Lead-free bonding in high-temperature electronic components is desirable for realizing eco-friendly technology. A pressureless process is more appropriate for electronic packaging because it enables a more automated manufacturing process and avoids any potential damage caused by application of pressure. Recently, Ag nanoparticles were used without pressure to join materials for high-temperature electronic applications. In this study, a Ag paste of micro-sized particles was proposed for electroless nickel immersion gold (ENIG) finished Cu pressureless bonding owing to its advantages of both cost effectiveness and easy manufacturing process compared to Ag nanoparticle paste. The micro-sized Ag paste was composed of both chestnut-burr-like (CBL) and spherical particles. The weight ratios of CBL to spherical particles were 10:0, 7:3, and 5:5. The bonding process was carried out at 573 K for 60 min in a nitrogen atmosphere. The experimental results showed that all of the sintered layers had an open porous structure. ENIG-finished Cu joint using the Ag paste of the 5:5 weight ratio exhibited the shear strength of 18.6 MPa, which is comparable to that of a conventional Pb-5Sn joint.