The ‘IMC rings’ growth mechanism in Sn–Ag–Cu solder ball and NiAu-plated BGA ball pad

Abstract

Due to environmental pollution concerns, the law says the lead (Pb) inside electronics devices must be eliminated. Lots of lead-free materials have been introduced and been used for electronic products and Sn–Ag–Cu (SAC) is one of most popular lead-free representatives and has been used in high-volume production. The most popular IC packages, BGA packages which have higher I/O counts, and better thermal and electrical performance than lead-frame type packages, use solder balls of SAC for lead-free applications to connect with printed circuit boards. A particular phenomenon, so-called ‘IMC rings’, is only observed on BGA solder ball pad surfaces after the SAC solder balls are mounted on BGA ball pads which are plated with NiAu. It has not been found in either eutectic solder or Sn–Ag solder welding on plated NiAu pads. No significant evidence exists to show that ‘IMC rings’ degrade the strength of solder joints or cause earlier failures in mechanical tests. ‘IMC rings’ appear to be an inevitable outcome after the SAC is soldered onto a plated NiAu ball pad. This study is to find the growth mechanism of ‘IMC rings’ on the ball pad which is created between SAC ball and plated NiAu pad during solder ball temperature reflow. The design of the experiment and data have been discussed.