A Comparison of Electroless and Electroplated Ni/Au UBM Structures for Board Level Reliability of WLCSP Devices

  The use of Ni/Au as an under bump metallization (UBM) structure for WLCSP devices is commonplace in the industry today. The Ni/Au structures can be applied through electroless (ENIG) or electrolytic methods.

The electrolytic method for applying Ni/Au relies on expensive metal sputtering and photolithography processes to deposit a current carrying seed layer and uses photoresist defined bond pad openings for the electroplating of Ni/Au metals. The ENIG plating of a Ni/Au UBM relies on chemical reactions between the base bond pad metal layer and the chemical baths that contain the Ni and Au.

The two processes are significantly different, but they can both be used to obtain the same general result of providing a suitable surface for solder ball placement.

This study focuses on the differences between the electroless and electroplated Ni/Au UBM structures, and how they relate to overall board level performance. Daisy chain samples with a 0.4mm bump pitch were designed and built with Sn-Ag-Cu solder alloys of varying Ag content.

The drop test and thermal cycle performance of each Ni/Au UBM type and solder alloy combination was compared to determine the best combination of Ni/Au UBM and solder alloy. In addition to board level testing, intermetallic compound growth of each combination was measured by thermal aging at 150degC for up to 1680hrs.

In drop testing the electroplated Ni/Au UBM samples with high Ag solder alloy performed similar to the electroless Ni/Au samples with low Ag solder alloy. Decreasing the Ag content in the solder using the electroplated Ni/Au UBM maximized the overall drop test performance.

Different drop test failure modes were observed for each UBM type. High Ag solder alloys performed the best in thermal cycle testing, and the electroplated UBM structure was shown to be significantly more robust.