Wafer-Level Packaging; WLP

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Wafer-Level Packaging (Page 2 of 3)

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.

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