TSV reaps headlines, MVP reaps profits
While high-density through-silicon vias (TSV) hold the limelight in advanced packaging development, a low-density via-through-pad version of TSV has moved past them to production. Tessera’s Micro Via Pad (MVP) package, introduced a year ago as an economical way to package image sensors and MEMS, cleverly avoids conventional TSV problems.
Tessera’s on-site technical papers give an excellent summary of the difficulties faced in commercial development of high-aspect ratio TSV. Etching through silicon to form high-aspect-ratio vias is a slow process that requires costly capital equipment. Lining and filling the vias adds to the complexity and cost.
Traditional TSV are blind vias, stopping at the oxide layer, which is then removed and replaced with metal. An ohmic contact to the back side of the bond pad at the bottom of a blind via is difficult to create and to verify. TSV reliability over temperature is challenged by the sharp bends that the conductors must make at the ends of the via.
Overcoming all of these challenges is the “holy grail” of current programs to commercialize TSV. That analogy may be apt, since in the Arthurian legend many quested after the grail for years, but only one eventually found it.
MVP, intended for low-density interconnects, is not the TSV holy grail, but with some of Merlin’s magic, it helps to bring the cost of a camera sensor module under $1.00. The task is to redistribute bond pads to a ball grid array (BGA) on the back of the die. This could be done with edge connections, but they increase the effective size of the die, reducing die per wafer and imposing special design rules. It could be done with blind TSV, but with the cost and reliability limitations stated above.
Via-through-pad forges a new path — edge connectors without die edges. A via-through-pad connection might be best visualized as a through-silicon edge-connector. Or you might picture it as an edge connector not located on the edge of the die.
Openings are made through the silicon and the redistribution conductors pass through those openings to the pads. The polygonal openings have rounded corners to eliminate 90° turns. A single opening may be large enough to serve several pads.
The process creates “internal” edge connectors in the openings, rather than constraining them to the die edges. Consequently, no additional real estate is needed, and no special die layout rules are required. The process works with virtually any scribe line width, bond pad location, and bond pad size.
The via continues through the oxide and penetrates the bond pads (Merlin won’t say how), exposing a ring of virgin metal for a radially symmetric ohmic contact. Creating a through via avoids the TSV problem of cleaning/depositing a buried connection at the bottom of a long narrow hole.
To complete the magic, another whisk of Merlin’s wand replaced expensive conventional dielectrics with a mature newcomer to micropackaging — automobile paint. The electrophoretic process perfected to give automobiles a glossy finish is used to deposit the polymer dielectric for this assembly. This material is orders of magnitude lower in cost, allows the finished module to exceed automotive reliability standards, and as an added bonus can be obtained in a wide range of designer colors.
In summary, MVP is a low-cost, high-reliability package for image sensors and MEMS, borrowing some characteristics from TSV and some from edge connectors. Perhaps we should call the result a TSVEDGE connection?
GEORGE A. RILEY, Ph.D, contributing editor, may be contacted at FlipChips Dot Com, 210 Park Ave, 300; Worcester, MA 01609 USA; 508-753-3572; firstname.lastname@example.org.