Nanotechnologies / Materials
Power device, EV/HEV car, Drone, Aerospace, 5G/6G communication

Failure mechanisms of the bonded interface between mold epoxy
and metal substrate exposed to high temperature
SANKEN (The Institute of Scientific and Industrial Research)Specially Appointed Associate Professor Chuantong Chen

https://researchmap.jp/chenchuantong?lang=en

Specially Appointed Professor Katsuaki Suganuma		https://researchmap.jp/ksuganuma?lang=en

Nanotechnologies / Materials

Abstract
The fast development of electric vehicles promoted the development of next-generation power modules. Along with this trend, theencapsulation techniques are also transforming from previous gel
encapsulation to epoxy encapsulation because epoxy encapsulation reduces the module size significantly. However, the dissimilar
bonding between the epoxy and the metal substrate is a weakpart of the entire module. In this study, a high-temperature storage
test (HST) was performed at 200 °C until 1000 h for encapsulated
packages. We found that the fracture happened inside the epoxy
rather than the copper/epoxy interface. More importantly, we foundthat copper atoms diffused into the epoxy reaching approximately
100 nm. The diffused copper atoms and the long-time high-temperature heating promoted the epoxy pyrolysis, forming a 100 nm
thick weak layer at the epoxy side, which is the key reason for thehigh-temperature failure.

▍ Significance of the research and Future perspective
Our study provided a fresh understanding of the failure mechanisms of the bonding between encapsulation epoxy and the copper substrate under HST, which will contribute significantly to future
power module design and material development. This result willcontribute greatly to the future power module design and the development of new epoxy materials. In the future, we will investigate
the copper’s catalytic effect on the epoxy with density functional
theory calculation in detail.

▍ Background & Results
With the application of wide-band gap semiconductors, encapsulation techniques are also transforming. Previously widely used
gel encapsulation, which requires fixing the substrate in a casemechanically and pouring liquid silicon gel on the substrate, isreplaced by epoxy encapsulation because epoxy encapsulation
reduces the volume size greatly. In addition, epoxy encapsulation
showed good reliability during the thermal cycling test and thepower cycling test. However, study on the failure mechanisms
from the perspective of improving epoxy properties and reducing
thermal stress was not enough. It is necessary to reveal the failure
mechanisms from the perspective of the interfacial interaction especially under the required reliability test conditions for next-generation power modules.
This paper aims to reveal the failure mechanisms between theencapsulation epoxy and the copper substrate by investigating
the change in the interfacial interaction. The high-temperature storage test (HST) conditions (200 °C lasting for 1000 h), meeting therequirements for next-generation SiC power modules, were used.
Different from previous studies, which used simplified samples(cylinder-shaped epoxy bonded with copper plate), real packages
used in electric industrials were used in this study. we revealed thefailure mechanisms of the bonding between encapsulation epoxy
and the copper substrate under HST with solid evidence. The
nano-scale weak epoxy layer formed by the copper diffusion andlong-time high-temperature heating was identified for the first time.
Under a long-time high-temperature condition, the diffused copper
promoted the pyrolysis of the epoxy and the pyrolysis of epoxy
reduced its strength, forming a 100 nm thick weak layer. This weaklayer is the key reason for the failure of the epoxy encapsulation
under HST.

Fig. 1 (a) The bonding strength results for different lead frames under various
reliability tests. (b) 3D render images of epoxy fragment C15H26O- for the copper
surface before encapsulation and the fracture surface of the copper substrate
after (c) 0 and (d) 1000 h HST; (e) depth profile of C15H26O- for the copper surface
before encapsulation and the fracture surface of the copper substrate after 0 and1000 h HST.

Fig. 2 (a)TEM HADDF image of the copper/epoxy interface after 1000 h HST, (b)minor copper diffusion happens after curing; (c) large extent copper diffusion
happens during HST; (d) diffused copper promotes the epoxy pyrolysis resulting in a weak layer; (e) weak layer fracture under internal or external stress.

P a t e n t
Tr e a t i s eZhao, Shuaijie; Chen, Chuantong; Suganuma, Katsuaki. Failure mechanisms of the bonded interface between mold epoxy and metal substrate exposed tohigh temperature. Composites Part B: Engineering. 2023, 254, 110562. doi: 10.1016/j.compositesb.2023.110562

URL
K e y w o r d power device, EV/HEV car, drone, aerospace, 5G/6G communication