Electrochemical migration

Electrolytic metal migration is a mechanism that affects microelectronic devices causing failure (1). The failure consists of dendritic growth bridging across adjacent conductors due to metallic ions. The migration occurs between biased lands and under conditions conducive to electrocrystallization (2)(3). 

A number of conditions must be satisfied to form a dendrite 1) the current density must reach high levels at the tip of the dendrite (e.g., 10 A/cm ), e.g., achieved through spherical diffusion 2) a sufficient liquid medium (polar), such as condensed water, must be present in the migration path so that adequate ionic flux is developed to sustain the required dendritic growth rate (3)(4) and 3) the applied voltage must exceed the sum of the anodic and cathodic potential in equilibrium with the electrolyte. The conditions dictate that the materials surfaces or interfaces must have the physical and chemical properties for adequate water condensation to the extent that the current density requirement at the tip of the growing dendrite is satisfied (2). 

Based on literature findings, there is a minimum rate below which a dendrite cannot continue to grow because of lattice poisoning at the surface by contaminants which deposit on the crystallographicfacets and obstruct the process. 

To produce a metal dendrite, the current density at the whisker s tip must be orders of magnitude greater than the average current. The growth is possible only through spherical diffusion of the ionic current on the dendrite tip, which can be expressed in terms of its radius of curvature, r. (The current density, however, must be above a critical value, Jc.) 

To achieve dendritic growth successfully, the mass flux density must be equal to or greater than the minimum rate below which dendrite poisoning by contaminants occurs  

A basic electronic assembly consists of a series of circuit elements connected by conductor traces. The substrate material is chosen to serve as an insulator, preventing current flow between adjacent traces. This resistance to current flow is termed insulation resistance.  

Ohm s law describes the linear relationship between voltage, current, and resistance  

Resistance is not an intrinsic property of a material. Rather it depends on geometric factors, as well as the intrinsic resistivity of the substrate. For surface resistance. 

Both resistivity and conductivity can be affected by processing chemicals. 

Electrochemical migration (Fig. 2) is the movement of an ionic species under the influence of a DC voltage. It can lead to electrical failure if (a) a short occurs due to surface dendrites or (b) an open circuit condition occurs due to depletion of the anode. 

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G. DiGiolomo, Electrochemical Migration, J. McHardy and F. Ludwig, eds., Noyes, Park Ridge, NJ (1992). 

This economical test exposes die climatically unstable points of electronic components. Due to the nature of the test, the entire board is evaluated. This test accelerates the mechanisms of electrochemical migration. Consequently, faults that previously would appear after months or even years can be detected during the development process. To identify potential weak points, the assembly is operated in standby mode and immersed in deionized water. Testing while the assembly is in full operation is even more effective. The sensitivity of the circuit to moisture exposure is assessed on the basis of flie recorded test current, combined with a subsequent examination of the assembly. Through weak point analysis, a Yes/No decision can be determined concerning the expected service life, of the assembly. 

Atmospheric—Atmospheric corrosion is responsible for a large fraction of the total corrosion in the world. Factors that affect the atmospheric corrosion of materials in a marine environment are the time of wetness, temperature, material, atmospheric contaminants and pollutants, solar radiation, composition of the corrosion products, wind velocity, and biological species [fO]. Atmospheric corrosion of a passive alloy tends to be localized. For electrochemical processes related to corrosion to occur, an electrolyte must be present to allow current to pass via diffusion and electrochemical migration of cations and anions. Seawater is a very conductive electrolyte. The severity of corrosion in an atmospheric environment is related to the time of wetness during which electrochemical processes and corrosion take place. There is also a direct relationship between atmospheric salt content and measured corrosion rates [/O]. 

PTH Hole Fill Exposed Copper after Soldering Post-Assembly Corrosion of Exposed Surfaces Handling Defects Solder Bridging Plugged Holes Equipment Safety Paste misprints Black-Pad, Black-Line Nickel Brittle Fracture Solder Mask Attack RF Signal Loss Post-Assembly Corrosion of Exposed Surfaces Tin Whiskers Ionic cleaning Failures Solder Mask Attack Post-Assembly Corrosion of Exposed Surfaces Post-Assembly Corrosion of Exposed Surfaces Tarnish Solderjoint Microvoids Electrochemical Migration... 

Brous, I, Electrochemical Migration and Flnx Residnes Causes and Detection, Proeeedings of NEP-CON West, February 1992, pp. 386-393. 

Do not use saponified water with no-clean unless the process has been tested for materials compatibility and subsequent corrosion. Surface insulation resistance (SIR) testing and electrochemical migration (ECM) testing are in order. 

FIGURE 56.8 Electrochemical migration showing the movement of positive ions toward the cathode and negative ions toward the anode. 

Moisture is essential to electrochemical migration. In the presence of moisture, metal ions form at the anode and migrate toward the cathode, where they plate out to form dendrites. When a dendrite bridges the gap, a short occurs that may bum out quickly due to the high current within the fragile dendrite. 

Ionic contaminants, which dissolve in the moisture on the surface, increase the conductance (lower the resistance) of the insulating layer between the anode and cathode. Their presence enhances electrochemical migration. How much the migration is enhanced depends on several factors the solubility of the ion, the mobility of the ion, the effect of pH on solu-bihty, the reactivity of the ion, temperature, and relative humidity. 

In summary, several factors affect electrochemical migration. These include the nature of substrate aud metallizatiou, the preseuce of contaminants, the voltage gradient, and the presence of sufficient moisture. This last element is critical, for without sufficient monolayers of water on the surface, ion mobility is impossible. 

D. Q. Yu, W. Jdlek, E. Schmitt, Electrochemical migration of Sn-Pb and lead free solder... 

DerMarderosian A (1978) Electrochemical migration of metals, Proc. Int l Microelectronics Symp., ppl34-141... 

Fig, 2 Electrochemical migration occurs under a bias voltage. 

In electrochemical migration, failure can occur due to dendrite growth, open circuit short, or conductive anodic filament (CAP) formation. Dendrites can form on the surface due to con-taminahon left by the solder flux (paste) or other residues. Under a bias voltage, the metal at the anode goes into solution, migrates toward, and plates-out at the cathode. The nature of the dendrite will depend upon the surface metallization. The following oxidation reactions can occur at the anode ... 

The electrochemical migration test requires that any degradation in insulation resistance value be less than a decade. The Telcordia Electrochemical Migration test method has also become an IPC test method (TM 2.6.14.1). Table 1 compares the variation in SIR and electrochemical migration test conditions for telecommunications test versus the non-telecommunication applications. 

Many of the factors discussed previously for the SIR test are equally applicable to the corrosion test. The spacing between the anode and cathode will affect the corrosion rate because it affects the electric field driving the electrochemical migration. Temperature and humidity both provide accelerated conditions as described previously. Contamination will be more strongly coupled to electrochemical migration, which results in the increase in corrosion factor over time. This test is so sensitive that corrosion can be measured electrically before corrosive residues are visible microscopically. This results be-... 

K. Sauter, Electrochemical Migration Testing Results Evaluating PWB Design, Manufacturing Process and Laminate Material Impacts on CAF Resistance, Circui-Tree, July 2002... 

J. Brous, Electrochemical Migration and Rux Residues—Causes and Detection, Proceedings of NEPCON West, 1992, p 386-393... 

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