Electroplating is a relatively fast process, inexpensive and simple, although fairly messy and limited in applicability. The wafer is dipped into a solution with dissolved salts of the metal (e.g. CuSO4 + H2SO4) and is connected to a negative voltage. A positive metal electrode (anode), also in the solution, completes the circuit. Anywhere that current can flow into the wafer surface, metal will be deposited. Plating has several advantages: it will deposit metal very quickly (batch processing is possible), the film is conformal and suitable masking can ensure that it grows only as directed. However, there are also considerable difficulties. It cannot easily grow a film on top of an insulator. A seed layer is usually required for good adhesion, which must be deposited by one of the techniques described in sections 7.3.2 to 7.3.6. Also, purity and grain structure are poor, so recrystallisation by annealing is needed. These disadvantages are usually dominant, so plating is commonly used only for large-scale and thick copper layers, e.g. in PCB (printed circuit board) manufacture. Most other metallisation applications rely on one of a range of vacuum deposition techniques.