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Differential thermal expansion in plated through holes (PTHs) of multilayer boards (MLBs), due to soldering or to changes in ambient temperature, produces thermal strains and possibly failure of the electroplated copper. In addition, repeated thermal cycling may produce mechanical ratcheting.
A first requirement for the design of PTHs is a knowledge of the mechanical properties at elevated temperature of the electroplated copper, potentially the weakest link in the structure. To provide such information, the creep, creep-rupture, stress relaxation, and monotonic tensile properties for CuBath® copper, electroplated under controlled laboratory conditions, were studied as a function of temperature and thickness. In addition, strain ratcheting at elevated temperature was determined from repeated stress relaxation tests. The material studied was found to exhibit adequate localized ductility for the design requirements of our MLBs, although, as is generally the case for electroplated copper, its final elongation after fracture was lower than that for wrought, annealed copper.
Associate member of technical staffMember of ASTM, Bell Laboratories, Murray Hill, N.J.
Stock #: JTE10510J