Reliability assessment of sintered nano-silver die attachment for power semiconductors

Abstract

For decades soldering has been the technology of choice in die bonding. However, due to worldwide health protection regulations, the most common solder alloys, which contain lead, have been banned. Furthermore, standard solders cannot fulfil the reliability requirements of future power electronic devices. New interconnection technologies have to be developed. One of them is pressure sintering (p=30..50 MPa) of silver flakes below 300 °C. It forms a strong, highly electrically and thermally conductive bond. In order to lower the level of pressure, silver nanoparticles can be used. Shear tests have shown that even 5 s of sintering, a temperature of 225 °C, or a pressure as low as 2 MPa is sufficient to generate bonds comparable to solder and high pressure sinter joints if the remaining parameters (p, t and T, respectively) are set correctly. However, strength is only a necessary criterion as aging comes into play. Therefore, reliability tests using thermal cycling and pow er cycling were run. These returned superior reliability of the sintered samples. 160 million of the power cycles between +45 and +175 °C run in this work can be extrapolated using a Coffin-Manson model. Solder joints failed at about 40,000 cycles. ©2010 IEEE

Are Sintered Silver Joints Ready for Use as Interconnect Material in Microelectronic Packaging?

“…The larger change in temperature that accompanies a lower minimum temperature, T min , can severely reduce the number of cycles to failure-by as much as 60 %, when comparing items (3) and (4) of Table III, 63 or 34 %, 64 when comparing items (1) and (2) of Table III. A porous sintered Ag joint behaves like a viscoplastic material at high temperature.…”

“…In other words, the nano-Ag joints were subjected to more stressful conditions than the micron-Ag joints, but the former still performed better than the latter. One possible reason for this difference in performance is that a nano-Ag joint has a higher die shear strength than a micron-Ag joint, 26,73 which leads to a higher fatigue resistance for the nano-Ag joint. This simple correlation may be flawed, however, as high-strength materials typically have low fracture toughness, which is a measure of the resistance to crack growth during cyclic loading.…”

“…Further analysis reveals that the die-attach thicknesses of these nano-Ag and micron-Ag joints were approximately 35-40 lm, while the die sizes used in the nano-Ag and micron-Ag joints were 14 mm 9 14 mm and 5.4 mm 9 3.4 mm, respectively. 63,64 A larger die is more susceptible to thermal fatigue failure than a smaller die because of the higher stress present in the larger die. In other words, the nano-Ag joints were subjected to more stressful conditions than the micron-Ag joints, but the former still performed better than the latter.…”

“…In the case of ''life cycle to failure'' (N f ), the criterion for failure differs from one work to another; some researchers have used the loss of 20 % of the die-bonded area the failure criterion, while others have used 10 % of the die-bonded area. [62][63][64] Some researchers have defined a crack length equal to the thickness of the die-attach layer as the detection limit for observing crack lengths and failure criteria. 71 Therefore, caution should be exercised when comparing results from different sources.…”

A diffusion-viscous analysis and experimental verification of defect formation in sintered silver bond-line

“…[1][2][3][4][5][6] The sintered silver joint in chip attachments also shows higher thermal reliability than traditional solders. [6][7][8][9] A bond formed by sintering is significantly different from that formed by the solder reflow process. In the latter, the alloy starts out as spheres immersed in a flux.…”