Microstructure and Mechanical Properties of Electroless Copper Deposits

Abstract

The plated through-hole (PTH) represents an important structural element of today's multilayer printed circuit boards (PCBs). It connects the circuit layers to provide a continuous electrical path, and a typical PCB contains thousands of PTHs. The electrical conduction in the PTH is provided by either electroplated or electroless copper. Because of the small diameters and high aspect ratios (the term aspect ratio refers to the ratio of the board thickness to the hole diameter) required in today's PCBs, it is becoming increasingly difficult to produce copper with a uniform thickness within the through-hole through the electrolytic method. The so-called full-build electro less methodi is more attractive, in principle, because it does not require the use of an external source of electric current. One serious concern about the integrity of PTHs arises from the fact that PTHs can be exposed to a thermal shock and/or a thermal cycle during and after manufacture of PCBs. At the elevated assembly temperature, the laminate materials expand more than the copper in the PTHs. This differential thermal expansion in the direction of the board' s thickness causes the copper to deform plastically and sometimes to crack (2). The cracking is believed to occur by the thermally-induced tensile stress. Solder-