Shape evolution of electrodeposited bumps with deep cavity

Electrodeposited bumps are the indispensable microconnectors for high-density interconnection in the latest microelectronics applications. The deep cavities are especially important for the solder bumps for ball grid arrays. This investigation discusses the relation between cavity shapes and current distributions of deep cavities. The role of convection and diffusion within the cavities is calculated at diffusion-limited overpotentials with numerical fluid dynamics computations. The current distributions become symmetric and peak profiles become sharper for the deeper cavities of large aspect ratios and of negative photoresist angles, theta. For 30 mu m cathode length, the current at the center is larger than that at the edges for photoresist angles of theta less than or equal to 0. For these deep cavities, the convection outside the cavity is not related to the current distribution and the current distribution is determined by the cavity shape. The mass transport within the deep cavities is controlled mainly by diffusion. This is because the convection outside the cavities is not effectively stirring inside the deep cavities.