Page 50 - Chip Scale Review Sep Oct_2022-digital

failure mode is also often observed
for Ag-containing AMB substrates.
However, there is no failure within
the reaction zone of the substrate
with the copper-bonded technology.
This is proven by energy dispersive
spectroscopy (EDS) phase mapping
(Figure 6) that shows most of the
r e m a i n i ng m at e r ia l on t he p e el
s t r ip o r ig i n a t e s f r o m t h e Si 3 N 4
Figure 6: EDS phase mapping of the peeled strip from above: a) (left) The red color indicates the Si 3 N 4 cer a m ic a nd not f rom t he br a ze
phase; and b) the blue color indicates phases from the braze metal. metal, indicating the strong bonding
mechanism achieved by the copper
bonding process.
Thermal resistance
The ther mal resistance (R th ) of
the metal ceramic substrate is an
important factor for the design of
the power module. A lower thermal
resistance enables a higher power
density, allowing reduced chip size.
Major contributors to the R th are
ceramic thickness as well as ceramic
t y p e . C o n d u r a .u l t r a i n t er f ac e
®
for m at ion is d if fe re nt t ha n Ag-
containing AMB and, therefore, it is
required to rule out any potential R th
contribution from the braze metal or
reaction zone itself.
T h e t r a n s i e n t d u a l i n t e r f a c e
m e t ho d wa s u s e d t o a s s e s s t h e
t he r m al re sist a nce of t he met al
ceramic subst rates. Diodes were
sintered onto both A MB and the
co p p e r- bo n d e d su b s t r a t e s, a n d
the cumulative structure function
was calculated from cooling curve
me a su re me nt s af t e r he at i ng t he
assembly by power ing the diode
with 40A. T he R th of the system
wa s t he n a s se s se d by det e c t i ng
t h e p oi n t of d ive r ge n c e of t wo
structure functions measured under
different boundary conditions, i.e.,
the assembly was connected by two
different thermal interface materials
(TIM) – graphite foil and thermal
g rea se – to t he cool i ng system.
The resulting structure functions
are shown in Figure 7. The point
of divergence is similar for both
substrate types, demonstrating that
there is no difference in ther mal
performance between Ag-containing
A MB subst rates and the copper-
bonded subst r ates i n t he i n it ial
st at u s. A f t e r wa rd s, t he copp e r-
bonded substrates were aged in a
high-temperature storage (HTS) test

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