Wafer-Level Packaging (WLP)

Wafer-Level Packaging

Wafer-Level Packaging (WLP) refers to the technology of packaging an integrated circuit at wafer level, instead of the traditional process of assembling the package of each individual unit after wafer dicing. WLP is essentially a true chip-scale packaging (CSP) technology, since the resulting package is practically of the same size as the die. Furthermore, wafer-level packaging paves the way for true integration of wafer fab, packaging, test, and burn-in at wafer level, for the ultimate streamlining of the manufacturing process undergone by a device from silicon start to customer shipment.

Wafer-level packaging basically consists of extending the wafer fab processes to include device interconnection and device protection processes. However, there is no single industry-standard method of doing this at present. In fact, according to an article in www.future-fab.com, there are at least four major WLP technology classifications in existence today, based on a study by Prismark and TechSearch International.

The four (4) WLP technology classifications according to Prismark and TechSearch International* are the:

1) Redistribution Layer and Bump technology, which is used by: Amkor (Ultra CSP™), Apack, Aptos, ASE (Ultra CSP™), ASAT Chipbond, Dallas Semi (2 lead), FCT (Ultra CSP™), Fraunhofer Institute, FuPo, Hitachi, Hyundai, National Semi (µSMD™), PacTech, Sandia Labs, Seiko Epson, SPIL (Ultra CSP™), Unitive (ExtremeCSP™) ;

2) Encapsulated Copper Post technology, which is used by: Casio, Fujitsu (SuperCSP™), IEP, Oki Electric, TI, Shinko (SuperCSP™ license), Toshiba;

3) Encapsulated Wire Bond technology, which is used by: Form Factor (Wow™, MOST™), Shinko, Hyundai, Infineon (Wow‰ licensees); and

4) Encapsulated Beam Lead technology, which is used by: ChipScale (Intarsia, M-Pulse Microwave), ShellCase (ShellBGA™), Tessera (WAVE™).

*source: www.future-fab.com

Redistribution Layer and Bump technology, the most widely-used WLP technology, extends the conventional wafer fab process with an additional step that deposits a multi-layer thin-film metal rerouting and interconnection system to each device on the wafer. This is achieved using the same standard photolithography and thin film deposition techniques employed in the device fabrication itself.

This additional level of interconnection redistributes the peripheral bonding pads of each chip to an area array of underbump metal (UBM) pads that are evenly deployed over the chip's surface. The solder balls or bumps used in connecting the device to the application circuit board are subsequently placed over these UBM pads.

Aside from providing the WLP's means of external connection, this redistribution technique also improves chip reliability by allowing the use of larger and more robust balls for interconnection and better thermal management of the device's I/O system.

Figure 1. Photos of two wafer-level packaged devices from

Dallas/Maxim; source:www.maxim-ic.com

Different companies using redistribution technology implement it using different materials and processes. Nonetheless, the sequence of steps required are more or less similar. The first layer put over the wafer to 'package' the device is usually a benzocyclobutane (BCB)-based polymer dielectric, to isolate the device circuitry from the rewiring system.

The rewiring metallization layer, usually Cu, Al, or a specially-developed alloy, is then deposited over this dielectric. This layer is then covered by another BCB dielectric layer serving as the solder mask. Underbump metallization is then put over the positions to be subsequently occupied by the solder balls. After the balls have been attached, flip-chip techniques are used to mount the WLP device onto the circuit board.

Encapsulated Copper Post technology is very similar to redistribution and bump technology in the sense that the chip's bond pads are also rerouted into an area array of interconnection points. In this technology, however, these interconnection points are in the form of electroplated copper posts, instead of pads.

These copper posts provide enough stand-off for the active wafer surface to be encapsulated in low-stress epoxy by transfer molding, exposing only the top portions of the posts where the solder balls will be attached. This WLP technology is supported mainly by Japanese companies.

Encapsulated Wire Bond technology mainly pertains to the Wire-on-Wafer (WOW) technology developed by Form Factor. Again, this technology employs a redistribution layer to reroute the device peripheral I/O's (bond pads) to meet the desired pitch. Using a modified gold ball bonder, gold ball-bumped micro-spring bond wires are formed on the redistributed pads.

The technology for the micro-spring structures employed by this system originated from Form Factor's earlier efforts to come up with highly compliant contactors for fine-pitched probe cards. The micro-spring bond wires are then overcoated with electroless Ni/Au to make them more robust without sacrificing compliancy.

Encapsulated Beam technology includes a very diverse class of WLP techniques from wafer lamination to glass technology. Examples of this technology are Shellcase's ShellBGA™ and ShellOp™, and Tessera's WAVE™.

Shellcase's patented ShellBGA™ is a true chip-size BGA type of wafer-level package wherein the silicon chip or die is sandwiched between two glass layers, resulting in a thin glass-silicon-glass structure. The glass sheet covering the active surface of the wafer has openings for the solder balls. The glass sheet covering the die backside completes the total enclosure of the die. Note that the die is completely encapsulated in epoxy prior to its being laminated between the two glass sheets.

Shellcase's ShellOP™, is similar to ShellBGA™, except that the solder balls are located on the die backside. This allows a clear view of the die's active circuit through the clear glass sheet protecting the top surface. This type of package was designed for image sensing and light detection applications, which is why it was designed to provide the die circuit with unobstructed exposure to external light. The die's bond pads are routed to the backside glass sheet which has openings to accommodate the solder balls.

Tessera's WAVE™ package ('WAVE' stands for "Wide Area Vertical Expansion") is a WLP technology targeting high I/O applications that require short assembly cycle time. The interconnection routing used by this architecture comes in the form of a compliant polyimide film-based copper circuit.

This polyimide film circuit is accurately aligned and brought into contact with the wafer, after which a low-modulus encapsulant is forced into the small space between the polyimide film and the wafer. This process causes both the polyimide film and wafer to expand, resulting in the polyimide bases' copper conductors to transform into its intended shape, i.e., as a stress-absorbing structure that links the die to the interposer. The wafer-level packaging is then completed with encapsulation and solder ball attachment steps.

The main drivers of WLP technology in the semiconductor industry today are cost, size, test, and burn-in. The advantages offered by wafer-level packaging include:

1) space savings from attainment of the smallest package possible for a device, i.e., a true chip-size package;

2) lowest cost per I/O since the traditional package assembly processes that are independent of wafer fab have been replaced by wafer-level interconnection processes;

3) lowest cost of electrical testing since this is done more efficiently at wafer level;

4) lowest cost of burn-in since this is done more efficiently at wafer level;

5) enhancement of device performance because of its minimum-length interconnections;

6) elimination of the need for underfilling of solder joints with organic materials; and

7) easier inventory management since fab, assembly, test, and burn-in can essentially be housed under one production floor.

As of this writing (2004), wafer-level packaging technology still has lots of room for improvement. Its range of applications is not yet too encompassing, being currently applied primarily to small packages with low I/O count, such as those used in analog/linear IC's, certain types of memories, integrated passive devices, and certain types of controllers.

Also, given the multitude of technologies available as WLP solutions today, we might not see a single industry-standard 'best-known' WLP process in the near future. Most experts likewise agree that WLP will not be the exclusive solution of choice for the packaging requirements of the future. Still, there's reason to believe that WLP technology will advance and become more cost-effective to eventually find its way into more complex, higher I/O applications in the electronics industry.

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