
Electrically conductive contact ball 62 may be constructed from a material that has high electrical conductivity, for example metals such as copper, copper-alloys, gold, gold-alloys, silver, silver-alloys, nickel, and nickel-alloys or a mixture thereof. A relatively high electrical conductivity may compensate for some of the end-to-end conductivity previously provided in part by conductive layer 32 in previous figures. A Copper-iron alloy, for example copper-iron alloy number 194, may have sufficiently good conductivity in comparison to pure copper when used for electrically conductive contact ball 62. Likewise, a copper-tin alloy, as another example, may also function well, and may be less sensitive to oxidation. Since electrically conductive contact ball 62 is not attached by solder means, electrically conductive contact ball 62 may be constructed from materials that do not melt during soldering such as, for example, copper-based metals. Thus after the curing process electrically conductive contact ball 62 may be firm, but elastically affixed to electronic package 16 by means of elastic conductor 22. In general, electrically conductive contact ball 62 should preferably be compatible with solder processing, and have electrical conductivity that is at least within about 10% of the conductivity of copper.
A person skilled in the art will recognize that combinations of the above exemplary embodiments may be formed. For example, electrically conductive contact ball 62 may be used in combination with conductive layer 32. OSP 72 may be used in combination with conductive layer 32. OSP 72 may be used in combination with electrically conductive contact ball 12. Electrically conductive contact ball 12 may be a solder ball and may be directly affixed to elastic conductor 22 without solder or a conductive layer. Electronic package 16 may be, for example, any type of IC package including, for instance, wafer level packages.
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
