The normal lead length to still obtain a satisfactory solder fillet is 1mm. This does depend on the pad size as the joint will tent to form a ball if the lead is short and the pad large. On a plated through hole the pad size is not as significant as a single sided board on reliability. As the lead size becomes longer the incidence of solder balls increases. This is regardless of thickness of the PCB. The lead length should not be longer than 3mm as then we start to have a less than optimum solder wave st up. You may be interested in the SMART Group training video on "Conventional and Surface Mount Design" Bob Willis SMART Group Tel 01245 351502 Fax 01224 496123