<p>Hot air solder leveling printed circuit board (HASL PCB) with Sn–Pb solder is becoming increasingly unable to meet the demands of the electronics industry, particularly as devices are required to operate in long-term, high-temperature environments. Sn–Zn solder, long considered a promising alternative to Sn–Pb, has emerged as a potential solution to this challenge. In this paper, we systematically investigated the intermetallic compound (IMC) formation in Sn–9Zn based HASL PCB boards and evaluated the shear strength of solder joints under varying reflow profiles. Our findings demonstrated that the IMC layers transitioned from Cu₆Sn₅ to ε-CuZn₅ and γ-Cu₅Zn₈ post-reflow. Notably, the thickness of the ε-CuZn₅ layer exhibited a negative correlation with both prolonged reflow time and elevated peak temperatures. The shear strength exceeded 45&#xa0;MPa, demonstrating a 20% enhancement over conventional Sn–37Pb solders, primarily attributed to the IMC layer transformation. Furthermore, thermodynamic analysis based on the Miedema model indicated that the Gibbs energy of γ-Cu₅Zn₈ is the lowest among Cu₆Sn₅, ε-CuZn₅, and γ-Cu₅Zn₈, confirming that the transformation of IMC conformed to the laws of the thermodynamics.</p>