Priyanka Dobriyal, Anil Kurella, and Suzi Southwick
Hillsboro, OR, USA
With the increasing demand for sleeker and light weight devices, printed circuit boards (PCBs) are getting very crowded due to numerous components mounted within a limited space. This also reduces the number of probing test points available for failure analysis/fault isolation (FA/FI) processes. Hence, FA engineers usually extrapolate conclusions about the missing signals without concrete evidence.
In many cases, these analyses end with component level isolation, without deep dive investigation of the failure signature. In addition, package decapsulation is a commonly used technique for die level analysis but in the cases where package removal from the PCB destroys the failure signature, on-board decapsulation is more appropriate. On-board decapsulation allows precise system level FA/FI and gives engineers the ability to literally look inside the components by exposing the die and wire bonds of the component while they are still intact on the motherboard.
In the present work, various on-board decapsulation techniques to remove package overmold while preserving wire bonds either gold (Au) or Copper Cu) were evaluated. From an electrical FA standpoint, this technique allows precise FA execution by enabling in situ probing of the wire bonds. Hence, previously inaccessible data can be collected by simply using a microprobe station.
Various on-board decapsulation techniques were evaluated. Chemical decapsulation provided a successful demonstration for a component on-board decapsulation while preserving all wire bonds and retaining device functionality. This was also found to be only standalone technique. The proof of concept was shown on rigid or flex PCBs common to wearable and mobile devices. Decapsulation performed with microabrasion was shown to be helpful for pre work for chemical decapsulation experiments.
Upon manual chemical etching, the packages with Au wire bonded devices were preserved. However, non-uniform etching, board corrosion and wire bond damage were some of the issues which were encountered. By developing automated etchers which could fit entire PCBs, successful etching was performed on the packages mounted on both rigid and flex PCBs. Cu and Au wire bonds were preserved while performing experiments at room temperature and without application of electrical bias. Successful functionality test post chemical decapsulation on automated etchers further validated the process.
Initially Published in the SMTA Proceedings