Search results

The mechanisms for void formation are investigated for applications involving solder paste in surface mount technology. Generally, voids are caused by the outgassing of entrapped flux in the sandwiched solder during reflow. The voiding is dictated mainly by the solderability of metallisation, and increases with decreasing solderability of metallisation, decreasing flux activity, increasing metal load of powder, and increasing coverage area under the lead of the joint. Decrease in the solder powder particle size has only a slightly negative effect on voiding. The data indicate that voiding is also a function of the timing between the coalescing of solder powder and the elimination of immobile metallisation oxide. The sooner the paste coalescence occurs, the worse the voiding will be. Increase in voiding is usually accompanied by an increasing fraction of large voids, suggesting that factors causing voiding will have an even greater impact on the joint reliability than shown by the total‐ void‐volume analysis results. Preliminary data suggest that certain predry treatment and flux solvent with higher boiling point appear to cause increased voiding.

Voiding in BGA assembly using SN63 solder bumps is primarily introduced at board‐level assembly stage. On the pretinned PCBs, voiding of BGA joints increases with increasing solvent volatility, increasing metal content and increasing reflow temperature, and with decreasing powder size. This can be explained by a viscosity dictated flux‐exclusion‐rate model. In this model, a higher viscosity in the fluxing medium at reflow temperature could hinder the exclusion of flux from the interior of the molten solder, hence increase the chance of outgassing due to the increasing amount of entrapped flux, and consequently result in higher voiding in BGA assembly. Flux activity and reflow atmosphere appear to have a negligible effect on voiding when the solderability of the immobile metallisation is not a concern. An increase in void content is accompanied by an increase in the fraction of large voids. This suggests that, similar to voiding phenomena in the SMT process, factors causing voiding in BGA will have an even greater impact on joint reliability than shown by the total‐void‐volume analysis results.

Thermal management of high-power (HP) light-emitting diodes (LEDs) is an essential issue. Junction temperature (T_J) and thermal resistance (R_th) are critical parameters in evaluating LEDs thermal management and reliability. The purpose of this paper is to study thermal and optical characteristics of ThinGaN (UX:3) white LED mounted on SinkPAD by three types of solder paste (SP): No-Clean SAC305 (SP1), Water-Washable SAC305 (SP2) and No-Clean Sn42/Bi57.6/Ag0.4 (SP3).

Thermal transient tester (T3Ster) machine is used to determine T_J and total thermal resistance (R_th–JA). In addition, the LED’s optical properties are measured via thermal and radiometric characterization of power LEDs (TeraLED) system. The LED is mounted on SinkPAD using SP1, SP2 and SP3 by stencil printing to control a thickness of SP and reflow soldering oven to minimize the number of voids. The LED with SP1, SP2 and SP3 is tested at various input currents and ambient temperatures.

The results indicate that at high input current, which equals to 1,200 mA, R_th–JA and T_J, respectively, are reduced by 30 and 17 per cent between SP1 and SP2. At same current value, R_th–JA and T_J are minimized by 42 and 25 per cent between SP1 and SP3, respectively. In addition, at an ambient temperature of 85°C, R_th–JA and T_J are decreased by 34 and 7 per cent between SP1 and SP2, respectively. Similarly, the reduction in R_th–JA and T_J between SP1 and SP3 is 44 and 10 per cent, respectively. Luminous flux, luminous efficacy and color shift of the LED with the three types of SPs are compared and discussed. It is found that the SP1 improves the chromatic properties of the LED by increasing the overall light efficiency and decreasing the color shift.

Thermal and optical performance of ThinGaN LEDs mounted on SinkPAD via three types of SPs is compared. This investigation can assist the research on thermal management of HP ThinGaN-based LEDs.

12 mil pitch processing is achievable with solder paste. It may also be the limit of solder paste printing technology, mainly due to the scooping problem associated with thin stencils. With decreasing pitch size, both smear and insufficiency rate increase. Tapering of stencil aperture helps thick stencil prints, but has an adverse effect on thin stencil printing. Apertures with orientation parallel to squeegee movement result in a higher print defect rate. Overall, the use of fine powders is the most effective means to meet most challenges. It helps in achieving high performance in printability, tack and non‐slump, with acceptable trade‐offs in rheology and tack time. Solder balling may be the primary drawback. The problem may be resolved by using inert reflow atmosphere or via flux chemistry improvements. A metal load of 90.5 to 91% seems to be the optimum for most properties.

As one of the major approaches to addressing the CFC issue, no‐clean solder paste has received rapidly increasing attention. Although currently the industry seems to accept full residue paste as a temporary solution, low‐residue no‐clean paste technology using inert or reactive atmosphere is advancing quickly to meet the challenge. At present no consensus has been established regarding how low a residue level could be achieved and how inert the atmosphere needs to be. In this study, a semi‐empirical model is proposed to predict the soldering performance of low‐residue solder pastes under various levels of inert reflow atmosphere. The model predicts that the soldering performance would improve rapidly, then gradually level off with decreasing oxygen content. The soldering performance vs oxygen content curves are superimposable, with the lower residue one levelling off at lower oxygen level. In general, the experimental data match this model fairly well. However, the data also indicate that, although inert atmosphere improves soldering performance, the optimum condition for bond strength performance seems to demand the presence of some oxygen. This unexpected behaviour suggests that a very tight low oxygen level control may not be required. The mechanism responsible for this phenomenon can be attributed to oxidation‐induced resin crosslinking. This slows down the flux drying rate as well as hindering the permeation of oxygen through the flux layer.

The purpose of this paper is to employ data mining as a new diagnosing scheme for investigating void formation to the thermal pad in quad flat non‐lead (QFN) assembly. Occurrences of voiding in various scenarios of component design, materials selection and manufacturing process are analyzed.

This research investigates the process yield of a PCB assembly for a handheld device in the electronics manufacturing industry using the chi‐square automatic interaction detection (CHAID) algorithm and chi‐square test. Practical data generated by an X‐ray apparatus from the shop floor are collected. The critical attributes to the void formation (in the solder joint) of the QFN component are identified.

Stocking the PCB material beyond ten days may increase the level of voiding by 1%. Using PCB provided by vendor U helps decrease the level of voiding by 1.6%. Stocking the component material above 43 days may increase the level of voiding by 1.9%. In addition, reflow soldering profile with time above liquid (TAL) less than or equal to 62 sec and with peak temperature higher than or equal to 241°C generate less voids. Finally, the via‐in‐pad design causes a concave geometry on the surface of thermal pad which contributes to the voiding formation. The amount of voiding can be further diminished by plugging the via with plated copper.

This research implements CHAID that extracts useful knowledge from a huge amount of manufacturing data in order to realize the complex interaction effects through automated analysis. The extent of voiding in the samples using the optimal process suggested through CHAID algorithm can be reduced from 16% to 10.2%.

Tombstoning and voiding have been plaguing the surface mount assembly industry for decades. The recent global move toward lead‐free soldering and the extensive adoption of microvia technology further aggravate the problems. The present study investigates the impact of SnAgCu (SAC) alloy composition on these important issues.

In this study, tombstoning and voiding at microvias are studied for a series of SAC lead‐free solders, with an attempt to identify a possible “composition window” for controlling these problems. Properties which may be related to these problems, such as alloy surface tension, alloy melting pattern, and solder wetting behaviour, were investigated in order to assess the critical characteristics required to control these problems.

The results indicate that the tombstoning of SAC alloys is greatly influenced by the solder composition. Both the wetting force and the wetting time at a temperature well above the melting point have no correlation with the tombstoning frequencies. Because the tombstoning is caused by imbalanced wetting forces, the results suggest that the tombstoning may be controlled by the wetting at the onset of the paste melting stage. A maximum tombstoning incidence was observed for the 95.5Sn3.5Ag1Cu alloy. The tombstoning rate decreased with increasing deviation in Ag content from this composition. A differential scanning calorimetry (DSC) study indicated that this was mainly due to the increasing presence of the pasty phase in the solders, which result in a slower wetting speed at the onset of solder paste melting stage. Surface tension plays a minor role, with lower surface tension correlating with a higher tombstoning rate. The voiding rate at the microvias was studied by employing simulated microvias. The voiding level was lowest for the 95.5Sn3.8Ag0.7Cu and 95.5Sn3.5Ag1Cu alloys, and increases with a further decrease in the Ag content. The results indicate that voiding at microvias is governed by the via filling and the exclusion of fluxes. The voiding rate decreased with decreasing surface tension and increasing wetting force, which in turn is dictated by the solder wetting or spreading. Both low surface tension and high solder wetting prevents the flux from being entrapped within a microvia. A fast wetting speed may also facilitate reducing voiding. However, this factor is considered not as important as the final solder coverage area.

In general, compositions which deviate from the ternary eutectic SAC in Ag content, particularly with a Ag content lower than 3.5Ag, exhibit a greater solid fraction at the onset of melting, resulting in a lower tombstoning rate, presumably due to a slower wetting speed. The SAC compositions with an Ag content lower than 3.5 per cent, such as 2.5Ag, resulted in a lower tombstoning rate with minimal risk of forming Ag₃Sn intermetallic platelets. On the other hand, ternary eutectic SAC exhibits a lower surface tension resulting in an easier solder spread or solder wetting, and consequently exhibit a higher tombstoning frequency and a lower incidence of voiding.

Provides a solution to the tombstoning problem in lead‐free soldering.

The present study provided a solution to the tombstoning problem encountered in lead free soldering by controlling the SAC solder alloy compositions.

The purpose of this paper is to study the solder beading phenomenon (referring to larger-sized solder balls) of surface-mounted electrolytic capacitors. Solder beading could induce failures by violating the minimal electrical clearance on the printed circuit board (PCB). In modern lead-free reflow soldering, especially in high-reliability industries, such as automotive, aeroplane and aerospace, detecting and preventing such defects is essential in reliable and cost-effective manufacturing.

The large size of the involved components may block the view of automatic optical inspection; therefore, X-ray inspection is necessary. To detect the failure mode, X-ray imaging, cross-section grinding, optical microscopy and Fourier transformed infrared spectroscopy were used. High-resolution noncontact profilometry and optical microscopy were used to analyse component designs. The surface mounting process steps were also analysed to reveal their dependence on the issue. Test methods were designed and performed to reveal the behaviour of the solder paste (SP) during the reflow soldering process and to emphasise the component design relevance.

It was found that the reduction of SP volume only reduces the failure rate but does not solve the problem. Results show that excessive component placement pressure could induce solder beading. Statistical analysis revealed that differences between distinct components had the highest effect on the solder beading rate. Design aspects of solder beading-prone components were identified and discussed as the primary source of the problem.

The findings can be applied in surface-mount technology production, where the total failure count and resulting failure costs could be reduced according to the findings.

This paper shows that component design aspects such as the low distance between the underside of the component and the PCB and blocked proper outgassing of volatile compounds of the SP can be root causes of solder beading under surface-mounted electrolytic capacitors.

Presents over sixty abstracts summarising the 1999 Employment Research Unit annual conference held at the University of Cardiff. Explores the multiple impacts of globalization on work and employment in contemporary organizations. Covers the human resource management implications of organizational responses to globalization. Examines the theoretical, methodological, empirical and comparative issues pertaining to competitiveness and the management of human resources, the impact of organisational strategies and international production on the workplace, the organization of labour markets, human resource development, cultural change in organisations, trade union responses, and trans‐national corporations. Cites many case studies showing how globalization has brought a lot of opportunities together with much change both to the employee and the employer. Considers the threats to existing cultures, structures and systems.

Globalisation is generally defined as the “denationalisation of clusters of political, economic, and social activities” that destabilize the ability of the sovereign State to control activities on its territory, due to the rising need to find solutions for universal problems, like the pollution of the environment, on an international level. Globalisation is a complex, forceful legal and social process that take place within an integrated whole with out regard to geographical boundaries. Globalisation thus differs from international activities, which arise between and among States, and it differs from multinational activities that occur in more than one nation‐State. This does not mean that countries are not involved in the sociolegal dynamics that those transboundary process trigger. In a sense, the movements triggered by global processes promote greater economic interdependence among countries. Globalisation can be traced back to the depression preceding World War II and globalisation at that time included spreading of the capitalist economic system as a means of getting access to extended markets. The first step was to create sufficient export surplus to maintain full employment in the capitalist world and secondly establishing a globalized economy where the planet would be united in peace and wealth. The idea of interdependence among quite separate and distinct countries is a very important part of talks on globalisation and a significant side of today’s global political economy.