Understanding the reliability of microelectronic devices is becoming increasingly important as modern technologies require higher performance from smaller components. A crucial factor leading to failure of these devices is thermal expansion, which is the movement of atoms within the solder material under temperature gradients. Recently, researchers have delved deeper into analyzing the microstructure and properties of electrical Ni-P/SN2.5AG0.7CU0.1RE microjoints during heat transfer, providing valuable insight into the formation and mechanical integrity of intermetallic compounds (IMCs).
This study, conducted by RQ Hou and Kk Zhang, focused on microjoints manufactured using electroless nickel phosphorus (NI-P) plating on copper substrates. This plating method forms a diffusion barrier and minimizes side effects with solder materials. The investigation revealed changes in solder interfaces under various conditions, highlighting the challenges posed by heat transfer.
One of the major findings of the study was the variation in IMC structures that demonstrated both needle-type and block-type formation of (Ni, Cu)3SN4 within the Ni-P/solder transition zone. These IMCs initially averaged 1.1-1.5 μm thick, but their growth dynamics undergo a significant change in response to the temperature gradient applied during the heat transfer test.
Temperature gradients of 450°C/cm, 550°C/cm, and 650°C/cm were set across the microsolder joint, followed by microstructural changes. This dynamic indication was the evolution of the Ni-P layer, which transitioned two stages of Ni-Sn-P compounds in the initial conversion of Ni-P to Ni3p, and in combination with Ni3p and solder. The cold end exhibits faster structural evolution compared to the hot ends during these experiments, highlighting the effect of temperature on solder joint integrity.
Notably, after 140 hours under the highest temperature gradient, the cold-end IMC region becomes significantly thicker to 14.8 μm, 12.5 μm more than the thickness observed at the hot-end. This asymmetric growth underscores the importance of accurate temperature management within solder joint applications.
Shear tests have pointed out that performance is reduced under thermal measurement conditions, and initially the thin force was reduced from 16N. Importantly, the location of the shear fractures changed over time. This shows a decrease in structural integrity under prolonged heat transfer, from that of solder sutures to the interlayer interface region. The authors stated that “the shear fracture location of the microsolder joint shifts from the solder joint to the Ni-SN-P/IMC layer junction,” highlighting the need to closely monitor these transitions.
Regarding fracture modes, the initial stages of fracture were ductile and indicated by the dimples pattern of the fracture surface. Over time, these evolve into brittle fractures characterized by cutting and slip steps as the solder joints are disassembled, and “fracture mode changes from dimples-controlled ductile fractures to brittle fractures dominated by cutting and slip steps.” Such insights provide important information on the average life expectancy of micro-solder joints and critically link their performance to thermal fluctuation phenomenon.
This study uncovers unique complications intertwined with the latest microelectronic packages, especially as devices continue to integrate and integrate more features. The nature of heat transfer and its impact on interface adhesion and composite growth have been highlighted as data highly relevant to both engineers and designers.
This study not only provides theoretical insights, but also serves as the basis for future modifications and innovation within solder joint technology. Understanding temperature transitions and addressing the consequences can lead to more reliable electronic devices that can withstand the strictest performance requirements.
