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NEW GENERATION REFLOW
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Silver-Free Thick Film Copper Bonds Reliable Metal-Ceramic Substrates
Continued from page 52
SAM after 3,000 and 5,000 cycles of thermal shock. The results proved that no delamination between the ceramic and the braze metal occurred. This indi- cates that the TFCB technology can fully leverage the mechani-
cal robustness of the Si3N4 ceramics, the same as AMB tech- nology.
Copper Peel Strength
The copper peel reflow soldering
vacuum soldering
temperature treatment
customized solutions
Machines for Thermal Processes from –50 °C up to +450 °C
www.smtna.com
strength character- izes the adhesion strength of the cop- per foil to the ceram- ic. A simple peel test is used in which a copper peel strip is brazed onto a piece of
Si3N4 ceramic using TFCB technology. The copper is peeled off and the force is measured. The peel strength strongly de - pends on the sub- strate type and tech- nology being tested. Closer inspec-
failure within the reaction zone of the TFCB substrate. This is proven by EDS phase mapping that showed most of the remain- ing material on the peel strip
originated from the Si3N4 ceram- ic and not from the braze metal, indicating a strong bond achieved by the TFCB process.
Thermal Resistance The thermal resistance of the
metal ceramic substrate is an
Cumulative structure functions of AMB and RFCB substrates prior to HTS. “S” shows the region under test.
tion of the failure mode of the peel sample can pro- vide valuable information about the bonding quality. A dark color on the peel strip and on the coun- terpart was visible, which indi-
cates pulling out Si3N4 particles from the ceramic substrate. This failure mode is also often
observed for Si3N4 AMB sub- strates. However, there was no
important factor for the design of the power module. A lower ther- mal resistance enables a higher power density, allowing reduced chip size. Major contributors to thermal resistance are copper and ceramic thickness as well as ceramic type. TFCB interface for- mation is different than AMB and, therefore, it is required to rule out any potential thermal resistance contribution from the braze metal or reaction zone itself. A transient dual interface
method was used to assess the thermal resistance of the metal ceramic substrates. Diodes were sintered onto both AMB and TFCB substrates, and the cumu- lative structure function was cal- culated from cooling curve meas- urements after heating the assembly by powering the diode with 40A. The thermal resistance of
the system was then assessed by detecting the point of divergence of two structure functions meas- ured under different boundary conditions, i.e. the assembly was connected by two different ther- mal interface materials (TIM) — graphite foil and thermal grease — to the cooling system. The point of divergence was
similar for both substrate types, demonstrating that there is no difference in thermal perform- ance between AMB substrates and TFCB substrates in the ini- tial status. Afterwards, the TFCB substrates were aged in a high-temperature storage (HTS)
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