Rheological characterization of a solder paste for surface mount applications

Solder pastes used in surface mount soldering techniques (SMT) are very complex suspensions containing high volumes of metallic powder in a carrier fluid. The rheological complexity results largely from the carrier fluid itself, which is a suspension of colloidal particles. In this work, we have characterized the rheological properties of a typical carrier fluid and its solder paste containing 64 vol.% metallic powder. A six-blade vane geometry was used to avoid wall slip and sample fracture. All measurements were carried out following pre-shearing and rest time in order to obtain reproducible results. Steady shear experiments showed that the solder paste was highly shear-thinning and thixotropic. In oscillatory shear, the linear viscoelastic domain was found to be very narrow for both the suspending fluid and the paste. Frequency sweep tests in the linear domain revealed a gel-like structure with a nearly constant G′ for the suspending fluid and a slightly increasing G′ for the solder paste. From creep experiments, a yield stress of about 40 Pa was determined for the suspending fluid at temperatures between 25 and 40°C, and of 100 Pa at 4°C. A much larger yield stress, 480 Pa, was determined for the solder paste at 25°C.     

Synthesis of boron suboxide (B6O) with ball milled boron oxide (B2O3) under lower pressure and temperature

Boron reacted with ball milled boron oxide under pressures between 1 and 5 GPa and at temperatures between 1300 and 1700 °C to afford boron suboxide (B₆O). Icosahedral B₆O grains with diameters ranging from 100 nm to 1.3 μm were prepared. The factors that affect the synthesis of B₆O are investigated. The best sample with crystal size up to 1.3 μm is obtained at 2 GPa and 1400 °C for 6 h. The indentation experiment gave an average Vickers hardness of 32.3 GPa for bulk B₆O sample, which is consistent with previous reports. Bulk B₆O sample exhibits oxidation resistance in air up to 1000 °C and mild oxidation in the temperatures of 1000-1200 °C, which is more oxidation resistant than diamond. It is possible that B₆O could be used as a substitute for diamond in industry because of its relatively mild synthesis conditions, high thermal stability and high hardness.     

红外光谱检测阻焊油墨固化程度试验报告

为从本质上分析阻焊剂剥离的原因,即光固化成分和热固化成分含量的差异,开展此试验。通过采用红外光谱检测阻焊油墨中环氧基以及丙烯基的含量,以衡量油墨的固化程度。并以此分析生产流程各环节对油墨固化程度的影响。通过研究不同的固化成分对阻焊剂结合力的影响,在业界开创了一种新的量化方法。     

干膜与阻焊油墨对化镍金厚度的影响

感光阻焊与干膜析出物,是化镍金表面工艺金、镍缸控制寿命的原因之一,旨在改善黑焊盘缺陷。由于阻焊、干膜种类繁多,且阻焊、干膜与化镍金药水属于不同公司或厂家,鉴于配方专利等原因,单方对其相互影响原理甚至现象鲜有研究,此方面文献更是屈指可数。本文简述了"黑焊盘"发生的基本原理,并利用干膜以及不同阻焊油墨进行试验,通过测量镍金层厚度评估干膜与油墨对化镍金工艺的影响。通过机理验证试验,验证阻焊油墨析出物抑制化镍反应,而非目前业界所认知的加速反应。本实验一则为PCB厂家提供经验数据,利于生产控制;二则抛砖引玉,促进阻焊油墨、干膜、化镍金药水及PCB厂家对其进一步研究。     

Application of Ball Milling Technology to Carbohydrate Reactions: I. Regioselective Primary Hydroxyl Protection of Hexosides and Nucleoside by Planetary Ball Milling‡

Dry ball milling of hexosides with trityl chloride in the presence of DABCO or Na₂CO₃ has been found to result in their complete conversion to the respective 6‐O‐trityl ethers. Further wet grinding of the reaction mixture with Ac₂O in the presence of DMAP led to the respective fully protected hexosides in good to excellent yields after isolation. It has been found to be an effective one‐pot two‐step synthesis under solvent‐free condition. The speed of homogenization has been shown to highly influence the rate and outcome of the reaction, and commercially available planetary ball mill has been proved to be very convenient for carrying out the reaction under standardized and reproducible conditions.     

Influence of transient flow and solder bump resistance on underfill process

The underfill flow process is one of the important steps in Microsystems technology. One of the best known examples of such a process is with the flip-chip packaging technology which has great impact on the reliability of electronic devices. For optimization of the design and process parameters or real-time feedback control, it is necessary to have a dynamic model of the process that is computationally efficient yet reasonably accurate. The development of such a model involves identifying any factors that can be neglected with negligible loss of accuracy. In this paper, we present a study of flow transient behavior and flow resistance due to the presence of an array of solder bumps in the gap. We conclude (1) that the assumption of steady flow in the modeling of the flow behavior of fluids in the flip-chip packaging technology is reasonable, and (2) the solder bump resistance to the flow can not be neglected when the clearance between any two solder bumps is less than 60-70 μm. We subsequently present a new model, which extends the one proposed by Han and Wang in 1997 by considering the solder bump resistance to the flow.     

BGA封装技术与SMT／SMD

文中就ＢＧＡ与ＳＭＴ／ＳＭＤ之间有关的主要问题进行了较为详细的介绍和分析。     

0.5 mm间距CSP焊接工艺研究

随着对各种电子产品，尤其是消费类电子产品的便携性和多功能的追求，CSP等新型封装器件(封装尺寸约为芯片本身尺寸的1．2倍)便应用到这些产品的设计中去。CSP器件的引脚间距有0．8mm、0．75mm、0．65mm、0．5mm等。为了便于以后产品设计和生产的需要，就CSP器件在PWB设计和焊接两方面进行研究，侧重于焊接方面。     

Effect of silver content on thermal fatigue life of Sn-xAg-0.5Cu flip-chip interconnects

The thermal fatigue properties of Sn-xAg-0.5Cu (x=1, 2, 3, and 4 in mass%) flip-chip interconnects were investigated to study the effect of silver content on thermal fatigue endurance. The solder joints with lower silver context (x=1 and 2) had a greater failure rate compared to those with higher silver content (x=3 and 4) in thermal fatigue testing. Cracks developed in the solders near the solder/chip interface for all joints tested. This crack propagation may be mainly governed by the nature of the solders themselves because the strain-concentrated area was similar for tested alloys independent of the silver content. From the microstructural observation, the fracture was a mixed mode, transgranular and intergranular, independent of the silver content. Higher silver content alloys (x=3 and 4) had finer Sn grains before thermal cycling according to the dispersion of the Ag₃Sn intermetallic compound, and even after the cycling, they suppressed microstructural coarsening, which degrades the fatigue resistance. The fatigue endurance of the solder joints was strongly correlated to the silver content, and solder joints with higher silver content had better fatigue resistance.     

Strength of Soldered Joints Formed under Microgravity Conditions

Experiments have been carried out to compare the mechanical strength of joints soldered with Sn-Ag-Cu under different gravitational conditions. Joints soldered under microgravity (produced during a parabolic flight) have lower strength (by 32% in this case) than similar joints formed under normal gravity. Electron microscopy has shown that this is due to a larger volume of residual porosity (14%) in the joints formed in microgravity compared with <1% for joints formed in normal gravity. The residual porosity in joints formed in microgravity is mainly within the bulk of the solder, with some microporosity in the Cu₆Sn₅ intermetallic layer near the copper interfaces. The porosity not only weakens the joint, but also biases the failure path away from the intermetallic layer and into the bulk of the solder. These observations show that gravitational buoyancy is important for the expulsion of flux and flux residues from soldered joints.