Electrodeposited Copper Recrystallization Mechanics in High Density Circuit Stackups
Thermal cycles in high density stackups drive copper recrystallization, causing grain growth and vacancy coalescence that trigger microvia interface failures.
IPC-TM-650 Method 2.4.18
Regulatory Standard
Standardized test procedure used to determine the tensile strength and elongation properties of copper foil. The ipc-tm-650 method 2.4.18 specifies the preparation of test specimens and the parameters for the pulling equipment. It ensures that the copper used in printed circuit board fabrication meets the minimum mechanical requirements for the intended application.
The test applies to both electrodeposited and wrought foils before or after they are bonded to the laminate.
Sample Preparation
The technician removes a sample of copper foil from the production batch or the finished board. This sample must be free of surface defects or nicks that could act as stress concentrators during the test. For foils already bonded to a substrate, a chemical etching process removes the laminate without damaging the copper.
The sample is then cut into a precise dog bone shape to ensure that the fracture occurs within the measured gauge length. A tensile testing machine pulls the sample at a controlled rate until it breaks. This machine records the force applied and the increase in length to calculate the stress and strain.
Ductility Verification
Results from this test confirm whether the copper foil can survive the thermal shock of soldering. A failure to meet the minimum elongation percentage often leads to cracked traces or barrel cracking in plated through holes.