As has been stated the effect is due to hole to lead ratio as a basic guide the hole to lead ratio is calculated from the lead and is the lead size plus 0.010" plus any tolerance. This ia requirement for auto insertion not soldering. The sunken joint is also caused by outgassing when it appears on the base of the board. If the outgassing occurs as the board exits the wave either a pin hole or a sunken joint will occur. Mounting parts flush to the board surface without any stand off from the board also causes blow back which looks like outgassing but it is not the printed board. Flux vapour do not escape through the board due to the component body masking the top of the hole. Check out my DFM video tape on the home page it has all the good design rules for assembly and soldering. Have A Great Day. Our Home Page solves your SMT process Problems Bob Willis Electronic Presentation Services 2 Fourth Avenue, Chelmsford, Essex CM1 4HA. England. Tel: (44) 01245 351502 Fax: (44) 01245 496123 Home Page: http://ourworld.compuserve.com/homepages/bwillis Email: [log in to unmask]