TECHNICAL FIELD
The present invention relates to integrated circuit packaging and, in particular, to a system and method for mitigation of thermally-induced stress in chip-scale package applications.
BACKGROUND OF THE INVENTION
In use, integrated circuit packages (IC packages) are exposed to a variety of environmental stresses. For example, high ambient temperatures can contribute to heat accumulation in the IC package and consequent shorter life or less reliability. Impact shock can affect internal connections or plastic package integrity. Changes in ambient temperature can cause mechanical stress that may break connections between the IC package and the circuit board upon which the IC is mounted.
Consequently, system designers have devised test protocols intended to eliminate less reliable proposed component designs. Such test protocols are intended to mimic, in an enhanced and typically accelerated form, the stresses that the proposed component would experience in use.
The widening use of ICs in more ubiquitous and numerous applications has increased the electronic and environmental performance demands on packaged integrated circuitry. Even so, the simultaneous trend of rising complexity has made reliability more difficult to achieve.
Temperature variation precipitates material expansion or contraction. All materials do not expand or contract at the same rates when exposed to the same temperature gradient. Thermally induced expansion or contraction in a material is quantified by an attribute known as the coefficient of thermal expansion or CTE. Consequently, when physically connected dissimilar materials expand or contract differently or, have different CTE's, mechanical stresses are induced between the dissimilar materials.
It is well known that IC packages typically have CTE's that differ from the CTE of the board upon which such IC packages are typically mounted. Package durability and package-board connection integrity under rapid temperature variation is one attribute that that is closely scrutinized in evaluating proposed component package designs. In traditional surface mount devices such as quad flat packages or thin small outline packages (TSOPs) the leads and solder joints provide the compliance needed to absorb the mismatch between the package and the board upon which the package is mounted.
In area array devices however, the small joint between the package and the board must absorb the bulk of the stresses arising from the mismatched CTE's between board and package. This is particularly true as circuit complexity increases and such packages exhibit smaller and smaller contacts. Chip scale packages typically exhibit an increased number of contacts as circuit complexity increases. However, the major package surface area across which the increased number of contacts is distributed tends to stay about the same if not diminish. Thus contact density increases and individual contact size decreases. With smaller contact diameters, the distance from the CSP body to the mounting circuit board typically decreases and the physical demands on the contacts increases while their quality becomes critical. In a board-CSP connection, it is the contacts that realize not only the electrical connection, but the physical connection as well and thus the connections provide the bulk of the coefficient of thermal expansion mismatch compliance. Consequently, higher complexity CSPs with smaller contacts may not exhibit sufficient reliability for high demand applications when exposed to rapid temperature variation or “temperature cycling”, as this characteristic is commonly called.
What is needed, therefore, is a technique and system for individual integrated circuit packages packaged in chip scale technology that provides a thermally-efficient, reliable system that performs well at higher frequencies, does not add excessive height to the device and allows production at reasonable cost with readily understood and managed materials and methods but exhibits enhanced performance under temperature variation regimes.
BACKGROUND OF THE INVENTION
The present invention attaches a mount to a CSP to provide a CSP module with improved temperature cycle performance. The present invention can be used to advantage with CSP packages of a variety of sizes and configurations where an array of contacts is distributed on a major package surface. Although in a preferred mode, the present invention will be applied most frequently to chip scale packages that contain one die it may be employed with chip scale packages that include more than one integrated circuit die in any of several configurations whether flip-chip or chip-on-board (COB) or board-on-chip (BOC).
In a module devised in accordance with a preferred embodiment of the present invention, a flexible circuit is attached to a CSP that is comprised of a die attached to a substrate. The flexible circuit exhibits one or more and preferably two metal layers while providing not only an increased stand off dimension but compliance flexibility between the CSP and the board upon which the CSP is mounted. One major side of the flex circuitry is attached to the CSP while the other major side of the flex circuitry exhibits contacts for attachment of the module to a circuit board. The substrate body of the CSP then stands off from the board by the sum of the heights of the CSP contacts, the flex circuitry and the diameter of the contacts distributed along the flex circuitry. Consequently, the forces arising from CTE mismatch between the circuit board and CSP are distributed along a longer axis thus improving temperature cycle performance.
SUMMARY OF THE DRAWINGS
FIG. 1 is an elevation view of a prior art CSP mounted on a circuit board.
FIG. 2 is an elevation view of module 10 devised in accordance with a preferred embodiment of the present invention.
FIG. 3 is an elevation depiction of another prior art CSP mounted on a circuit board.
FIG. 4 is an elevation depiction of a module 10 devised in accordance with a preferred embodiment of the present invention.
FIG. 5 depicts, in enlarged view, the area marked “A” in FIG. 4.
FIG. 6 depicts an interposer employed in a preferred embodiment of the present invention;
FIG. 7 depicts an interposer employed in an alternative embodiment of the present invention;
FIG. 8A illustrates placement of a CSP on a printed circuit board;
FIG. 8B depicts an exemplary placement of a CSP on a printed circuit board in accordance with a preferred embodiment of the present invention; and
FIG. 9 represents an exemplary computer system in accordance with an embodiment of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS:
FIG. 1 is an elevation view of a CSP 12 mounted on a circuit board 14. For purpose of this disclosure, the term CSP is to be broadly construed to include all sizes of array contact IC packages, whether larger BGA's or smaller micro-BGA's and other array contact devices. CSPs include chip scale packages that include one or more integrated circuit die in any of several configurations whether flip-chip or chip-on-board (COB) or board-on-chip (BOC) on a substrate having an array of contacts on a major surface.
In FIG. 1, CSP 12 is shown in a cross-sectional depiction to illustrate IC 16 contained within casing 18. IC 16 is depicted as a die flip-chip with contact bumps 17 bonded to substrate 15, but IC 16 may be connected to the substrate in any of the several modes known in the art including but not limited to flip-chip, chip-on-board (COB) or board-on-chip (BOC). For purposes of understanding the invention, substrate 15A and the lower planar floor 15B of casing 18 should together be considered, for this example to be substrate 15 of the CSP.
Casing 18 exhibits a lid 19 having upper major surface 21 and a lower major surface 20 along which are found CSP contacts 22 each having a height “H-CSP” as shown more clearly in later FIG. 5.
FIG. 2 depicts a rugged module 10 devised in accordance with a preferred embodiment of the present invention. In module 10, CSP 12 of FIG. 1 is attached to interposer mount 24. In a preferred embodiment, unitary mount 24 is comprised of a flexible circuit and has upper surface 25 and lower surface 26. Within the preferred modes implemented with a flexible circuit, a flexible circuit that exhibits flexibility across its extent, or a rigid-flex that is flexible in sectors and more rigid in other sectors either with or without defined enhanced relief areas may be used. However, other materials that provide stand off and, optionally, compliance attributes, may also be employed to advantage. The invention is best adapted when a mount of unitary structure (a unitary mount) is employed. There are known prior art interposer systems, but such systems do not employ unitary structures such as those disclosed here which, despite having one or multiple constituent layers, are attachable to a CSP as a single aggregate piece thus providing significant manufacturing and cost advantages. For example, a business may acquire CSPs and attach a flexible circuit having appropriate contact fields on each of its two major sides to the acquired CSPs and after contact population of the flex circuit, be able to efficiently provide CSP modules having improved thermal cycling performance without engaging in managing the structural and connective complexities of earlier systems and methods.
In a preferred embodiment, unitary mount 24 will be a flexible circuit that exhibits multiple layers to provide flexibility for high contact count devices and optional opportunities to mitigate ground bounce phenomena by, for example, balancing signal and ground in custom applications. An array of contacts 28 is distributed across the lower surface 26 of mount 24 to provide connective facility for attachment to circuit board 14. Typically, contacts 28 will be solder balls or other substantially spherical contacts most often comprised of solder.
FIG. 3 depicts a prior art CSP device 13 comprised of a die 16 flip chip bonded to substrate 15. Optional underfill 21 is shown at the juncture of die 16 and substrate 15. CSP 13 is attached to board 14 through the array of contacts 22. FIGS. 1 and 3 are meant to depict only a couple of examples of the many types of CSPs that may be employed with the present invention. Additional, but non-limiting examples include, CSPs comprised of: a die flip chip bonded to a substrate, underfilled and overmolded with plastic, a die mounted face up and wire bonded to a substrate, and a die mounted face down and wire bonded to a substrate as well as the other variations of die and array contact populated substrates known to those in the art as array devices or, in this disclosure as CSPs.
FIG. 4 is an elevation depiction of another module 10 devised in accordance with a preferred embodiment of the present invention. As shown in FIG. 4, CSP 13 is mounted on mount 24 which is populated with contacts 28 which are connected to circuit board 14. Consequently, both die 16 and substrate 15 are set off from board 14 by the thickness of CSP contacts 22, contacts 28 and mount 24.
FIG. 5 depicts in enlarged view, the area marked “A” in FIG. 4. As shown in FIG. 5, the distance or stand off (SO) from the lower major surface 20 of substrate 15 to the surface 30 of board 14 includes the height “HCSP” of CSP contacts 22 plus the thickness “HI” of mount 24 plus the height “HR” of contacts 30.
In typical applications, board 14 is composed of what is known as FR4 laminate that is familiar to those of skill in the art. However, board 14 may be any circuit or other board upon which a CSP is mounted. The thermal expansion coefficient for typical FR4 is as follows in Table 1.
TABLE 1
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Coefficient of Thermal Expansion
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x-axis14 ppm/° C. Ambient to the transition
temperature (Tg)
y-axis13 ppm/° C. Ambient to the transition
temperature (Tg)
z-axis175 ppm/° C. Ambient to 288° C.
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FIG. 6 depicts one type of mount 24 that may be employed in a preferred embodiment of the present invention. Depicted mount 24 is a two metal layer flex circuit of unitary structure having contact sites 32A and 32B on upper and lower major sides 25 and 26 respectively through which are conveyed signals and current between exemplar CSPs 12 or 13 and board 15.
FIG. 7 depicts another substrate mount 24 as may be employed in a preferred embodiment of the present invention. Depicted substrate mount 24 is of unitary structure and exhibits expansion relief channels 34 that provide particular areas for dissipation of thermally induced expansive forces. The mount 24 shown in FIG. 7 may also be comprised of rigid sections R delineated from each other by relief channels 34 but interconnected with a flexible base such as polyimide.
FIGS. 8A and 8B compare a first order heuristic illustration of relative deformative forces between substrate 15 and board 14 for the case of FIG. 3 (as illustrated in FIG. 8A) and in the case of module 10 shown in FIG. 4. FIG. 8A designates an arbitrary fixed point a1 on lower surface 20 of substrate 15 for the CSP shown in FIG. 3 and a fixed point b1 on upper surface 30 of board 14 with a1 being directly above b1. The illustration of FIG. 8A postulates relative movement of point a1 to point ax relative to point b1 on upper surface 30 of board 14 when the CSP of FIG. 3 is exposed to a thermal gradient of arbitrary characteristic. The degree of movement is delta (Δ). Those of skill will recognize that neither the absolute characteristic of the thermal gradient nor the absolute magnitudes of the distances illustrated are important to the exposition here offered to illustrate, in magnified form, thermally induced stresses when the stand off increases between board 14 and substrate 15. Therefore, point a1 has moved relative to board 14 by Δ. Thus an imaginary angle θ1 describes the degree of angular movement and may be considered a proxy for the amount of mechanical stress induced by the thermal gradient.
In FIG. 8B, the same thermal gradient is applied to the system shown in FIG. 4 and the relative displacement of a1 is considered in light of the greater distance SO between substrate 15 and board 14. Thus, a1 moves again to ax but the angle θ2 now describes the relative displacement between board 14 and substrate 15. Angle θ2 may be described by Equation 1:
where, Δ is the lateral movement of the substrate 15 with respect to the board 14, HCSP is the height of the CSP contacts, HI is the thickness of the interposer mount, and HR is the height of contacts 30.
As is shown, angle θ2 is less than angle θ1 for the same degree of absolute displacement between substrate 15 and board 14. Thus, the displacement is less per unit of stand off. Those of skill will further note that this displacement is now distributed across the three structural features CSP contacts 22, mount 24 and contacts 28 rather than being concentrated in CSP contacts 22.
As can be appreciated by those of skill, embodiments of the present invention may be implemented in various electronic devices. FIG. 9 depicts a computer system 200 including a processor 210. Processor 210 is coupled to a bridge logic device 220 via a host bus 225. Host bus 225 preferably controls the flow of data between processor 210 and other devices in system 200. Bridge logic 220 further couples to an I/O controller 230 via system bus 235 and solid state memory 240 via memory bus 245. I/O controller 230 may include connections to various I/O devices. For example, storage device 250 may be an integrated drive electronics (IDE) type hard disk drive, and therefore its connection to I/O controller 230 is preferably via an IDE bus as shown. Memory 240 may be implemented using variety of ways, such as, solid state dynamic random access memory (DRAM) or static random access memory (SRAM). In general, the various devices in the computer system 200—i.e., processor 210, bridge logic 220, I/O controller 230, and memory bus 240—are packaged ICs. Accordingly, each of the devices in computer system 100 is capable of being mounted on a printed circuit board according to the methods disclosed herein.
Although the present invention has been described in detail, it will be apparent to those skilled in the art that the invention may be embodied in a variety of specific forms and that various changes, substitutions and alterations can be made without departing from the spirit and scope of the invention. The described embodiments are only illustrative and not restrictive and the scope of the invention is, therefore, indicated by the following claims.
Patent Application
20060033187
