IC Leadframe Plating: A Reliability Risk

As the electronics industry has transitioned to RoHS compliant products there was (and continues to be) much research on reliability issues. Much of the early research was focused on solder joint reliability under temperature cycling and vibration.  Only in the past several years has the question of corrosion and PCB surface finishes been studied extensively due to several companies experiencing failures with immersion silver PCB finishes.  One potential failure mechanism has not received much attention and that is the potential of sulfur-driven corrosion failures of palladium plated leadframes (1) used in IC packaging.  This leadframe plating makes the component very susceptible to failures due to copper sulfide creep corrosion. This is a result of 1) the palladium being a very noble and accelerating the copper corrosion and 2) the copper sulfide corrosion films can creep quickly across the palladium (or gold) surface.  This failure mechanism is certainly not unknown but has not received much attention.  This is surprising since ICs packaged using the leadframe plating are more sensitive to sulfur-driven corrosion than the more publicized failures of immersion silver PCBs.  The palladium plated leadframe method has been utilized by Texas Instruments for many years but with the search for a RoHS compliant leadframe plating which is guaranteed not grow tin whiskers, many companies have joined TI in utilizing the leadframe plating .  Samsung, Toshiba, ST Micro, Infineon and others are using this leadframe plating for some products (2).
However the underappreciated danger of this type of leadframe is corrosion in environments with airborne sulfur.  Under these conditions the Pd plated leadframe component will fail quickly and will be the weak link in terms of corrosion resistance.  A vulnerable area for creep corrosion to originate is the tie bar cut-off area in the gull wing part.  In this area there is exposed copper adjacent to the palladium and the creep kinetics across the palladium surface are quite high. Even conformal coating on the part can not guarantee corrosion resistance since most coatings will tend to run off the knee of the gull wing lead thereby exposing the vulnerable area.  Considering the sensitivity of these parts to creep corrosion and the increasing use of the devices in RoHS designs, I predict that the electronic industry will see more instances of sulfur-driven component failures with Pd plated leadframe components. Stand by….

References
1. Assessment of Ni/PdAu-PD and Ni/Pd/Au--Ag Pre-plated Leadframe Packages Subject to Electrochemical Migration and Mixed Flowing Gas Tests, P. Zhao and M. Pecht, CALCE
2. Reality of Pb-Free Reliability, C. Hillman, DfR Solutions, SMTA Lead-Free Academy, Toronto, May 2010