Mark, Dendritic growth is a corrosion mode generically refered to a electromigration. Conductor will dissolve into a water film on your board. These metal ions will move under the influnce of the circuits applied voltage and "plate" onto the opposing conductor. As more metal dissolves, moves and plates out, a metal crystal begins to grow along the lines of potential, eventually causing a short circuit. Electromigration needs three components to occur: 1.) A voltage difference 2.) Ionic contamination 3.) Moisture Voltage differences can be controlled via design. Keep grounds and the high voltages seperated as much as possible. Avoid layouts which concentrate electric fields, such as sharp corners, points, etc. Use the lowest voltages required for the application requirements. Keep in mind that the closer your conductors, the greater the electric field strength, the more chance of dendritic growth. Ionic contamination can be controlled via process and materials selection. Keep you products clean. Check ionic levels of incoming material. Check your handeling procedures (finger prints, flux residues, etc.). Moisture is often controlled via conformal coatings. These should be considered stop-gaps until proper design and process controls can be implimented. IPC has test metheod, coupon designs and guidelines. I don't know all the document numbers, if someone else does, please respond.