Thermal fatigue generally occurs in ultrasonically bonded copper joints in electronic devices as the bonding substrate is composed of plural materials, leading to differences in the coefficient of thermal expansion. In this study, we found that the thermal fatigue resistance of the ultrasonically bonded copper joints was influenced by the grain size and hardness of the bonding substrate through the evaluation of the thermal fatigue properties. Copper alloys C1020 and C1940 were used as substrate materials to investigate the influence of the initial properties of the bonding material on the thermal fatigue resistance. We evaluated the crack propagation due to thermal fatigue via thermal cycle tests. Microstructural observations of the region fractured because of thermal fatigue revealed that cracks resulting from thermal fatigue did not progress in the fine grain region formed at the bonded interface. It was inferred that grain boundaries were an obstacle to crack propagation. C1940 has higher hardness and finer grains than C1020, and showed a lower decreasing rate of the peel strength and bonding area after the thermal cycling test than C1020 joints. Thus, a hard copper material with fine grains is effective in suppressing thermal fatigue fracture of ultrasonically bonded copper joints.
概要 本論文では,Cu 端子と Cu 基板との超音波接合について,継手の機械的特性評価や組織解析により,初期接合強度に 影響を及ぼす因子や高信頼な接合部の形成機構について検討した。接合中の荷重,振幅を制御した継手に対する初期接合強度 試験や,Cu 基板側に高強度な Cu 合金を用いた継手の熱疲労試験を行った。その結果,超音波接合部の初期信頼性向上には, 接合開始時の初期凝着核の形成および高荷重,高振幅な条件で接合することが重要であることが示された。また,Cu 基板側 に高強度な Cu 合金を用いることで,通常の Cu 基板の場合に比べて端子側の塑性変形領域が増え,接合界面近傍の結晶粒が微 細化し長期信頼性が向上することが示された。
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