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Background
Yellowing has been observed on the Al₂O₃ ceramic portions of Direct Bonded Copper (DBC) substrates, with varying degrees of discoloration. Recently, some customers have expressed concerns that this phenomenon might affect the performance of DBC ceramic substrates and their subsequent use in products. Therefore, Shenzhen QFPCB factory conducted this experimental analysis to investigate the yellowing issue.
Investigation and Analysis
1.Sintering Principle of DBC Substrates
Al₂O₃-DBC substrates are formed by directly bonding copper plates to Al₂O₃ ceramic sheets under high temperature and an inert atmosphere. The bonding mechanism involves the oxidation of the copper surface in a controlled-oxygen atmosphere at elevated temperatures, forming a thin layer of Cu₂O. When the temperature exceeds the eutectic point, a Cu–Cu₂O liquid phase appears. The Cu₂O phase exhibits good affinity with Al₂O₃ ceramic, reducing interfacial energy and enabling the eutectic liquid phase to wet both copper and ceramic effectively.
Simultaneously, a chemical reaction occurs between Cu–O in the liquid phase and Al₂O₃, generating complex oxides such as CuAlO₂, which act as a brazing filler material, thereby creating a strong bond between the two materials.
2.Interfacial Layer in DBC Substrates
Based on the sintering principle described above, a CuAlO₂ layer forms at the interface between the copper plate and the Al₂O₃ ceramic after sintering. CuAlO₂ is a dark red ceramic oxide with good resistance to acids and alkalis. After standard copper etching treatment using typical etching solutions, this interfacial layer is generally difficult to remove completely. The figure below shows the ceramic surface after the copper layer has been etched away. It clearly reveals that the areas previously bonded to copper exhibit noticeable yellowing compared to unbonded regions.
3.Elemental Analysis of Yellowed Areas
The following figures present EDX (Energy Dispersive X-ray Spectroscopy) analysis results comparing untreated ceramic regions (marked by red lines) and yellowed areas after sintering. The results indicate that the yellowed regions show increased concentrations of elements such as Si, Ca, and Mg. These elements originate from the Al₂O₃ ceramic itself, where they are used as sintering aids (in 96% Al₂O₃ ceramics, minor amounts of Mg, Si, and Ca oxides are added alongside the primary Al₂O₃ component). This suggests that during the high-temperature DBC sintering process, these sintering aids migrate toward the ceramic surface. According to literature on Al₂O₃ ceramic sintering, an increase in sintering aids—particularly SiO₂—can lead to yellow discoloration of the ceramic.
4.Non-Uniformity of Yellow Discoloration
Based on the above analysis, it is inferred that yellowing arises either from the CuAlO₂ interfacial layer or from the migration of sintering aids within the ceramic during the DBC sintering reaction. Both mechanisms involve inherent variability: for instance, the CuAlO₂ layer, formed via interfacial reaction, may exhibit nanoscale thickness variations; similarly, the amount of sintering aid migrating to the surface can also vary. These minor inconsistencies in reaction extent result in non-uniform yellowing across the ceramic surface.
5.Impact of Yellowing on DBC Substrate Performance
To verify whether yellowing affects the performance of DBC substrates, a series of tests were conducted comparing substrates with severe yellowing and those with minimal or no yellowing. Detailed test procedures and results are documented in our Process Technology Department’s “Experimental Test and Analysis Report on Ceramic Yellowing.” The findings confirm that yellowing has negligible impact on the functional performance of DBC substrates.
In summary, yellowing of the Al₂O₃ ceramic in DBC substrates is caused by either the CuAlO₂ interfacial layer or the migration of sintering aids from within the ceramic during the high temperature bonding process. Minor variations in CuAlO₂ layer thickness or in the quantity of migrated sintering aids lead to inconsistent yellow discoloration. However, this yellowing phenomenon does not significantly affect the performance or usability of DBC substrates.
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