Role And Measurement Of Trimming Shock On Component Lead

Lead trimming is a standard operation performed during printed circuit board assembly. The trimming activity is generally conducted on discrete components before attachment to the board, or after solder attachment of through hole mounted components. The purpose of the trimming operation is to cut any excess lead to a specified length. This paper describes the detrimental effects of trimming shock on some component types and presents a method for qualitative measurement of trimming shock. During the trimming operation the cutter wedges itself into the lead resulting in a local tensile stress in the lead material between the two cutter faces. The magnitude of this tensile stress is a function of the cutter geometry. The shock wave, or more properly the stress wave, that is generated in the cutting action is related to the catastrophic crack propagation or tensile failure 'of the lead upon completion of the cut. Tensile failure or fracture instability occurs, when upon the creation of new surface area or crack extension, the elastic energy released remains larger then the crack resistance. This surplus of released energy can be converted into kinetic energy and is associated with the rapid movement of the material at each side of the crack or cut. During lead trimming the lead is split into two entities, one which remains attached to the component, and another which is removed. The energy released during the catastrophic tensile failure or crack propagation ahead of the cutting edge induces a stress wave in the trimmed lead which is still attached to the component. Similarly the trapped stress wave in the free end of the lead results in it flying across the room. Failures can be induced in the component due to the stress wave created in the trimming operation. Table-1 lists the potential failure sites and associated the failure mechanisms. This paper presents a means to qualitatively measure the stress wave generated in the trimming operation and presents the results for three different type of commonly used cutters. Trimming shock is a commonly overlooked failure mechanism. Production yield can be improved by considering some of the methods suggested in the paper to reduce the shock magnitude. Two different failure sites are considered for the stresses induced by the trimming shock; the lead seal and the wire bond area at the lead termination within the package.