Abstract: Exposed surfaces of a layer of a copper thick film paste on a multilayer substrate are cleaned after being fired so that such surfaces are readily solderable to the extent that conventional tin/lead solders will wet said surfaces. The substrate with the exposed surfaces of a layer of a fired copper thick film paste is cleaned by being immersed in a warm dilute solution of an acid containing a fluoride ion, such as HF or HBF.sub.4 for a short period of time. After the short period of time of immersion has elapsed, the substrate is removed from the dilute acid solution and rinsed to remove substantially all traces of the cleaning solution.