Question

Please list the important practical consequences of fatigue of sintered nano-Ag traces on flex. Explain why they are useful.

Answer 1

Behaviour of fatigue varies with combination of substrate modulus and viscoelastic deformation properties. Sintered Ag particles are considered ideal for usage in electronic from late 1980s.

Lead containing materials were not god for health so introduction of eco friendly die-attaching material became necessary due to increasing requirements of reduction in weight, miniaturization and high thermal dissipation.

Sintered Ag technology has high thermal and electrical conductivities, and lead-free composition but limited market demand. Reason being, the reliability of sintered Ag joints depends on the density of the sintered joint, selection of metallization/plating schemes, formulation of Ag pastes/laminates, types of substrates, substrate roughness, joint configurations and testing conditions (different pressure and temperature).

Silver is used widely in microelectronic packages as a promising interconnection material between substrates and chips because of its superior electrical/thermal conductivity, high melting temperature (around1,232 K), and good reliability. The aided pressure might be destructive for brittle silicon chips and ceramic substrates, even slightest irregularities. To raise the sintering driving force of this interconnection material, the paste formed by nano sized silver powder. With the assistance of mechanical pressure of about 40MPa, micro-sized silver powder could be sintered at temperature below 300℃.

Sintered silver film imposed a limitation to consider the thermally induced strain due to thermal expansion coefficient mismatch between substrate and chip in actual applications. Following this lap-shear structure, further research on fatigue and dwell-fatigue property of sintered nano-silver joint were conducted.

Experiment conducted to check fatigue behavior of nano-Ag :-

To obtain the shear stress-strain relations of sintered lap shear joints as the base for cyclic tests, a series of shear tests were conducted at four different ambient temperatures of 25℃, 225℃, 275℃ and 325℃. For fatigue and dwell-fatigue tests. All the shear and cyclic tests are conducted under stress-controlled mode with loading rate of 2MPa/s.

Shear strain amplitude increases with the loading amplitude increasing under fully reversed loading situation. The increasing of hysteresis loops with loading amplitudes demonstrates the larger energy dissipating per unit volume during a cycle.

Obtain true strain-stress hysteresis loops of the nano-silver joint at load amplitude of 6MPa and 8MP. Other values can be taken based on sample and requirement. The enclosed area of the hysteresis loop represents the cyclic plastic energy consumed in each cycle. At certain loading amplitude, with the increasing of the cycles, the enclosing area increases gradually. The enclosed area of hysteresis loop experiences a rapid increase before the nano-silver joint comes to the final failure.

It can be found that at higher temperature of 325℃, more plastic strain is present and the loops are wider, which causes the cyclic plastic deformation in the joint to be more severe resulting the fatigue life at the temperature of 325℃ is shorter than that at the temperature of 225 ℃. It should not be ignored that higher temperature could mobilize the dislocation of the nano-silver joints, which leads to a significant reduction of fatigue life.

(1) Shear behaviors are temperature dependent. The shear strength decreases with the increase in temperature. The shear strength at room temperature is around 28MPa, which reduces to half of its value at the temperature of 325℃.

(2) Basquin’ s equation

Δτ2=σft'(2Nf)b

Use Basquin’ s equation to predict the fatigue life of the nano-sintered lap-shear joints at different temperatures, where the fatigue strength exponent b is a constant and the fatigue strength coefficient σft'  , is a value dependent on temperature, which decreases with the elevated temperature

(3) With the increasing of the dwell time, the effect of creep becomes more and more significant. It can be concluded from the dwell-fatigue tests at the temperature of 325℃ that creep is the main factor that lead to the final failure of the specimen.