The weldment design must provide three key prerequisites for successful ultrasonic welding. 

● Small initial contact surface. In order to ensure the deformation (strain) of the welding interface (fusion surface), the initial contact surface on the welding interface is required to be small, so as to ensure that the whole surface is heated evenly and quickly reach the melting temperature of thermoplastic material. 

● Ensure proper alignment. The weldment design and sometimes solder joint design should be designed with alignment structure to ensure proper positioning of the weldment. The alignment of welding parts is generally carried out by positioning pins, pre-assembly of welding parts or design of welding joints, such as tongue and tenon joints. The weldment can also be aligned with the weld head and fixture, but this is not a good method. 

● Energy conversion. The weldment design should maximize the contact area of the weldment to minimize the local high stress induced by the welding head/weldment contact surface, which also reduces the possibility of the weldment being scratched. In addition, the weldment shall be designed to minimize the distance between the weldment and the contact and welding surfaces and shall be maintained throughout the welding year. Care must be taken to avoid creating voids, which will prevent the ultrasonic vibration from spreading directly to the welding surface.

The first consideration in weldment design is the welding distance (the distance between the weldment/welding head contact surface and the welding surface). If the distance is less than 6mm, it is called "near field welding"; if it is greater than 6mm, it is called "far field welding". It should be noted that if the welding distance is smaller than the wavelength of the welding material, near-field welding is suitable. For near-field welding, it is generally assumed that the amplitude of the energy controller is similar to that applied by the welding head. In contrast, in far-field welding, the propagation of waves in the weldment must be considered when determining the amplitude experienced by the energy controller of the welded surface. Where possible, it is best to design welds for near field welding to reduce weldment scratches, energy loss, weldment deformation and equipment overloading. If it is not possible to design for near-field welding, the size of the weldment should be carefully selected to ensure maximum amplitude on the solder joint surface. As Menges and Potente [8] suggest, this generally means that the length of the weldment should be about half or the whole wavelength.