Thermo-Mechanical Reliability of Sintered-Silver Joint versus Lead-Free Solder for Attaching Large-Area Devices

This study mainly evaluated the thermo-mechanical reliability of lead-free packaging techniques for attaching large-area chip. With 3 MPa pressure, a low-temperature (<300oC) sintering technique enabled by a nano-scale silver paste was developed for attaching 100 mm2 silicon die. This new lead-free packaging technique for die-attachment was compared with soldering by vacuum reflow. Lead-free solder SAC305 and SN100C were selected and used in this work since they were widely used in electronic packaging industry.

Inspection of as-prepared die-attachments by X-ray and optical microscopy (observation of cross-section) showed that the voids percentage in solder joint was less than 5% and no voids was observed at the scale of hundreds of micron in sintered silver joint. Then these die-attachment were thermal cycled with the temperature range from -40oC to 125oC. Deduction of curvature and residual stresses were found for both soldered and sintered die-attachment. After 800 cycles, the residual stresses in silicon-solder-copper sample already decreased to around 0.

The SEM images of solder and silver joint after 800 thermal cycles showed that cracks longer than 2.5 mm already grew in both kinds of solder joint (die-attachment of Si-Solder-Copper). In contrast, no cracks or voids at the scale of hundreds of micron were defected in silver joint. Based on these observation, different mode of stress-relaxation were proposed for sintered silver and solder, respectively. While solder joint released stresses by crack growth, the silver joint relied on the deformation of porous structure, and plastic deformation may occur.

The pressure-sintering process with double printing and drying was proved to be a reliable process to produce sintered - silver bonding with high strength. The reliability of silver joint was better than that of SAC305 or SN100C. Besides, the technique of measuring the curvature by laser scanning, introduced in this work, showed its significance by directly reflecting the bonding integrity of die-attachment. As a nondestructive testing technique, It was a cheaper and faster way to examine the die-attachment. Additionally, it overcame the disadvantage of X-ray Inspection: it was of the ability to differentiate between layers of die-attachment.