November_EDFA_Digital 15 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 4 the laminate. The interposer and dies were overmolded and planarized before assembly. The failing structure is a multilevel perimeter via chain that travels around the entire interposer (Fig. 2). Due to the presence of overmold, traditional methods to isolate open or resistive fails in the chain are not practical. EBAC, OBIRCH, nanoprobing, and other methods cannot penetrate the overmold to exercise the chain, and the module laminate prevents backside access. Capacitance measurements [2] were used to approxi- mate the open fail location bymeasuring the capacitance from each end of the chain to a neighboring perimeter line. The ratioof the capacitancemeasurementswere then used to estimate the fail location. Inorder to further isolate the fail, the overmold needed to be removed. REMOVAL OF INTERPOSER OVERMOLD Overmolds consist of a base resin material and filler particles. Traditionally, overmold is removed with acid or an O 2 plasma or reactive ion etch (RIE) process. [3] In this case, because the overmold is coincident with the edge of the interposer, it would be difficult to mask off an area andeffectively remove theovermoldwithanacid. Theacid might also attack the laminate and other components of the module surrounding the interposer. A traditional O 2 plasma method was attempted, but the time needed to remove the overmold was prohibitive. An O 2 RIE with CF 4 added was developed, and was able to effectively attack the overmold. Modules were exposed to the RIE process for a period of time, removed and placed in an ultrasonic water bath to remove the exposed filler particles, and then the etch and clean cycles were repeated as necessary. Overmold removal times could be reduced to a fewhours. After the interposer surface began to be exposed though, the O 2 with CF 4 RIE began to attack the passivation over the metal links, and the perimeter chain was damaged (Fig. 3). Seeing as the overmold was being removed from the interposer edge at the same time as being removed from the top, it was not practical to start with an O 2 /CF 4 RIE process and finish with an O 2 plasma etch to protect the surface. For this analysis, the overmold was removed with a laser decapsulation tool followed by CF 4 free MIP spot etching. [4,5] A plasma is formed in a Beenakker cavity using amicrowave generator. The plasma consists of an Ar carrier gas andO 2 etchant gas direct- ed out of a nozzle. The sample is rastered under the nozzle on an XYZ movable stage. The rastering pattern, speed, andmicrowave power can be optimized for the sample. The systemincorporates anultrasonicwater bath to periodically dip the sample and remove the filler particles automatically duringmul- tiple etch and clean cycles. The regionof theovermold tobe removed was selected based on the approximate location determined from the capacitance data. The laser decapsulation process was used to thin the overmold to 200 µm. The Fig. 4 Overmold removed along one edgewith themicrowave-induced plasma process. The perimeter via chain is preserved. Fig. 3 Overmold partially removedwith O 2 /CF 4 reactive ion etch, causing damage to the perimeter via chain. Fig. 2 Diagramand image of the perimeter via chain on the interposer.