Conventional Reflow Research Articles

Reflowed solgel spherical microlens for high-efficiency optical coupling between a laser diode and a single-mode fiber

To improve the coupling efficiency between a laser diode and a single-mode fiber, we propose a two-microlens coupling scheme that uses two solgel spherical microlenses for high coupling efficiency. The conventional reflow technique was employed and extended to the inorganic-organic hybrid SiO2/ZrO2 solgel material to form the microlenses. Preliminary results show that the coupling efficiency was increased to--1.28 dB (74.5%) by the proposed scheme, compared with a coupling efficiency of--10.13 dB (9.7%) by the butt-joint method. The proposed fabrication technique demonstrates that use of a reflowed solgel spherical microlens is a cost-effective mass-production approach to application of micro-optical elements in optical communication.

Characteristics of the Sn-Pb eutectic solder bump formed via fluxless laser reflow soldering

In an attempt to develop a fluxless reflow solder bumping process, the effects of processing variables, which include energy input rate and time, and the shape of solder disk on the microstructure of the solder/Cu pad interface and the shear strength of the joints were investigated. It was demonstrated that a proper combination of the variables could lead to the formation of a spherical solder bump with shear strength comparable to that formed via the conventional reflow soldering process. In addition, the kinetics of Cu pad dissolution into the solder during laser heating was modeled numerically to elucidate intermetallic formation mechanism at the solder/Cu pad interface.

Effect of intermetallic compounds on the thermal fatigue of surface mount solder joints

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints during thermal cycling has been studied. The samples consist of components [leadless ceramic chip carrier (LCCC)] soldered onto FR-4 printed circuit board (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste. The specimens are subjected to thermal cycling between -35/spl deg/C and 125/spl deg/C with a frequency of two cycles per hour until failure. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of IMC's layer between Cu-pad and bulk solder, and the relation of the lifetime to the thickness can be described as a monotonically decreasing curve. The lifetime is very sensitive to the thickness of the IMC when the thickness is less than 1.4 /spl mu/m. During thermal cycling the thickness of the IMC layer increases and then the interface between IMC and solder becomes gradually flatter. The results of X-ray diffraction and scanning electron microscope (SEM) analysis show that cracks propagate along the interface between the IMC layer and the solder joint. The Cu/sub 3/Sn (/spl epsiv/-phase) is also found to form between the Cu-pad and /spl eta/-phase during thermal cycling. On the basis of the above results, the thick and flattened IMC layer is shown to responsible for the fatigue failure of solder joint during thermal cycling. The results of this paper can be used to optimize the reflow soldering process for the fabrication of robust solder joints.

Effect of intermetallic compounds on the shear fatigue of Cu/63Sn-37Pb solder joints

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints has been studied by means of shear cycling. The samples consist of leadless ceramic chip carriers (LCCC) soldered onto FR-4 printed circuit boards (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste and then aged at 150/spl deg/C for 1, 4, 9, 16, 25, 36, and 49 days. The specimens are subjected to low cycle fatigue shear tests controlled by the displacement. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of the LMC layer between the Cu-pad and bulk solder, and the quantitative relationship between the lifetime and thickness can be described as a monotonically decreasing curve. The greatest decrease is over the thickness range up to 2.8 /spl mu/m, when the IMC/bulk solder interface becomes flat, corresponding to a lifetime decrease to 62% of the as assembled value. For further increase in IMC thickness the lifetime decreases more slowly. Evidently, the effect of the Cu-Sn intermetallic compounds on the joint fatigue lifetime is not only concerned with the IMC thickness hut also the interface morphology. A thick and flat LMC layer has a deleterious effect. The results of X-ray diffraction and metallographic analysis show that cracks initiate underneath the component metallization, and propagate along the IMC/solder interface, then toward the fillet. The Cu/sub 3/Sn (/spl epsiv/-phase) is formed between the Cu-pad and p-phase, and grows more quickly than the /spl eta/-phase during storage and long term operation or aging tests. However, the Cu/sub 3/Sn makes only a small direct contribution toward fatigue failure.

Aluminum-germanium-copper multilevel damascene process using low-temperature reflow sputtering and chemical mechanical polishing

A low-temperature multilevel aluminum-germanium-copper (Al-Ge-Cu) damascene technology was developed using reflow sputtering and chemical mechanical polishing (CMP). The maximum processing temperature for the fabrication of multilevel interconnections could be reduced to 420/spl deg/C using Al-1%Ge-0.5%Cu, whereas the conventional reflow temperature was not less than 500/spl deg/C. No degradation due to reflow heat cycles was observed in terms of Al-Ge-Cu wiring resistance. Electromigration test results indicated that the mean time to failure (MTTF) of Al-1%Ge-0.5%Cu was longer than 10 years at the operating condition, which was equivalent to that of Al-1%Si-0.5%Cu. The Al-1%Ge-0.5%Cu triple-level interconnection was fabricated using reflow sputtering to fill vias and wiring trenches and subsequent CMP for Al-Ge-Cu films.

Fatigue life studies on defect-free solder joints fabricated from modified reflow soldering

This paper describes the improved characteristics of defect-free solder joints fabricated through modified reflow soldering. The modified method only requires that the solder joints are split for a very short time during a conventional solder reflow. The fatigue properties and microstructure of the solder joints are critically studied. It is found that the method of splitting is effective in eliminating pore formation (both gas and shrinkage pores) and inclusions in solder joints. The method is applicable to various solder pastes, whether it be no-clean or water-soluble. Thermal and mechanical fatigue cycling tests show that the fatigue life of the solder joints can be prolonged by more than 60% as compared to that without splitting. Fractographs illustrate that the fracture in the fatigued joints occurs quite often at the interfaces of printed circuit board (PCB) and copper pads when splitting has been applied to the joints. This affirms that the solder joints have been strengthened considerably by splitting. The proposed method is particularly suitable for specimens with a large pad area in each joint or when the pores or inclusions are likely to form during solder joint fabrication.

NON-CONTACT TEMPERATURE MEASUREMENT IN SM COMPONENT REWORK

Although surface mount assembly equipment has been rapidly developed during the last decade, surface mount device rework equipment is still largely basic and its development has not kept pace. The rework process is still viewed as a side issue, even when manufacturers consider the quality of the reworked board to be as good as the original board. An important consideration in rework is the control of the heating of the surface mount (SM) component, and although various attempts have been made to automate the heating process, the rework heating process is unfortunately still not as good and uniform as those for conventional SM reflow. Primarily, the techniques used for rework are manually assisted, and usually rely on the rework operator's judgement in the observation and control of the rework process. A real-time closed-loop controlled rework-reflow system, capable of reflowing all SM components regardless of their heat capacity, type of Printed Circuit Board Assembly (PCBA) and the position of components on PCBA, has been developed as part of a fully automated robotic rework cell. The reworkreflow system is a separate unit, which can be easily adapted for use in any manual rework system. An important part of the system is a non-contact sensor, for real-time measurement of the reflow temperature and its feedback to the device controller unit. The paper describes the closed-loop controlled reflow system employed in the automated robotic rework cell, highlights the problems associated with its use, and then summarizes the requirements for non-contact temperature measurement in reworking SM components. The principles of non-contact measurement and the importance of emissivity for temperature measurement are also highlighted, and the experimental results of the true emissivity of the SM components are presented.