Footprints of Fine Pitch SMDs

Most footprints of surface mounted components have been calculated on the basis of the sizes and tolerances of their electrodes, taking into account the various inaccuracies of the mounting processes to be applied. The introduction of components with very small connecting areas or leads (e.g., fine pitch IC packages) does not only imply smaller solder land dimensions, but also a different approach for the calculation of the footprint. This approach tackles the question of the required insulation spacing and lead/land overlap, and the ‘optimum’ solder land dimensions and the available mounting (i.e., placing and fixing) freedom are then obtained. The reduction of mounting freedom with decreasing pitch distances is quantitatively demonstrated. The ratio width of lead/size of pitch proves to be an important parameter in this respect. Requirements for mounting quality are linked with certain criteria for inspection of soldered joints (for example, regarding shift and rotation and vice versa), and some of these criteria are discussed in more detail.