Surface Finish Can Achieve Robustness Against Copper Creep Corrosion

Kunal Shah, Ph.D.
LILOTREE
WA, USA

Dem Lee, Jeffrey Lee
iST-Integrated Service Technology Inc.
Hsinchu, Taiwan

Due to tremendous growth in Information Technology equipment in the Big Data/server applications, 5G-6G-high frequency, Edge Computing, Artificial Intelligent (AI) applications recently, the hardware reliability of these equipment is critical and paid more attention in the industry. With harsh environment and severe air pollution, hardware equipment is subjected to severe corrosion and reduce lifetime. Among various materials used in the electronics assembly, reliability evaluation of surface finishes on printed circuit boards (PCBs) is critical for long terms optimal operation of the equipment. The failure mode due to exposure to long term harsh environment, air pollution, etc. involves creep corrosion which can potentially cause electrical short failure. It is important to evaluate for the robustness against creep corrosion occurrence in the electronic assemblies.

Mixed Flowing-Gas (MFG) test is an environmental stress laboratory test intended to simulate contaminated industrial ambiance. There are many controllable parameters, including temperature, relative humidity, corrosive gases type (e.g. H2S, Cl2, NO2, SO2, NH3 and O3…etc.), concentration and gaseous flowing rate, etc. Because many controllable parameters, complex equipment setup and continuous constant-flowing corrosive gases, MFG test is a popular accelerated corrosion test that has high availability to simulate the field environment corrosion. In order to observe the effective corrosive reactivity within a short period of time, a MFG test condition from International Electronics Manufacturing Initiative (iNEMI) was adopted in this research.

iNEMI MFG test condition has high concentration of hydrogen sulfide (H2S), mixed with multiple corrosive gases that is an effective accelerated condition for corrosion test. Various, surface finishes were evaluated including Electroless Nickel Immersion Gold (ENIG), Immersion Tin (ImmSn), Hot Air Solder Leveling (HASL), Organic Solderable Preservative (OSP)and novel Ni-free surface finish. Creep corrosion tendencies and robustness were compared among various surface finish options and long-term reliability in harsh environmental conditions is evaluated.

The novel Ni-free surface finish of cyanide-free immersion gold plated onto a nano-engineered barrier layer treated copper is a viable solution for high-frequency-HDI applications. The surface finish was tested for insertion loss and robustness against creep corrosion. The results showed that this surface finish performs better than ENIG in terms of extremely low insertion loss and robustness against creep corrosion after MFG test in 7 days of exposure. This Ni-less surface finish with nano-engineered barrier layer is an optimal solution to the current need for a reliable surface finish for high-frequency, HDI PCB applications.