Loss of Electrical Contact

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically cormect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 X 10 to 3 X 10 ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 X 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable cormections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

Increases in electrical resistance or complete loss of electrical contact (electrical opens) can result from several failure mechanisms, such as  

Loss of backside electrical contact on semiconductor devices. A 

Stability of solder replacements on non-noble metal snrfaces. A new area of concern for electrical stability arises because of the increasing use of conductive adhesives as replacements for solder. Some conductive adhesives show unstable electrical-contact resistance when used on nonnoble metal surfaces such as copper or tin-lead solder. Although stable on gold, palladium, platinum, and silver surfaces, the same adhesives were found to be unstable on tin, tin-lead, copper, and nickel surfaces, The 

The unstable behavior of some solder-replacement adhesives has been attributed to galvanic corrosion. Similar to most corrosion mechanisms, condensed or absorbed moisture on the surface and dissimilar metals are required to form a galvanic cell. The silver filler acts as a cathode while the substrate metallization acts as an anode and is oxidized. In the case of tin-lead solder surfaces, the solder, which has a lower electrochemical potential (0.13 V) than silver (0.79 V), becomes the anode at which corrosion and oxidation occur. A smaller potential difference between a copper surface and silver accounts for some improvement in contact resistance over the solder-silver couple. 

---

However, that is observed only within the range of the so-called safe voltage window of operation - for alkaline aqueous electrolytes it covers the range of l,12-l,24v, and depends on material purity and current density. Exceeding this voltage causes gassing, electrode swelling and loss of electric contact between particles in the electrode volume. 

Capacity fading — Loss of faradaic - capacity of the active mass in a -> secondary battery, i.e., reduction of the amount of electric charge which can be stored and retrieved. Numerous causes depending on the type of secondary battery maybe effective mechanical disintegration, loss of electrical contact between particles constituting the active mass, changes in chemical composition, and partial dissolution are only a few. 

Another common mode of failure of batteries is loss of electrical contact between the active material and the current collector. There are many other ways in which batteries can fail, such as the aging of separators and accidental contact between anode and cathode. These problems are not discussed here. 

Because of the high power setting necessary to atomize molybdenum, response was often variable. As in the case of vanadium (Chapter 14), the life of the carbon rod was short. Response dropped significantly because the furnace and support rods deteriorated, resulting in loss of electrical contact and therefore lower atomization temperatures. Replacement was necessary after every 20-25 measurements. 

The major failure modes of adhesives include loss of adhesion, high thermal impedance, loss of electrical contact, and corrosion. To a lesser extent, failures may occur because of metal migration causing high leakage currents and even electrical shorting. Sloughing of particles from the... 

Finally, a third possible effect in causing the capacity decline is the isolation of some of the IC electrode particles within the composite positive membrane, due to losses of electrical contact resulting from the repeated volume contractions and expansions associated with the intercalation-deintercalation cycles. 

Si, however, involves a fimdamental drawback that needs to be overcome first. When Li is doped into Si, Si expands hugely by 300%. The repeated volume expansion during cycling brings about pulverization of Si and a sharp capacity decay due to the loss of electric contacts between resulting Si fine particles. 

Although irrducing the loss of electrical contact of the Pt particles, the complete oxidation of the carbon support to form carbon dioxide should be distinguished from the particle detachmerrt process. As for the previously discussed degradation mechanisms, the complete oxidation of the carbon support is a complex function... 

---