RF devices: from WLSCP to smart interposers

Abstract

RF wireless devices such as 3G/4G, Wifi, Bluethooth etc... represent today 32% of the whole WLCSP products which know a steady grow above 10% per year [1]. However, a clear trend of combining multi RF chips in one WLCSP module is emerging. This allows shorter RF connections, reduction of BOM and diminution of the overall cost. Complex and heterogeneous RF systems will become possible allowing soon tunability. A silicon package directly reported on board is a good candidate to meet these next challenges of RF WLCSP. Indeed, low cost and multi-chips interposers will soon be produced by TSV fabs or OSATs and tunables devices could be reported on them. But the direct report on board as well as the high frequency signal integrity must be first evaluated carefully. In this work, we describe promising results of silicon package based on high resistive silicon interposer encompassing 2 levels of Cu Damascene, Tx/Rx antenna, one RF chip and TSV last for 60GHz applications. A new wafer-level molding [2] is performed on front side by lamination at wafer level. On the backside of the interposer, thick Cu RDL and soft layer passivation is deposited before the balling. Just like in WLSCP, different kinds of solder balls are mounted and analyzed with respect to their size (350 or 400 μm) or type (solder or polymer/solder). The passivated interposers are diced and then reported on a PCB, thus forming the silicon package. Low Rf loss (<1dB) attenuation is demonstrated for the routing layers while the antennas exhibit a suitable gain of 5dBi over the 55–65 GHz band. The assembly characteristics between the silicon package and the board will be discussed with respect to the different chosen design rules. These first results pave the way to a new kind of smart package based on Si interposers, moving forward to more complex WLCSP RF devices.