Solder electromigration

Many studies have been concerned with electromigration on PCB s (8-11). Metallic (dendritic) growth of metal from one conductor to another can cause microamp current leakage or electrical shorts. Since high insulation resistance lowers the rate of electromigration, the insulation resistance of a solder mask and the factors which influence it are of great interest. This study was performed to isolate the affects of temperature and humidity on the insulation resistance of photopolymerizable solder masks relative to conventional solder masks. 

Studies have shown that reliable lead-free solder joints, with proper grain structures and in-termetallics formation, can be produced using appropriate rework processes. Care must be taken to minimize any potential negative impact of the rework process on the reliability of the components and the PWB. Surface insulation resistance (SIR) tests must be performed to ensure the compatibility between the reflow/wave solder flux and the rework flux, i.e., to ensure that the rework flux and any products of reaction between the reflow/wave solder flux and the rework flux do not pose any unacceptable risk for electromigration and dendritic growth for noclean applications. 

Parameters IPC-TM-650 Solder flux Telcordia SIR TA-NWT-000078 Telcordia electromigration... 

Liu YH, Lin KL (2005) Damages and microstructural variation of high-lead and eutectic SnPb composite flip chip solder bumps induced by electromigration. 

However, the presence of an applied voltage can potentially accelerate corrosion activity in Pb-Sn solder joints. The predominant phenomenon is electromigration, which is illustrated in... 

Fig. 31 [99]. Electromigration occurs when a voltage dilference between neighboring solder joints sets up a cathode/anode pair. A corrosion cell is formed when an electrolyte is present. For example, the combination of moisture and flux residues can comprise an electrolyte. Metal ions generated by an oxidation reaction at the anode travel to the cathode where they are reduced to their elemental metal. The elemental metal forms filaments that grow back toward the anode. Once the filament contacts the anode, a short circuit occurs between the two joints. The tenacious oxides of both Pb and Sn slow the electromigration phenomenon compared to other metals that do not form tenacious oxides, the most notorious being Ag.

Liu, C.Y. Chen, C. Liao, C.N. Tu, K.N. Microstructure-electromigration correlation in a thin strip of eutectic SnPb solder stressed between Cu electrodes. Appl. Phys. Lett. 1999, 75(1), 58-60. 

The first ever observation of electromigration occurred long before it became an engineering problem. Fully a century before it was recognized as a reliability issue, electromigration was observed in a series of experiments that are remarkably relevant even today. In fact, the same reasons that electromigration was observed in these older experiments apply today to solders as well. 

Although there has been significantly more research carried out on Pb-based solders than Pb-free solders, the literature is scanty compared with other conductors such as Cu, Al, and their alloys. Lead-based solders are two-phase alloys (Fig. 6), usually combined with Sn, but sometimes alloyed with other metals depending on the specific application. In many applications, such as in microelectronics, where low melting points are desired, eutectic (63% Sn/Pb) or near-eutectic solders are used, but high-Pb (>90%Pb) solders are typically used for flip-chip solder bumps owing to their resistance to electromigration. 

Electromigration failure in Pb-alloy solders can be classified in two ways (1) the mass transport of Pb or one of the major alloying elements in a solder alloy, such as Sn in a traditional eutectic... 

Electromigration Failure in Flip Chip Solder Joints... 

As discussed, flip-chip solder joints (Fig. 7) can be susceptible to electromigration failure because the relatively high diffusion coefficient of solder alloys enables significant diffusion at relatively... 

Although electromigration data related to Pb-free solders is sparse in the published hterature, it is well known that Sn acts very much like Pb as a host for fast diffusers. The same metals that diffuse rapidly in Pb alloys also diffuse rapidly in Sn, and exhibit many of the same types of behavior. The differences are important, and they reside in the details. One important difference is that unlike Pb, Sn does not possess a cubic crystal structure so the important material properties are anisotropic. Metallic Sn is body centered tetragonal (BCT), whereas Pb is face centered cubic (FCC). For example, in Pb the diffusion coefficient is independent of the orientation, but in Sn there are marked diffusion rate differences parallel and perpendicular to the basal plane. This is true for self-diffusion as well, but nowhere is it more evident than in fast diffuser behavior where the ratio of the diffusion coefficient typically varies by a factor of 30-40 at 200°C. 

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