MSL 1 Failure Mechanism and Solution for Silicon Carbide (SiC) Power Package with Pressure-Less Sintering Die Attach process on Bare Copper Lead Frame

Rapid growth of Silicon carbide (SiC) market in recent years has made a significant evolution in power market. SiC is favored for many high voltage, high power and high temperature applications, especially in automotive industry due to its material advantages. With the much higher thermal conductivity of SiC, traditional die attach method with leadbased solders become less suitable and limits the overall thermal performance of power devices. Sintering process with silver paste is one of the improved die attach techniques for high voltage SiC power devices. Due to the downset design of TO package, pressure-less silver sintering appears as a more suitable die attach method without screen printing process limitation. However, the disadvantage of this method is long duration of high temperature curing condition, which lead to poor adhesion between EMC and copper lead frame; hence Moisture Sensitivity level (MSL) 1 becomes a huge challenge for this SMD device. This paper presents the studies of the delamination failure mechanism on bare copper lead frame and the solutions for TO-263 package with pressure-less silver sintering die attach condition at 220 °C. To investigate the effect of copper oxidation and outgassing of silver paste during silver sintering process, TO-263 lead frame samples after undergoing different processes (bare lead frame, thermal treatment at 220 °C / 2 hours and silver paste sintering at 220 °C / 2hours) were prepared. Adhesion button shear test, Energy-Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS) analysis were conducted on the copper surface of TO-263 lead frame. Due to the limitation of depth analysis, EDX method cannot distinguish the differences of all samples. In this case, XPS analysis appears to be a better surface sensitive technique to characterize the copper surface in depth of 0 - 10 nm and chemical state information of the elements could be identified by analyzing the binding energy. XPS analysis revealed that organic materials were deposited on the copper surface after the silver sintering process. This organic material was proven to be the main root cause of delamination, which results in significant degradation of adhesion between epoxy mold compound and lead frame surface based on button shear test. This organic residue originated from glycol ether solvent used in silver paste based on XPS analysis. Cuprous oxide (Cu2O) was observed on the surface of all samples but no degradation of adhesion between epoxy mold compound and lead frame surface despite higher intensity of Cu2o. A methodology to improve the delamination issue is then proposed based on the observed failure mechanism. Chemical cleaning, formic acid reduction process and plasma cleaning to remove the organic residues after silver sintering process were proven to be effective to improve the delamination and achieve zero delamination on bare copper lead frame after MSL 1. This solution successfully enabled a MSL 1 zero delamination SiC power package to operate at high temperature, voltage and humidity conditions for automotive applications while maintaining excellent package reliability during its product lifetime. The cost of dry packing can be saved as well due to the unlimited floor life of products rated MSL 1.