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The crystallinity of electroless nickel deposit was manipulated by applying bath stabilizer including lead acetate and thiourea. A crystalline deposit and a higher deposition rate of electroless nickel were achieved with thiourea than with lead acetate. The effect of crystallinity on the diffusion barrier performance of the electroless nickel deposit was studied between solder and Cu deposit. The thickness of the electroless nickel deposit investigated includes 1, 3, and 5 μm. It was found that both crystalline and amorphous deposits perform similarly in barrier performance except when the thickness of the deposit is as thin as 1 μm. Cross sectional elemental analysis results indicate that a 3 μm thickness deposit can withstand ten times of reflow without counter diffusion between Sn and Cu, although Ni-Sn intermetallic compounds were formed. The 3 μm thickness is also adequate for barrier function after 1000 hours of aging at 150°C.