Evaluation new corner stress relief structure layout for high robust metallization

For a high robust metallization it is necessary to solve different problems related to migration mechanisms and thermo-mechanical stress in the material. Extended operating conditions and challenging assembling processes influence stress behaviour in chip corners. Typically the corner area of the chip is excluded for use. For higher stress load the forbidden area increases. But effort for demanding mission profiles of a product should not cumulative in increasing chip size. Simulation can help to a better understanding of mechanical stress in the chip corner and chip-package interaction.Corner stress relief structures lower the influence of high thermo-mechanical stress. A high functional corner stress relief structure allows a more efficient chip design. In this work of corner stress relief structure is presented and an evaluation structure is shown which allows to prove the effectiveness of the stress relief.     

Influence of silver paste rheology and screen parameters on the front side metallization of silicon solar cell

Four commercially available silver pastes for silicon solar cells front side metallization were examined and compared. Both rheological and printing experiments were used to correlate pastes properties and screens characteristics with printing results. Theoretical printability and spreading behavior were first characterized by means of rotational and oscillatory rheological experiments. Further on, the pastes were printed, dried, and fired in a standard solar cell process on an industrial line using a full factorial design of experiments. This methodology allows to study the effect of the paste, the screen, the aperture, and their interactions on the printing results. Especially, the finger aspect ratio, the finger cross section and its relative standard deviation, which are respectively linked to solar cell efficiency, silver consumption, and process stability, were systematically measured. This work allows finding general guidelines for process optimization. Finally, a confirmation test was carried out, leading to 19% efficiency on monocrystalline silicon solar cells in an industrial environment.     

辐照对聚酰亚胺基板铜薄膜金属化TiN阻挡层的影响

采用物理气相沉积方法在聚酰亚胺基板上沉积Cu薄膜,利用TiN阻挡Cu元素向聚酰亚胺基板内部扩散。研究了在60Co-g射线辐照条件下,TiN阻挡层的阻挡效果,扫描俄歇微探针谱图分析表明:TiN层可以有效地阻挡Cu元素向聚酰亚胺基板内的扩散。当照射剂量大于2105 Gy后,TiN失去阻挡Cu元素扩散的效果。     

Development of under bump metallizations for flip chip bonding to organic substrates

Several under bump metallization (UBM) schemes using CuNi alloys as the solderable layer were investigated. Nickel slows down dissolution of the UBM into the solder and formation of intermetallics during reflow. To study the intermetallic reaction, CuNi foils of different concentrations were immersed in a eutectic PbSn solder bath for reaction times ranging from 30 seconds to 30 minutes. It was observed that when 10% and 20% Ni is added into copper, the intermetallic forms a continuous layer, instead of the discrete scallops seen in pure Cu/solder interfaces. However, the thickness of the intermetallic remained about the same. For 30% and 45% Ni alloys a definite decrease in the intermetallic thickness was observed compared to the lower Ni alloys. Actual under bump metallizations were also made on Si wafers to study the reactions when there is a limited supply of CuNi available. Cr or Ti was used as the adhesion layer, and the solderable layer was a copper-nickel alloy, instead of pure copper used in the conventional UBM scheme. The metal layers were deposited on a wafer by evaporation and patterned into contact pads. Eutectic PbSn solder balls were reflowed on top of the pads. SEM micrographs of the intermetallic that forms at the UBM/solder interface show the refining effect of Ni in the interfacial microstructure. Since nickel metallizations often have high stresses, stress in the UBMs was measured by the wafer curvature method. Stress vs Ni content plots show that while stresses increase somewhat with the Ni content, the adhesion layer under the CuNi layer has a much larger effect on the stress. UBMs with Cr/CrCu adhesion layer had stresses ranging from about 300 to 600 MPa, while the stresses in UBMs with Ti/TiNi layers were between 70 and 350 MPa.     

3D amino-induced electroless plating: a powerful toolset for localized metallization on polymer substrates

The "3D amino-induced electroless plating" (3D-AIEP) process is an easy and cost-effective way to produce metallic patterns onto flexible polymer substrates with a micrometric resolution and based on the direct printing of the mask with a commercial printer. Its effectiveness is based on the covalent grafting onto substrates of a 3D polymer layer which presents the ability to entrap Pd species. Therefore, this activated Pd-loaded and 3D polymer layer acts both as a seed layer for electroless metal growth and as an interdigital layer for enhanced mechanical properties of the metallic patterns. Consequently, flexible and transparent poly(ethylene terephtalate) (PET) sheets were selectively metalized with nickel or copper patterns. The electrical properties of the obtained metallic patterns were also studied.     

金属化光纤光栅高温失效及其光谱特性

光纤光栅(FBG)作为温度传感器在智能结构领域得到广泛关注,然而将其埋入金属基体的方法大多是焊接方法。由于焊接过程经历高温,对FBG传感特性将产生一定不良影响。为提高FBG在高温工况下的应用,采用化学镀结合电镀的方法,对FBG进行了镀铜、镀镍金属化试验。采用高温加热炉对金属化FBG进行了短时间高温失效试验,分析了300~900℃区间内金属化FBG的温度灵敏度、光谱特性以及功率衰减规律。结果表明:在300~800℃温度区间,镀铜、镀镍的FBG以及裸光栅的平均温度灵敏度分别为15.35 pm/℃、16.34 pm/℃、16.1 pm/℃;金属化FBG的失效温度约为900℃,与裸光栅相比提高了约100℃;获得了金属化FBG失效过程的光谱及反射峰功率衰减情况。     

第二主族氢化物的高压研究

富氢化合物的压致金属化和超导电性是实现金属氢和高温超导体的有效途径,已成为物理学、材料科学等学科的研究热点之一。从应用上看,富氢化合物是潜在的储氢材料,研究高压下富氢化合物结构和性质变化是提升其储氢性能的有效手段。以典型的第二主族氢化物为例,简要地介绍了第二主族氢化物在高压下的实验与理论研究成果,包括高压结构相变、新结构的稳定性以及金属化机制,并探讨不同的氢构型对压致金属化及超导电性的影响。

     

Mechanism of interfacial reaction for the Sn-Pb solder bump with Ni/Cu under-bump metallization in flip-chip technology

Nickel-based under-bump metallization (UBM) has been widely used in flip-chip technology (FCT) because of its slow reaction rate with Sn. In this study, solder joints after reflows were employed to investigate the mechanism of interfacial reaction between the Ni/Cu UBM and eutectic Sn-Pb solder. After deliberate quantitative analysis with an electron probe microanalyzer (EPMA), the effect of Cu content in solders near the interface of the solder/intermetallic compound (IMC) on the interfacial reaction could be probed. After one reflow, only one layered (Ni_1−x,Cu_x)₃Sn₄ with homogeneous composition was found between the solder bump and UBM. However, after multiple reflows, another type of IMC, (Cu_1−y,Ni_y)₆Sn₅, formed between the solder and (Ni_1−x,Cu_x)₃Sn₄. It was observed that if the concentration of Cu in the solders near the solder/IMC interface was higher than 0.6 wt.%, the (Ni_1−x,Cu_x)₃Sn₄ IMC would transform into the (Cu_1−y,Ni_y)₆Sn₅ IMC. The Cu contents in (Ni_1−x,Cu_x)₃Sn₄ were altered and not uniformly distributed anymore. With the aid of microstructure evolution, quantitative analysis, elemental distribution by x-ray color mapping, and related phase equilibrium of Sn-Ni-Cu, the reaction mechanism of interfacial phase transformation between the Sn-Pb solder and Ni/Cu UBM was proposed.     

Reactions of lead-free solders with CuNi metallizations

We have done experimental research on the dissolution rate and intermetallic growth on Cu, Ni, and CuNi-alloy substrates as a function of time and Cu/Ni ratio of the substrate. Reactions that occur when CuNi metallizations are soldered with lead-free solders were investigated. The experiments were performed using Sn-3.5Ag and Sn-3.8Ag-0.7Cu solders and different CuNi alloys. To determine the rate of dissolution of the substrate material into the solder, CuNi foils of different concentrations were immersed in Sn-3.5Ag and Sn-3.8Ag-0.7Cu solder baths for soldering times ranging from 15 sec to 5 min at 250°C. In addition, reflows of solder balls were made on top of bulk substrates to study the reaction when there is a practically infinite amount of CuNi available compared to the amount of solder. Thin film experiments were also done, where Ni containing under bump metallizations (UBMs) were fabricated and reflowed with eutectic SnAg solder balls. The nickel slows down the dissolution of the UBM into the solder and the formation of intermetallics during reflow compared to Cu metallizations. The solder/UBM interfaces were analyzed with SEM to find out how Ni concentration affects the reaction, and how much Ni is needed to obtain a sufficiently slow reaction rate.