Balázs Illés

Electrochemical migration behaviour of Cu, Sn, Ag and Sn63/Pb37

The Electrochemical Migration (ECM) behaviour of leaded surface finishes was compared to other surface finishes that are applied in the electronics manufacturing. The studied surface finishes were as follows: bare copper (bCu), immersion tin (iSn), immersion silver (iAg) and Sn63Pb prepared by HASL (Hot Air Solder Leveling). The results were evaluated by Water drop test with the calculation of the Mean Time to Failure (MTTF) and by the investigation of the composition of the dendrites. The results have shown some contradictions relating bare copper and HASL compared to the ECM ranking published previously (Harsányi and Inzelt in Microelectron Reliab 41:229–237, 2001; Yu et al. in J Mater Sci: Mater Electron 17(3):229–241, 2006). The copper and the HASL can change their places in the ECM ranking depending on the technological circumstances of the investigations. This impact can be caused by the composition of lead alloys (eutectic or not), the solubility parameters of the metal hydroxides; the oxidation state of the copper surface, etc. Further and theoretical explanations and the necessary fine adjustment of the migration models are discussed in the paper.

3D thermal model to investigate component displacement phenomenon during reflow soldering

Abstract This paper reports the development of a 3D component-level thermal model to investigate the displacement of the components during the reflow soldering process. One of the root causes of displacement is the temperature deviation between the contact surfaces of the components. Therefore, our model examines the temperature distribution at the level of discrete components and not at the level of whole assembly as is the case of general models used in reflow soldering [Whalley DC. A simplified reflow soldering process model. J Mater Process Technol 2004;150:134–44; Sarvar F, Conway PP. Effective modelling of the reflow soldering process: basis construction and operation of a process model. IEEE Trans Compon Pack Manuf Technol C: Manuf 1998;21(2):126–33; Inoue M, Koyanagawa T. Thermal simulation for predicting substrate temperature during reflow soldering process. In: IEEE Proceeding of the 55th ECTC; 2005. p. 1021–6]. Our model is based on the thermal or central node theory. This means that the investigated area is divided into thermal cells representing the thermal behaviour of the given material. During the model discretization a nonuniform cuboid cell grid is used, with this the resolution of the model can be refined to focus on areas of interest (e.g. the soldering pads) whilst reducing the involvement with the less important areas (such as the epoxy body of a component). This approach therefore allows to achieve a significant increase of spatial resolution of the calculated temperature distribution in some distinct areas with only a little increase of model complexity. The model is described by common heat conduction and convection equations and these are solved by the finite difference method in order to achieve high calculation speeds and easy implementation. The ability of our single purpose model is then compared with a general purpose FEM analyzer. It results that the calculation accuracy of our model is comparable with more detailed models. Also, its calculation speed and application is much faster and easier.

Effect of water condensation on electrochemical migration in case of FR4 and polyimide substrates

The effect of water condensation on electrochemical migration was investigated by water drop and thermal humidity bias tests. The investigations were carried out on immersion silver surface finish on glass fiber epoxy (FR4) and polyimide (PI) substrate. During the thermal humidity bias tests, water condensation was monitored by an in situ and real time optical inspection system. Simultaneously, electrical measurements were also done in order to validate the visual results. The water drop tests (without condensation effect) have not showed any dendrite growth time difference between the substrate materials. However, in the case of the thermal humidity bias tests (with condensation effect), it was observed that the water condensation intensity is significantly related to the electrochemical migration failure mechanism. In the case of FR4 substrate the mean time to failure was observed to be lower than in case of PI which effect was caused by the different water condensation intensity which depends on the surface roughness and the thermal diffusivity of the substrate materials. Therefore, the water condensation effect has to be considered in the classical electrochemical migration model.

Characterization of vapour phase soldering process zone with pressure measurements

Purpose – The purpose of this paper is to present a novel approach on the process zone characterization for direct feedback regarding the state of vapour, in order to assure a better monitoring, control and understanding of the process.Design/methodology/approach – Different pressure sensors were applied in an experimental vapour phase soldering (VPS) station, where the hardware setup was dedicated to the current experiments. Static and dynamic pressure values were analyzed and correlated with additional thermal measurements.Findings – The results reveal the dynamics of the vapour blanket generation. The correlated measurements show different stages of the process initialization, highlighting better accuracy than sole temperature measurements of saturated vapour identification. It is possible to trace the height of the available saturated vapour blanket with static pressure measurements.Practical implications – The VPS process may benefit from the more precise saturation detection, giving better control o...

Lead-Free Soldering Technology Review - Evaluating Solder Pastes and Stencils

Lead-free solders differ from lead-bearing solders in several properties such as surface tension, wetting ability, etc. In paste form they have greater adhesion force, so they can block small stencil apertures more easily. Because of this fact, the printing parameters and stencils design criterias have to be changed. Since lead-free soldering has to be implemented in the electronic industry until 1st of July 2006, selection of the appropriate pastes and stencils is more important than ever. In our experiment six TSC (Tin-Silver-Copper) solders from different suppliers were evaluated from aspects of wetting properties, solderability and printability alike. Component self alignment was also investigated. In addition three kinds of stencils (lasercut stainless steel, lasercut nickel and electroformed nickel) were investigated as well by measuring transfer efficiency in function of area ratio of stencil-apertures. Results have shown that electroform stencils have outstanding printing properties while lasercut stencils (stainless steel and nickel) have similar capabilities independently of the material, thus it is not sure it is worth using lasercut nickel stencils for double the price. The evaluation of solders has revealed that the new lead-free pastes have equally excellent printing and soldering properties. Lead-free soldering with these pastes will not be problem.

Present status of transition to Pb-free soldering

The global electronics industry is adjusting to lead-free technology, and it is a surviving criterion for all industrial performers, in particular for small and medium-size enterprises (SMEs), to keep pace with this transition. This paper provides basic information on lead-free soldering, discussing the main topics as follows: lead-free alloys, finishes and coatings, components, fluxes for lead-free solders, soldering equipment and processes, the use of nitrogen atmosphere, reliability of lead-free solder joints, and causes of lead-free soldering faults. Finally those topics are summarized, which need to be taken into account and whose results and/or solutions should be implemented during the transition to the lead-free manufacturing process.

Comparative study on proper thermocouple attachment for vapour phase soldering profiling

Purpose – The paper aims to present a comparative study of various thermocouple (TC) attaching methods for the proper measurement of soldering temperature profiling during vapour phase soldering (VPS). The heat transfer process during VPS is different from common methods, while the required heat for reflow is provided by the condensation. The condensate is a flowing layer on the board, where the dynamic behaviour also affects the local conditions on the surfaces. Temperature measurements based on TCs are also affected this way; it is important to investigate the process for deeper understanding. Design/methodology/approach – Bare printed circuit boards (PCBs) were prepared for standard K-Type TCs attachment with industry standard materials: kapton polyimide tape, aluminium tape, SMD adhesive and high-temperature solder (HTS). Heating experiments were performed in a batch-type VPS oven with Galden LS240 fluid and fixed oven parameters. Findings – According to the specific attachment requirements, HTS and A...

Investigating the thixotropic behaviour of Type 4 solder paste during stencil printing

Purpose A measurement method has been developed to reveal the viscosity change of solder pastes during stencil printing. This paper aimed to investigate thixotropic behaviour, the viscosity change of a lead-free solder paste (Type 4). Design/methodology/approach The viscosity change of the solder paste during stencil printing cycles was characterised in such a way that the time-gap between the printing cycles was modelled with a rest period between every rheological measurement. This period was set as 15, 30 and 60 s during the research. The Cross model was fitted to the measurement results, and the η0 parameter was used to characterise the viscosity change. The number of printing cycles necessary for reaching a stationary state in viscosity was determined for various rest periods. Findings It was found that the decrease in zero-shear viscosity is significant (25 per cent) in the first cycles, and it starts to become stationary at the sixth-seventh cycles. This means a printing process can provide the appropriate deposits only after the 7th cycle with the investigated Type 4 solder paste. Originality/value Time-dependent rheological behaviour of solder pastes was studied in the literature, but only the viscosity change over continuous time at constant shear rates was examined. The time-gap between stencil printing cycles was not considered, and thixotropic behaviour of solder pastes was also neglected. Therefore, the authors developed a measurement set which is able to model the effect of time-gap between printing cycles on the viscosity change of solder pastes.

Comparing 2D and 3D numerical simulation results of gas flow velocity in convection reflow oven

Purpose – This paper aims to compare and study two-dimensional (2D) and three-dimensional (3D) computational fluid dynamics simulation results of gas flow velocity in a convection reflow oven and show the differences of the different modeling aspects. With the spread of finer surface-mounted devices, it is important to understand convection reflow soldering technology more deeply. Design/methodology/approach – Convection reflow ovens are divided into zones. Every zone contains an upper and a lower nozzle-matrix. The gas flow velocity field is one of the most important parameters of the local heat transfer in the oven. It is not possible to examine the gas flow field with classical experimental methods due to the extreme circumstances in the reflow oven. Therefore, numerical simulations are necessary. Findings – The heat transfer changes highly along the moving direction of the assembly, and it is nearly homogeneous along the traverse direction of the zones. The gas flow velocity values of the 2D model are...

Investigating of electrochemical migration on low-ag lead-free solder alloys

Among the conditions of high humidity, elevated temperature, and voltage applied, metals and alloys in electronic components and assemblies can cause insulation failures by forming so called dendrites due to the electrochemical migration (ECM) failure phenomenon. This effect causes short-circuit failures of the electronic circuits, which might lead to catastrophic failure. The ECM behavior of lead-free micro-alloyed low Ag content solder alloys (SAC0307 and SAC0807) were compared with common used SAC305, SAC405 and with the traditionally lead-bearing solder alloys: Sn63Pb37 and Sn62Pb36Ag2 as references. In order to carry out the ECM investigations, water drop (WD) tests were made on standard comb patterns in 1 mMol NaCl and 1 mMol Na2SO4 solutions as well. The results have shown that the SAC0807 low Ag content micro-alloyed solder alloy could have a relative weak resistance against ECM.