Traceless flip chip assembly and method

A method for forming a fine-pitch flip chip assembly interconnects fine pitch devices after they have been connected to a carrier substrate. A die having a plurality of conductive sections, such as solder balls, is attached to a conductive layer of the substrate. An interconnect pattern is then formed in the conductive layer to connect the conductive sections and generate electronic functionality to the assembly. By forming the interconnect pattern after the device have been connected to the carrier, the invention provides precise alignment between the devices and the interconnect pattern without actually aligning the two components during the assembly process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices, and more particularly to an assembly and method for constructing chip assemblies having fine pitch interconnections.

2. Description of the Related Art

Currently, electronic devices require faster and more compact systems that pack a greater number of components into a smaller chip substrate area. The increased number of components on the chip increases the number of interconnections in the finite spaced offered by the chip. Interconnections are usually conducted via bonding pads or solder bumps through a flip-chip technique. Reducing the distance between the bonding pads, or the pitch increases the number of interconnects available on the chip, thereby increasing packaging density and reducing packaging weight. Fine pitch assemblies often rely on the ability to etch or deposit very fine traces onto a carrier material to create the interconnects.

FIGS. 1 and 2 illustrate conventionally-known flip-chip technology used to physically and electrically connect two microchips together. FIG. 1 shows two chips 100 and 102 that are bonded together via solder bumps or bonding pads 104 and 106 , respectively. As can be seen in the Figure, the solder bumps 104 , 106 are aligned together so that corresponding solder bumps 104 , 106 on each chip 100 , 102 touch only each other and not any other solder bumps 104 , 106 . If the solder bumps 104 , 106 are spaced a relatively large distance apart, that is, if the pitch P allows sufficient spacing in between the solder bumps, alignment is relatively simple even if an automated process is used. The pitch P of the solder bumps 104 , 106 using this method cannot be reduced to less than 25 microns, making the structure and method shown in FIGS. 1 and 2 unsuitable for applications require very fine pitch structures.

More particularly, if the pitch is reduced beyond the alignment capabilities of the bonding pad structure, the likelihood of misalignment increases as can be seen in FIG. 2 . Misalignment can often occur simply because of the difficulty that automated systems have in aligning the solder bumps with the required precision, often causing a given solder bump or bonding pad to touch two other solder bumps or pads to form a undesirable bridge connection. Attempts to increase the precision of alignment between the solder bumps may slow the manufacturing process to such a degree that the overall yield is too low for cost-effective manufacturing.

There is a need for a fine pitch flip chip assembly process that allows cost-effective manufacturing of flip-chip assemblies without encountering the alignment problems present in known processes.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a flip chip method and assembly that is suitable for fine pitch devices. The invention includes applying a conductive layer on a substrate and then forming an interconnect pattern on the conductive layer of the substrate after the conductive portions of a die have been attached to the conductive layer on the substrate. In one embodiment, the die is attached to a substrate having a base layer that supports the conductive layer. A portion of the base layer is cut away to expose the conductive layer, and then the interconnect pattern is etched into the conductive layer via a laser. A sealing layer may be deposited over the exposed conductive layer after etching to protect the interconnect pattern and/or act as a heat sink for the assembly.

Because the interconnect pattern is formed only after the die has been attached to the substrate, the alignment between the conductive portions of the die and the pattern is automatically conducted during the pattern formation process. As a result, there is no need to precisely align the die with any portion of the substrate as the die and substrate are connected together, making the production of fine pitch devices more cost-effective without sacrificing accurate alignment between the conductive portions of the die and the interconnect pattern on the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 is a top view of a substrate 300 having a conductive pattern 302 disposed on a base layer 304 . The substrate 300 preferably begins as a two-layer structure with a conductive material disposed on the base layer 304 . Portions of the conductive material are then etched away from the base layer 304 to form a conductive pattern 302 . The conductive pattern 302 includes a die attachment area 306 on which a die can be placed. The conductive pattern 302 itself can be frame-shaped, as shown in FIG. 3 , or it can include a solid plane of conductive material in the center area of the pattern. The choice of using a frame-shaped or a solid die attachment area 306 depends on the specific interconnect pattern that will eventually be formed in the assembly. If the interconnect pattern will be used to connect solder balls or other conductive pads located only at the periphery of the die, then applying the conductive material in a frame-shaped pattern is sufficient and will reduce the total amount of conductive material that needs to be eventually removed to form the interconnect pattern. If the interconnect pattern is expected to cover areas away from the periphery of the die and closer to the center of the die, the conductive material can be distributed over a greater portion, or even over the entire area, of the die attachment area 306 .

As can be seen in FIG. 3 , the die attachment area 306 does not have an interconnect pattern and is not separated into discrete bonding pads or conductive traces; instead, the die attachment area 306 is a continuous plane of conductive material on the substrate 300 at the areas where the die will contact the conductive layer 302 .

FIGS. 4 and 5 are side cutaway views, along line 4 4 in FIG. 3 , of the substrate 300 after a die component 400 is attached to the die attachment area 306 . As can be seen in FIGS. 4 and 5 , the die 400 has a plurality of solder balls 402 or other conductive protrusions. During manufacturing, the die 400 is flipped and attached to the substrate 300 so that the solder balls 402 contact the conductive die attachment area 306 . Once the die 400 is attached to the substrate 300 , the entire assembly 404 is flipped to obtain the structure shown in FIGS. 4 and 5 , such that the die 400 is at the bottom of the assembly 404 and the base layer 304 of the substrate is at the top of the assembly 404 .

Referring to FIGS. 6 and 7 , once the assembly 404 has been flipped to place the base layer 304 at the top of the assembly 404 , a laser, such as a CO 2 laser, or other means is used to cut away a window 600 in the base layer 304 and expose the conductive die attachment area 306 on top of the die 400 and the solder balls 402 . As noted in FIG. 3 , the conductive material in the die attachment area 306 is preferably continuous at the locations where the solder balls 402 are attached and does not have any pattern thereon that requires alignment with the solder balls 402 .

After the window 600 is formed by removing a portion of the base layer 304 , a laser or other means cuts away portions of the conductive material in the die attachment area 306 to form a fine pitch interconnect pattern 700 . As can be seen in FIG. 7 , the laser removes the conductive material joining the solder balls 402 so that the solder balls are coupled to the substrate 300 in accordance with the specific interconnections designed to accomplish the electrical functions of the chip assembly 400 . The conductive material that remains after the fine pitch interconnect pattern 700 is etched creates the interconnection between the die 400 and the substrate 300 . Because conductive material 302 between the solder balls 402 is removed after the die 400 is joined to the substrate 300 , there is no need to align the solder balls 402 with any conductive pads on the substrate; instead, the solder balls 402 are automatically and precisely aligned with the fine pitch interconnect pattern 700 after unnecessary conductive material between the solder balls 402 is etched away. The alignment of the laser system with the interconnect pattern to be etched in the conductive material 302 can be conducted via fiducial marks on the base layer 304 of the substrate 300 to instruct the laser etching system which portions of the conductive material 302 to remove to form the interconnect pattern 700 and which portions to leave behind. For example, the location of the solder balls 402 can be obtained using an X-ray and then correlated with the location of the fiducial marks to guide the laser etching system.

Note that although the above description specifies using a laser to remove the base layer 304 and conductive material 302 , other material removal methods can also be used, such as chemical etching. Because precision is not as large of a factor in removing the base layer, chemical etching, an excimer laser, or a frequency-quadrupled YAG laser can all be used to create the window 600 . The interconnect pattern requires greater precision; therefore, a YAG laser at prime frequency or a CO 2 laser system may be more appropriate for removing the conductive material to form the pattern.

Once the fine pitch interconnect pattern 700 has been formed, a sealing material 800 is deposited in the window portion 600 of the base layer 304 over the interconnect pattern 700 , as can be seen in FIG. 8 . The sealing material 800 can be used to insulate and protect the conductors of the assembly 400 . The sealing material 800 itself can be any material that can flow easily around and through the interconnect pattern 700 and surround the solder balls 402 and pattern 700 . For added functionality, the sealing material 800 can be a high thermal conductivity material, thereby acting as a heat sink to direct heat away from the die 400 .

By forming the fine pitch interconnect pattern 700 after the die 400 has been connected to the substrate 300 rather than attempting to align solder balls 402 on the die 402 with the interconnect pattern 700 , the inventive method allows assembly of fine pitch flip chips without requiring any precision alignment steps between the solder balls and pattern on the substrate. The conductive material removal process in essence creates precise alignment between the conductive portions of the die and the interconnect pattern without actually carrying out an alignment process. As a result, the yield from the inventive process tends to be larger than other fine pitch chip assembly methods and can be automated more easily.