Jeong-Won Yoon

Interfacial reactions and mechanical strength of Sn-3.0Ag-0.5Cu/Ni/Cu and Au-20Sn/Ni/Cu solder joints for power electronics applications

Abstract The mechanical strength of Sn-3.0Ag-0.5Cu (SAC305) and Au-20Sn solder joints and their interfacial reaction with a Ni-plated ceramic substrate were evaluated to assess their suitability for use as die attach materials in power module applications. The compatibility between the two solder alloys and the Ni substrate was assessed during isothermal long-term aging, while the mechanical strength of the two solder joints was measured by die shear testing. A higher intermetallic compound (IMC) growth rate and Ni consumption rate was observed in the SAC305 solder joint, with the formation of a thick IMC layer and weak interface resulting in brittle fracture. The Au-20Sn solder joint, on the other hand was found to exhibit superior high temperature interfacial stability and joint strength.

Lead-free Solder for Automotive Electronics and Reliability Evaluation of Solder Joint

Automotive today has been transforming to an electronic product by adopting a lot of convenience and safety features, suggesting that joining materials and their mechanical reliabilities are getting more important. In this study, a Sn-Cu-Cr-Ca solder composition having a high melting temperature (>230℃) was fabricated and its joint properties and reliability was investigated with an aim to evaluate the suitability as a joining material for electronics of engine room. Furthermore, mechanical properties change under complex environment were compared with several existing solder compositions. As a result of contact angle measurement, favorable spreadability of 84% was shown and the average shear strength manufactured with corresponding composition solder paste was 1.9kg/mm 2 . Also, thermo-mechanical reliability by thermal shock and vibration test was compared with that of the representative high temperature solder materials such as Sn-3.5Ag, Sn-0.7Cu, and Sn-5.0Sb. In order to fabricate the test module, solder balls were made in joints with ENIG-finished BGA and then the BGA chip was reflowed on the OPS-finished PCB pattern. During the environmental tests, resistance change was continuously monitored and the joint strength was examined after tests. Sn-3.5Ag alloy exhibited the biggest degradation rate in resistance and shear stress and Sn-0.7Cu resulted in a relatively stable reliability against thermo-mechanical stress coming from thermal shock and vibration.

Ultrasonic Bonding Technology for Flip Chip Packaging

최근 들어, 다양한 형태의 휴대형 전자기기 및 디지털 컨버전스 제품들이 출시되고 있으며, 그 종류와 수요는 해마다 급속도로 증가하고 있다. 전통적으로 기계 부 품의 집합체로 알려진 자동차를 비롯한 운송 수단에도 엔진 효율 증대, 배기가스 감소, 안정성 및 편의성 향상 을 위해서 전자 부품의 양이 증가하고 있는 추세에 있 다. 이러한 전자 부품 및 제품들의 성능을 극대화하기 위해서는 고성능 마이크로프로세서, 대용량의 저장 매체 및 다양한 수동소자들의 개발 뿐 만 아니라, 각 요소들 이 최적 상태로 상호 유기적인 구동이 가능하도록 시스 템화하는 전자 패키징 기술의 개발이 요구된다. 전자 패키징 기술은 칩이나 각종 부품 또는 제품들을 외부의 환경으로부터 보호하고, 필요한 요소들에 전원을 공급하며, 전기적 신호의 원활한 통로 역할을 하는 동시 에 핫 스팟 (hot spot)에서 발생한 열을 외부로 방출시 키는 역할을 수행한다. 하나의 전자 제품이 완성되기 위해서는 다양한 전자 패키징 기술이 요구된다. 이 중에 서, 실리콘 칩을 패키징하는 기술을 1차 레벨이라 하며, 신뢰성이 높고, 작업 속도가 빠르며, 자동화가 용이한 와이어 본딩 (wire bonding) 기술이 일반적으로 사용되 어 왔지만, 최근 기하급수적으로 증가하는 논리 소자들 의 전송 속도와 발열량을 대응하기에는 한계가 있다. 이 에 따라, 솔더 범프나 금속 범프를 사용하여 실리콘 칩 을 기판 상에 실장시키는 플립칩 (flip chip) 기술의 시 장이 점차 증가하고 있다. Prismark 조사에 의하면, 세 계 플립칩 시장 규모는 2001년 411만 웨이퍼에서 2005 년 2,460만 웨이퍼로 급격하게 증가하였다. 플립칩 기 술은 기존 와이어 본딩 기술 대비 임피던스가 1/10 이 하로 낮고, 다이 크기에 유사한 크기로 패키지 크기를 줄 일 수 있는 동시에 우수한 방열 효과를 기대할 수 있기 때문에, 하기와 같은 분야에 필수적으로 요구되는 기술이다.

Cu–Sn and Ni–Sn transient liquid phase bonding for die-attach technology applications in high-temperature power electronics packaging

Power electronics modules in electric vehicles and hybrid electric vehicles, particularly those containing next-generation power semiconductor devices such as silicon carbide and gallium nitride are operated at high temperatures exceeding 200 °C. Consequently, the reliability requirements for such modules have become highly stringent and new packaging materials and technologies are required to meet the demands of power electronic modules. Some good candidates for high temperature applications include high-temperature solders such as Au–20Sn, Ag or Cu sinter pastes, and transient liquid phase (TLP) bonding materials. In particular, the TLP bonding technology is suitable for use in high temperature environments owing to its low cost and simplicity of the bonding process. In this study, the feasibility of Cu–Sn and Ni–Sn TLP bonding technologies as die-attach methods for power electronics packaging applications is examined. The results of the study indicate that the Cu–Sn and Ni–Sn TLP bonding processes transform the joints fully into Cu6Sn5/Cu3Sn and Ni3Sn4 intermetallic compounds (IMCs), respectively. Further, the mechanical strength and reliability of the two TLP bonding joints are reduced owing to the formation of brittle IMCs.

Flip-chip Bonding Technology and Reliability of Electronic Packaging

IT (Information Technology) 산업의 급속한 발전과 새로운 서비스 요구로 인해, 최근 반도체는 무게와 크기 측면에서 기존 반도체에 비해 더욱 작고 가벼운 방향으 로 진화하고 있으며, 다양한 데이터의 처리 및 멀티미디 어 기능을 구현하기 위해 한 개의 패키지 내에 복수의 소자 또는 칩을 통합시키는 새로운 방식의 반도체 패키 징 기술의 수요가 증가되고 있다. 특히, 크기와 무게가 작은 패키지를 요구하는 휴대용 통신기기 및 멀티미디 어 기기 시장의 급격한 증가는 고집적의 새로운 반도체 패키지 기술의 수요가 비약적으로 증가될 것임을 보여 준다. 전자.정보통신 산업이 이렇게 발전함에 따라 카메라, 게임기, 전화, PDA (Personal Digital Assistants) 등과 같은 멀티미디어 시스템 전자 제품들이 휴대전화로 빠 르게 융합화 되고 있다 (Fig. 1참조). 이러한 전자 제품 의 진화에 따라 빠른 신호처리가 가능한 고성능 반도체 칩의 개발 및 칩과 칩 또는 칩과 다른 주변 장치들간의 상호 신호전달을 위한 전자패키징 (electronic packaging) 기술의 발전 또한 요구되고 있다. 특히, 전 자 및 반도체 패키징 분야에 있어서 재료, 부품, 기판 및 모듈 (module)의 고성능화 (high performance), 고집적 화 (high density integration), 다기능화 (multi functionality) 및 소형화 (miniaturization)에 대한 요구 가 점차 증대되고 있다. 기술적인 측면에서 90년대 중반 까지의 패키징은 칩의 물리적인 보호와 보드와 칩 간의 전기적 연결을 제공하는 것이 중요한 문제였으나, 최근 들어 칩의 고성능화뿐만 아니라 다기능화와 칩 성능의 최적화, 발열처리 문제 등에서 패키징의 역할이 더욱 중 요해지고 있다. 최종 시스템에서 패키지의 역할은 IC 칩들이 동작할 수 있도록 파워 (power)와 시그널 (signal)을 전달하며 다른 시스템과 전기적 연결이 가능 하도록 채널을 공급하여 인터패이싱 (interfacing)이 가 능하도록 하는 동시에, 전체 시스템을 물리적으로 보호 하여 신뢰성을 보장하는 역할을 수행한다. 최근 전자 제 품의 성능은 칩 자체보다는 패키징 구조에 의한 신호지 연에 의해 결정되고 있으며, 전체 신호지연의 약 50% 가량을 차지하고 있다. 따라서 패키징 기술 개발의 발전 이 점차 중요한 문제로 대두되고 있으며, 이를 개선하기 위해서 많은 연구들이 진행되고 있다. 본 고에서는 최신 마이크로 시스템 패키징 분야에서 주목받고 있는 플립칩 (Flip-chip) 본딩의 기술 및 종류, 접착제를 이용한 플립칩 본딩, 초음파를 이용한 플립칩 본딩, 플립칩 패키지의 일렉트로마이그레이션 (electromigration) 현 상, 3차원 실장 (3 Dimensional Packaging) 기술 및 금주석 (Au-Sn) 솔더를 이용한 패키징기술에 대해서 간 략히 소개하고자 한다.

Cu-Sn Intermetallic Compound Joints for High-Temperature Power Electronics Applications

Cu-Sn solid–liquid interdiffusion (SLID) bonded joints were fabricated using a Sn-Cu solder paste and Cu for high-temperature power electronics applications. The interfacial reaction behaviors and the mechanical properties of Cu6Sn5 and Cu3Sn SLID-bonded joints were compared. The intermetallic compounds formed at the interfaces in the Cu-Sn SLID-bonded joints significantly affected the die shear strength of the joint. In terms of thermal and mechanical properties, the Cu3Sn SLID-bonded joint was superior to the conventional solder and the Cu6Sn5 SLID-bonded joints.

Die-attach for power devices using the Ag sintering process: Interfacial microstructure and mechanical strength

The sintering reactions and mechanical reliability of Ag sinter paste with three different surface finishes, Cu, Ag, and electroless nickel-immersion gold, were evaluated during the sintering process. We compare the sintering reaction behaviors of the three sintered joints and identify the relationship between the sintering behavior and the sintered Ag/substrate compatibility. Inter-diffusion behaviors result in of good metallurgical bonding during the Ag sintering process in the three sintered joints. The shear strength increases on increasing the bonding pressure, irrespective of the surface finish. The surface finish material of the substrate strongly affects the shear strength of the Ag sintered joints. The Ag finished joint exhibits superior interfacial stability and shear strength compared to the Cu and Au finished joints.

Effect of Plasma Surface Finish on Wettability and Mechanical Properties of SAC305 Solder Joints

The wetting behavior, interfacial reactions, and mechanical reliability of Sn-Ag-Cu solder on a plasma-coated printed circuit board (PCB) substrate were evaluated under multiple heat-treatments. Conventional organic solderability preservative (OSP) finished PCBs were used as a reference. The plasma process created a dense and highly cross-linked polymer coating on the Cu substrates. The plasma finished samples had higher wetting forces and shorter zero-cross times than those with OSP surface finish. The OSP sample was degraded after sequential multiple heat treatments and reflow processes, whereas the solderability of the plasma finished sample was retained after multiple heat treatments. After the soldering process, similar microstructures were observed at the interfaces of the two solder joints, where the development of intermetallic compounds was observed. From ball shear tests, it was found that the shear force for the plasma substrate was consistently higher than that for the OSP substrate. Deterioration of the OSP surface finish was observed after multiple heat treatments. Overall, the plasma surface finish was superior to the conventional OSP finish with respect to wettability and joint reliability, indicating that it is a suitable material for the fabrication of complex electronic devices.

Effect of Sintering Conditions on the Mechanical Strength of Cu-Sintered Joints for High-Power Applications

In this study, the feasibility of low-cost Cu-sintering technology for power electronics packaging and the effect of sintering conditions on the bonding strength of the Cu-sintered joint have been evaluated. A Cu paste with nano-sized Cu powders and a metal content of ~78% as a high-temperature bonding material was fabricated. The sinter-bonding reactions and mechanical strengths of Cu-sintered joints were evaluated at different sinter bonding pressures, temperatures, and durations during the sintering process. The shear strength of the Cu-sintered joints increased with increasing sintering pressure. Good interfacial uniformity and stable metallurgical microstructures were observed in the Cu joints sintered at a high sintering pressure of 10 MPa, irrespective of the sintering time. It was confirmed that a high-pressure-assisted sintering process could create relatively dense sintered layers and good interfacial uniformity in the Cu-sintered joints, regardless of the sintering temperatures being in the range of 225–300 °C. The influence of the sinter bonding pressure on the shear strengths of the Cu-sintered joints was more significant compared to that of the sintering temperature. Durations of 10 min (at 300 °C) and 60 min (at 225 and 250 °C) are sufficient for complete sintering reactions between the Si chip and the direct bond copper (DBC) substrate. Relatively good metallic bonding and dense sintered microstructures created by a high sintering pressure of 10 MPa resulted in high shear strength in excess of 40 MPa of the Cu-sintered joints.

Comparative study of ENEPIG and thin ENEPIG as surface finishes for SAC305 solder joints

A new multilayer metallization, electroless-nickel electroless-palladium immersion gold (ENEPIG) with a thin 0.1-μm-thick Ni(P) layer (thin-ENEPIG), was plated onto a Cu printed circuit board substrate for fine-pitch package applications. We evaluated the interfacial reactions and mechanical strengths of Sn–3.0Ag–0.5Cu (SAC305) solder on thin ENEPIG-coated substrates over various reflow times and compared them to those of a conventional ENEPIG-coated substrate with a 6-μm-thick Ni(P) layer. Thin Au, Pd, and Ni layers on the thin ENEPIG substrates were exhausted during the initial reflow time, which brought the underlying Cu layer in direct contact with the molten SAC305 solder, resulting in the formation of scallop-shaped (Cu,Ni)6Sn5 intermetallic compounds (IMCs) at the interface. The interfacial IMC layer for the thin ENEPIG substrate was thicker than that for the normal ENEPIG substrate due to the direct contact between the SAC305 solder and the Cu layer. In the low-speed shear test, all the fractures occurred in the bulk solder regardless of the different substrates and reflow times. In the high-speed shear test, the fracture mode was changed from ductile to brittle on increasing the reflow time. The P-rich Ni layer and thick Cu–Sn IMC formation deteriorated the shear strengths of the normal and thin ENEPIG joints, respectively. The thin ENEPIG joints showed better mechanical strength in the solder joint than the normal ENEPIG joints, despite the thick interfacial IMCs.