Nickel contact resistance values

Different nickel deposits show a great variety of contact resistance values. This is particularly so after the deposits have been exposed to the atmosphere for an extended period of time. The differences between these values may be best explained in terms of variations in plated texture. Nickel electrodeposits with polycrystalline nature have been observed to behave as single crystals ( ) when their grains were oriented such that the (100) plane was parallel to the surface. Not surprisingly, the oxidation rate in (lOO)-oriented single crystals is self-limiting at ambient temperature. 

Comparison of ACAs with Hard and Soft Fitters. Kishimoto and coworkers reported (15) ACA pastes using two different fillers Au-coated rubber particles (soft) and nickel particles (hard). The ACAs were used to bond a flip chip with Au plated bumps to a board with copper metallization. With the application of pressure, the soft particles were brought into contact with surface pads and were deformed, which lowered this contact resistance. The hard particles, however, deformed the bumps and pads, and thus were also in intimate contact with the surfaces to help reduce this contact resistance. Their study showed that their choice of both hard and soft flllers in ACA materials had similar voltage-current behavior, and both exhibited stable contact resistance values after 1000 cycles of thermal cycling and 1200 h of 85°C/85% RH aging conditions (15). 

Volume conductivity measurements indicate that the composition of the electroplated substrates is of prime importance with an increase in resistivity covering 5-6 orders of magnitude. The conductive adhesives have volume resistivities in the range of 6.5 X 10 -1.6 X 10 " fi cm but the values measured for the joined assembhes extend from 9 X 10 " to 1 X 10 fi cm. The general trend observed with all adhesives is that the increase in resistivity is related to the ease of oxidation of the plated substrates, in particular when nickel and aluminium are compared to noble metal plating. Other studies support these results indicating that the best silver-filled adhesives have a volume resistivity of 5 X 10 fl cm approaching the value of solders, typically 2 X 10 fi cm. The contact resistance at the interfaces with the metallic conductors is, however, more important than the bulk conductivity [155-157]. 

Silver is often preferred as an undercoat for rhodium by reason of its high electrical conductivity. A further advantage of silver in the case of the thicker rhodium deposits (0-0025 mm) applied to electrical contacts for wear resistance is that the use of a relatively soft undercoat permits some stress relief of the rhodium deposit by plastic deformation of the under-layer, and hence reduces the tendency to cracking , with a corresponding improvement in protective value. Nickel, on the other hand, may be employed to provide a measure of mechanical support, and hence enhanced wear resistance, for a thin rhodium deposit. A nickel undercoating is so used on copper printed connectors, where the thickness of rhodium that may be applied from conventional electrolytes is limited by the tendency of the plating solution to attack the copper/laminate adhesive, and by the lifting effect of internal stress in the rhodium deposit. 

In acidic media, the metals iron, nickel and chromium have passivation current densities that increase in the order Cr < Ni < Fe. In Figure 6.11, the anodic polarization curves for the three metals in 0.5 M sulfuric acid (25 °C) are compared. Chromium has lower values of both ip and Ep than the other two metals. By alloying increasing amounts of chromium to steel one therefore improves the corrosion resistance. Experience shows that above a chromium concentration of 12 to 13%, a steel passivates spontaneously in contact with aerated water. It becomes "stainless", meaning it does not rust easily. Figure 6.12 gives the corrosion potential of different... 

---