Bálint Medgyes

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.

The effect of chloride ion concentration on electrochemical migration of copper

The effect of chloride ion concentration of electrochemical migration (ECM) was investigated on copper applying in situ electrochemical, optical and scanning electron microscopy—energy dispersive X-ray spectroscopy methods. An unexpected phenomenon was found: copper dendrite grows not only at low chloride concentration, but also at high and even saturated chloride concentrations, although it is generally suggested that the growth of copper dendrite through ECM does not take place in high chloride concentration solutions. Reactions have been proposed to explain the ECM of copper under different chloride concentration levels.

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.

Electrochemical migration of silver on conventional and biodegradable substrates in microelectronics

Water drop (WD) test and a contact angle measurement method were used to investigate the electrochemical migration (ECM) behavior on different microelectronic substrates with silver conductor lines in all cases. Environment friendly substrates were also investigated in this study as possible candidates of the conventional ones. ECM is kind of short failure phenomenon, which can lead to series defect in case of operating microcircuits, when moisture appears on/in a conductor-dielectric-conductor system. The results show that the novel biodegradable substrate patterns can have a significant higher ECM risk comparing to the conventional ones. The main causes can be the different wetting behavior and the different conductor line accuracy of the technological processes.

Electrochemical migration of Sn and Sn solder alloys: a review

Sn and Sn solder alloys in microelectronics are the most susceptible to suffer from electrochemical migration (ECM) which significantly compromises the reliability of electronics. This topic has attracted more and more attention from researchers since the miniaturization of electronics and the explosive increase in their usage have largely increased the risk of ECM. This article first presents an introductory overview of the ECM basic processes including electrolyte layer formation, dissolution of metal, ion transport and deposition of metal ions. Then, the article provides the major development in the field of ECM of Sn and Sn solder alloys in recent decades, including the recent advances and discoveries, current debates and significant gaps. The reactions at the anode and cathode, the mechanisms of precipitates formation and dendrites growth are summarized. The influencing factors including alloy elements (Pb, Ag, Cu, Zn, etc.), contaminants (chlorides, sulfates, flux residues, etc.) and electric field (bias voltage and spacing) on the ECM of Sn and Sn alloys are highlighted. In addition, the possible strategies such as alloy elements, inhibitor and pulsed or AC voltage for the inhibition of the ECM of Sn and Sn solder alloys have also been reviewed.

The effect of NaCl on water condensation and electrochemical migration

The effect of NaCl crystals on water condensation and on electrochemical migration processes (ECM) were investigated in case of different lead bearing and lead-free solder alloys on FR4 substrate during a special Thermal-humidity Bias (THB) test, called dew point test. The dew point test simulated the water condensation process and ECM, which processes were followed by in-situ optically and simultaneously electrically as well. The results showed that NaCl crystals can accelerate the condensation intensity, hence they played role as condensation cores. Furthermore, water droplets have formed locally “water bridges” between adjacent conductor lines. On the other hand, after the dissolution of NaCl crystals the ECM processes were accelerated. Finally precipitates and dendrites were formed resulted in shorts.

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.

Real-time monitoring of electrochemical migration during environmental tests

More and more reliability and quality problems of electrical assemblies have to be solved due to the trend of miniaturization and the even higher level of integration. In some cases these failures can lead to catastrophic failure. The phenomenon of electrochemical migration is one of the most dangerous failure mechanism which usually results in short resistive circuits. In this paper electrochemical migration (ECM) failure phenomena is be investigated which was real-time monitored by the measurements of electrical parameters of conductor patterns with immersion Ag and immersion Ag finish coated with Sn60Pb on Printed Wiring Boards (PWB.) Highly Accelerated Stress Test (HAST) and Thermal Humidity Bias (THB) tests were carried out and Mean Time To Failure (MTTF) comparison was investigated between immersion Ag and immersion Ag coated with Sn60Pb.Only preliminary investigations were presented in order to determine the steps of the full ECM process on different substrates, surface finishes, solder alloys under various climatic conditions. The key findings were that the MTTFs during HAST tests were significantly shorter than in case of THB tests.

Qualifying Methods of Conformal Coatings used on Assembled Printed Circuit Boards

Increased protection of printed circuit boards (PCB) is required in some fields of the electronic industry, because of the diverse climatic circumstances in which these devices are used. This increased protection is reached by the use of different conformal coatings. The article summarizes the temperature-related and the electric parameters of these materials, and classifies them from the viewpoint of their chemical resistance. Furthermore, it demonstrates the influences of two environment tests (thermal shock and thermal humidity tests) on the insulation resistance of two coatings (Dow Corning: 3-1953 silicone elastomer [1] and PETERS:TWIN-CURE DSL 1600 E-FLZ polyurethane-polyacrilate copolymer [2]), in function of the duration of exposing. After that, it demonstrates the formation of polycondensational cross-linking coatings in function of time and layer thickness, and it touches on the measurement of the dielectric strength as well.

Corrosion investigations on lead-free micro-alloyed solder alloys used in electronics

The corrosion properties of different lead-free solder alloys (including novel low Ag content solder alloys) were investigated using the potentiodynamic polarization test method in 3.5 wt% NaCl solution. The results showed that most of the investigated lead-free micro-alloyed solders have better corrosion properties compared to the commonly used SAC305 lead-free solder alloy. The main causes of the different corrosion properties could be the different composition types and concentration of the alloying components.