Investigations have been carried out on the phase transformation and interactions among the multilayers Pb-Sn/Ni-Cu-P/Al and Pb-Sn/Ni-Co-P/Al. Ni3P was formed in both multilayers after thermal treatment at 400 degrees C. Ni3Sn4 was found by means of XRD within the Pb-Sn/Ni-Cu-P/Al multilayer after thermal treatment, whereas the Pb-Sn/Ni-Co-P/Al multilayer formed Ni3Sn4 and CoSn. Interdiffusion, investigated with the aid of S-EM and Auger spectra, between Al and solder was effectively retarded by the Ni-Cu-P layer. Ni-Co-P, was, however, unable to exhibit such barrier behavior.