PRINTED CIRCUIT BOARDS. RELIABILITY OF INTERCONNECTIONS

Abstract

Stability of metallization of holes to thermomechanical pressure is provided with durability and plasticity of electrodeposited copper.

Distinctions in factors of thermal expansion of copper and dielectric of bases of printed circuit boards create powerful thermomechanical factors of rupture of metallization of apertures, destructions of internal interconnections in multilayered structures of printed-circuit boards. Standard norms of requirements for the depth of metallization of apertures, its durability and plasticity of copper were established in the course of manufacture of ordinary printed-circuit boards with reference to use of traditional technologies of soldering by tin-lead solders. Return to consideration of the copper plasticity problem has been caused first of all by transition to lead-free solders initiated by the all-European Directive RoHS [1], featuring a high temperature of soldering. Higher temperatures create large deformations of metallization of holes, which forces us to reconsider the requirements for plasticity of copper. At the same time, there is a general tendency to reduction of the diameter of metalized holes, and consequently to reduction of the area of metallization cross-section. Smaller sections have smaller resistance to rupture. Therefore, along with good plasticity, metallization of holes of printed-circuit boards should provide higher resilience to rupture as well. In this reference, deformation of metallization of holes when heating to soldering temperatures has been studied. The purpose of researches is to revise norms as regards plasticity of copper in holes of printed-circuit boards. It has been shown that the plasticity of copper deposition in holes of modern printed-circuit boards should not be less than 6% [2]. Current copper plating electrolytes allow us to reach plasticity of copper of 12-18% [3].