Semiconductor chip package using flexible circuit board with central opening

[0002] 1 Field of the Invention

[0003] The present invention relates generally to a semiconductor device. More particularly, the present invention relates to a semiconductor chip package in which an active surface of a semiconductor chip is attached to a bottom surface of a flexible circuit board having a central opening, and in which input/output pads on the active surface of the chip are electrically connected to a circuit layer on a top surface of the flexible circuit board through the opening, so that the package can be reduced to a size on the order of that of the chip.

[0004] 2 Description of the Prior Art

[0005] As semiconductor integrated circuit (IC) chips become increasingly sophisticated, the component density increases as does the number of input/output (I/O) pads per chip, resulting in larger chips. However, there is a continuing need to reduce the size of the semiconductor chip package to meet the need for smaller devices.

[0006] One of the packages that has been developed to meet this need is a ball grid array (BGA) package. The BGA package provides a relatively high surface-mount density and improved electrical performance as compared to a conventional plastic package. A distinct difference in the structure of the BGA package from the conventional plastic package is that the electrical interconnection between the semiconductor chip and a mother board is provided by a printed circuit board (PCB) having circuit patterns and solder balls instead of a lead frame.

[0007] The typical BGA package 10 is illustrated in FIG. 1. As shown in FIG. 1, the chip 11 is attached to the top surface of the PCB 12. The circuit patterns 13 on the top surface of the PCB 12 extend to the bottom surface through vias 15. The circuit patterns 13 on the top surface are electrically connected to the chip 11 by bonding wires 14, while those on the bottom surface are electrically connected to the mother board (not shown) by solder balls 16. Parts of the top surface of the PCB 12, the chip 11, and the bonding wires 14 are encapsulated by resin 17. The body of the PCB 12 is generally made of FR-4, FR-5, or BT (Bismaleimide triazine) resin. FR-4 and FR-5 are fire retardant epoxy glass-reinforced laminates.

[0008] The BGA package 10 is larger than the chip 11 because the PCB 12 used in the BGA package 10 requires an area without circuit patterns 13 to allow the chip 11 to be attached to the PCB 12. Furthermore, the width and the distance between adjacent circuit patterns 13 are limited to 70 μm, respectively. Therefore, it is difficult to manufacture a BGA package when circuit patterns 13 having a small width or a small degree of spacing there between are required.

[0009] It is therefore an object of the present invention to provide a semiconductor chip package of a size on the order of that of the semiconductor chip itself.

[0010] It is another object of the present invention to provide a semiconductor chip package employing a circuit board with fine circuit patterns.

[0011] It is still another object of the present invention to provide a semiconductor chip package which, by its design, prevents liquid resin from overflowing during the manufacture thereof.

[0012] It is still another object of the present invention to provide a semiconductor chip package in which the upper surface of an encapsulant is disposed below the upper surface of terminals.

[0013] It is still another object of the present invention to provide a semiconductor chip package having a coefficient of thermal expansion similar to that of a mother board.

[0014] The present invention achieves these and other objects by providing a semiconductor chip package which comprises a semiconductor chip having an active surface, and a plurality of I/O pads extending across central parts of the active surface. The package also includes a flexible circuit board having an electrically non-conductive flexible tape with a top surface, a bottom surface and a central opening therein. A circuit layer is patterned on the top surface of the flexible tape. The circuit layer has a plurality of circuit traces, bonding pads to which first ends of the traces are connected, and land pads to which the other ends of the traces are connected.

[0015] The active surface of the semiconductor chip is attached to the bottom surface of the flexible tape, so that the I/O pads on the chip are exposed through the opening of the flexible tape. Accordingly, a plurality of metal bonding wires are electrically interconnect the I/O pads and the bonding pads through the opening. Furthermore, the flexible circuit board can have an area nearly equal to that of the semiconductor chip. In other words, the present invention can provide a so-called chip sized package (CSP).

[0016] The I/O pads, the bonding pads, and the metal wires in the package of the invention are all encapsulated. A plurality of terminals are provided on and are electrically connected to the land pads on the top surface of the flexible tape. A polyimide tape is preferably used as the flexible tape of the invention.

[0017] The circuit layer may include multilayered circuit traces in which upper circuit traces are disposed over lower circuit traces with an insulating layer being interposed therebetween.

[0018] The encapsulant may be provided by a method of supplying liquid resin having a certain viscosity into the opening and over the bonding pads. A dispenser is preferably used for the supplying the resin. Moreover, the package may comprise a dam extending on the top surface of the flexible tape radially outward of the bonding pads and around the opening, so that the dam will prevent the liquid resin from overflowing as it is supplied onto the bonding pads. The dam may be also formed by a method similar to that used to form the encapsulant.

[0019] The encapsulant extends a first predetermined distance from the circuit layer, and each terminal extends a second predetermined distance from the circuit layer, the first predetermined distance preferably being less than the second predetermined distance.

[0020] The flexible circuit board may further include a protective layer on the top surface of the flexible tape so as to protect the circuit traces. In addition, the protective layer may have a groove extending between the bonding pads and the land pads. The aforementioned dam can be provided in the groove and rise above the protective layer so that the dam will prevent the liquid resin, used to form the encapsulant, from overflowing. A polyimide layer is preferably used as the protective layer.

[0021] The flexible circuit board may also further include a stiffener on the bottom surface of the flexible tape. The stiffener can be made of a material having the same coefficient of thermal expansion as that of a mother board to which the package will be mounted. Moreover, the flexible circuit board may further include an adhesive layer between the stiffener and the semiconductor chip. The adhesive layer can be made of an electrically non-conductive adhesive material, such as silicone resin or a nonsilicone resin having a low modulus of elasticity.

[0022] These and various other features and advantages of the present invention will be described with reference to the accompanying drawings, wherein:

[0023] FIG. 1 is a cross-sectional view of a conventional ball grid array (BGA) semiconductor chip package;

[0024] FIG. 2 is a partially cut away perspective view of one embodiment of a flexible circuit board having a central opening according to the present invention;

[0025] FIG. 3 is a cross-sectional view taken along line III—III of FIG. 2;

[0026] FIG. 4 is a cross-sectional view of the semiconductor chip package according to the present invention;

[0027] FIG. 5 is a cross-sectional view of a second embodiment of a flexible circuit board for a semiconductor package according to the present invention;

[0028] FIG. 6 is a cross-sectional view of a third embodiment of a flexible circuit board for a semiconductor package according to the present invention; and

[0029] FIG. 7 is a cross-sectional view of still another embodiment of a semiconductor chip package according to the present invention.

[0030] The present invention will now be described more fully hereinafter with reference to accompanying drawings, in which preferred embodiments of the invention are shown. In the drawings, like numerals refer to like elements throughout.

[0031] Referring first to FIG. 2 and FIG. 3, a flexible circuit board 20 has a patterned circuit layer 26 formed on the top surface of an electrically non-conductive flexible tape 21. The circuit layer 26 has a plurality of circuit traces 24, a respective bonding pad 23 connected to each of the traces 24 at one end, and a respective land pad 25 connected to each of the traces 24 at the other end. As shown in FIG. 4, each bonding pad 23 provides an electrical connection between a semiconductor chip 31 and a respective circuit trace 24 via metal wires 33, through a central opening 22. Each land pad 25 provides an electrical connection between a terminal 35 and the respective circuit trace 24.

[0032] The flexible tape 21 can be made of a material such as polyimides, which provide high thermal and mechanical stability. The circuit layer 26, such as a copper (Cu) a gold (Au) layer, or other conductive layer, can be formed into a pattern by etching. Specifically, copper or gold foil is laminated on the flexible tape 21 and then etched so that the pattern of circuit traces 24, bonding pads 23, and land pads 25 is formed. Preferably, the circuit layer 26 has a thickness of 18 μm or 35 μm and the circuit traces have both a width and interspacing of 50 μm. The circuit board 20 of the present invention occupies a comparatively small space because it is flexible, thin, and has fine dimensions when compared to the prior art PCB.

[0033] Multilayered circuit traces may be employed according to the present invention. For example, two layers of circuit traces 46aand 46bare shown in FIG. 5, with an insulating layer 4lbinterposed between the two layers of traces 46aand 46b. The flexible circuit board 40 having double-layered circuit traces 46aand 46bcan be made by the following method. First, a piece of metal foil for forming the lower traces 46ais laminated on a flexible tape 41aas in the above-described embodiment, and then etched. Then, another flexible tape 41bis attached to the lower traces 46a, and then paths or vias (not shown) for allowing the lower traces 46ato be connected with the upper traces 46bare etched in the flexible tape 41b. A second piece of metal foil for forming the upper traces 46bis then laminated on the flexible tape 41band etched. The circuit layer having multiple layers of circuit traces provides for a greater routing capability and better electrical performance.

[0034] Referring back to FIGS. 2 to 4, a protective layer 27 may be formed on the circuit traces 24 in order to protect the circuit traces 24 from oxidation, and a plating layer (not shown) may be formed on both the bonding pads 23 and the land pads 25 in order to enhance their bondability. A polyimide layer is preferably used as the protective layer 27, and nickel (Ni), gold (Au), or other metals may be used as the material of the plating layer (not shown). The protective layer 27 can be formed by coating a polyimide resin on the entire surface of the flexible tape 21, and then etching the resin above portions of both the bonding pads 23 and the land pads 25. The protective layer 27 which is formed on the entire surface of the flexible tape 21 as well as on the circuit traces 24 electrically isolates adjacent traces 24 from each other.

[0035] The flexible circuit board 20 of the present invention may further comprise a stiffener 28 on the bottom surface of the flexible tape 21. When the semiconductor chip package 30 is mounted to the mother board (not shown) after the completion of package assembling, if there exist some differences in the coefficients of the thermal expansion (CTE) between the package 30 and the mother board, the stress due to the differences in the CTE may result in problems such as cracks in the package or chip contamination. Therefore, a stiffener 28 having the same or similar CTE as the package-mounting pad of the mother board (not shown) can be used for reducing the difference in the CTEs. Moreover, the stiffener 28 may impart some rigidity to the flexible circuit board 20. The stiffener 28 may be divided into several parts or may have slits so as to maximize these effects. Various metals such as copper (Cu) or aluminum (Al) may be used as the stiffener 28.

[0036] The stiffener 28 is provided on the bottom surface of the flexible tape 21 because the circuit layer 26 lies on the top surface. In this case, an adhesive layer 29 may be interposed between the stiffener 28 and the chip 31. Silicone resin or a nonsilicone resin having a low modulus of elasticity is preferably used as the adhesive layer 29 because they are electrically non-conductive materials and can reduce the difference in the CTEs between the chip 31 and the stiffener 28. Of course, each of the stiffener 28 and adhesive layer 29 have central openings aligned with the central opening in the flexible tape.

[0037] The semiconductor chip 31 is attached to the surface of the flexible circuit board 20 opposite the circuit layer 26 and the terminals 35. Therefore, the circuit board 20 can have a size (area) nearly equal to that of the chip 31. The flexible circuit board 20 according to the present invention is particularly adapted for use with the chip 31 having central I/O pads 32. The central I/O pads 32 are formed across central parts of the active surface of the chip 31. The central opening 22 of the circuit board 20 permits the I/O pads 32 to be exposed therethrough. Further, the I/O pads 32 on the chip 31 can be readily interconnected to the bonding pads 23 on the top surface of the circuit board 20. These structural features allow the semiconductor chip package 30 according to the present invention to be of a size on the order of that of the chip 31, or a so-called chip size package (CSP).

[0038] The I/O pads 32 and the bonding pads 23 are connected by metal wires 33, preferably by gold wires. Thereafter, the I/O pads 32, the bonding pads 23, and the metal wires 33 are encapsulated by an encapsulant 34, so as to be protected from moisture, ion contamination, radiation, interference, and harsh environments. An epoxy resin compound may be generally used as the encapsulant 34. Although the method of potting a liquid resin compound having a certain viscosity is preferably used to provide the encapsulant 34, other methods for encapsulating can be used. For example, the encapsulant 34 can be formed by molding or lid sealing methods. The potting method encompasses both the dispensing method of injecting the resin from a syringe, and the screen printing method of applying the resin using a mask, with the dispensing method being more preferable.

[0039] Adopting the dispensing method requires forming a dam 36 on the protective layer 27, as shown in FIG. 4, for preventing the liquid resin compound from overflowing. This is because the bonding wire 33 has a height greater than that of the circuit layer 26 when the wire 33 forms an interconnection loop. Therefore, the encapsulant 34 should not only fill in the opening 22, but should also cover the bonding wire 33. However, in seeking to cover the bonding wire 33, the liquid resin compound used as the encapsulant 34 can overflow across undesired portions of the surface of the circuit board 20 despite its viscosity. In order to solve this problem, the dam 36 may be formed along outer portions of the bonding pads 23 extending around the opening 22.

[0040] The dam 36 may be formed by using a silk screening or tape adhesion method, each of which has problems. The silk screening method has a drawback of increasing the height of the dam 36, and the tape adhesion method has a problem in that it requires an expensive and less reliable tape. Therefore, a dispensing method similar to that used to form the encapsulant 34 is used to form the dam 36. Although the liquid resin compound used to form the encapsulant 34 may also be used to form the dam 36, the dam is preferably formed from a liquid resin having a viscosity greater than that of the encapsulating resin so that the dam 36 is definitely formed with the desired height and width.

[0041] Terminals 35 are formed on the land pads 25 of the circuit layer 26. The terminals 35 provide the electrical and physical connection between the package 30 and the mother board (not shown). Each terminal 35 is a metal layer in the shape of a ball or bump. Solder (Sn/Pb) balls or gold (Au) bumps are preferably used as the terminals 35. The solder balls can be formed using mounting/reflow or printing/reflow methods known to those of ordinary skill in the art, while the gold bumps can be formed using known plating or photo-masking methods.

[0042] The top of the terminal 35 is higher than the top surface of the encapsulant 34. In other words, the encapsulant 34 extends a first predetermined distance from the circuit layer 26, and the terminal 35 extends a second predetermined distance from the circuit layer 26, with the first predetermined distance being less than the second predetermined distance. Therefore, the encapsulant 34 will not contact the mother board (not shown) when the package 30 is mounted to the mother board. Furthermore, the height of the terminals 35 as formed must take into account that the overall height will be somewhat reduced when the terminals deform during mounting of the package 30. For instance, if the height of the solder ball 35 (i.e., the diameter of the ball) is 0.3 mm, it is reduced to about 0.25 mm after being provided on the circuit layer 26, and thereafter to about 0.2 mm when the package is mounted to the mother board. Accordingly, the height of the encapsulant 34 (as measured from the circuit layer 26) is preferably about 0.15 mm less than that of the original height of the terminals 35. Moreover, the height of the dam 36 (as also measured from the circuit layer 26) is preferably from about 0.1 mm to about 0.15 mm.

[0043] It is, however, difficult to form the dam 36 having a height of 0.1 mm by using the known silk screening method. It is also difficult to form a dam 36 having a height greater than 0.15 mm by using the tape adhesion method. Therefore, as described above, the dispensing method is preferably used to form the dam 36. When using the dispensing method, the minimum height and the minimum width of the dam 36 are about 0.2 mm and about 0.6 mm, respectively. Therefore, an appropriate dam 36 can be easily formed by the dispensing method when using solder ball terminals 35 having a diameter of 0.5 mm or 0.76 mm. However, difficulties arise using the dispensing method when the solder ball terminals 35 have a diameter of 0.3 mm because the dam 36 has a height (0.2 mm) similar to that of the solder ball terminals 35 after being mounted to the mother board. Another disadvantage is that the reduction in the size of the package is limited by the 0.6 mm width of the dam 36.

[0044] In order to solve the problems described above, a groove can be provided in the protective layer. FIGS. 6 and 7 show a flexible circuit board 50 of another embodiment of a semiconductor chip package 60 according to the present invention. A groove 57ahaving a certain width and depth is formed in the protective layer 57 of this flexible circuit board 50. The groove 57ais located between the bonding pads 23 and the land pads 25. The groove 57acan be formed by etching at the same time the protective layer 57 is being etched to expose the pads of the circuit layer 26.

[0045] The groove 57ahas two functions. One is to reduce the height of the dam 66, i.e., the extent to which the dam 66 extends above the circuit layer 26. Also, the groove 57acontrols the width of the dam 66. In other words, the width of the dam 66 is proportionate to that of the groove 57aand so can be predetermined by selecting an appropriate groove 57awidth. Without the groove 57a, the minimum width of the dam 66 is about 0.6 mm as previously stated. However, the groove 57acan allow a dam having a width of about 0.3 mm to be formed.

[0046] Th groove 57acould also serve the role of the dam. If the liquid resin of the encapsulant 34 were to overflow the opening 22, the liquid resin would flow into the groove 57a, thereby preventing the resin from overflowing onto the land pads. In this way, the groove 57acan act as a dam making the dam 66 unnecessary.

[0047] Although the present invention has been described above in connection with the preferred embodiments thereof, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, all such embodiments which fall within the scope of the appended claims are seen to be within the true spirit and scope of the present invention.