Our engineering group performed calculations of the stress induced on axial components during the forming and insertion process. Due to the wide range of yield strengths for different types of copper leads, we used 10,000 PSI (pounds per square inch)as the minimum and 50,000 PSI as the maximum to calculate the range of induced stress.
Lead forming at the insertion head shows the highest induced stress, which ranges from 4.1 pounds minimum, to a maximum of 20.5 pounds. (see calculation 1, below). Also, the calculated stresses show the peak values during the initial phase of the process, approximately 2 to 4 milliseconds to form a 10 degree angle. Please note that this calculation does not take into consideration stress propagation velocity or changes in stresses during forming.
Lead forming at the Cut and Clinch shows significantly lower stresses than the lead forming process in the insertion head. At the Cut and Clinch, induced stress ranges from .09 pounds minimum to a maximum of .44 pounds (see calculation 2, below). Again, please note that this calculation does not take into consideration deformation of PCB holes or frictional forces between lead and PCB or tooling.
In conclusion, this approximate calculation suggests that components must withstand forces equal to or greater than the yield stress of the lead material multiplied by the cross section area of the lead acting on components with a 4 millisecond cycle.