Lead Finish

Lead finish, or 'leadfinish', is the process of applying a coat of metal over the leads of an IC to: 1) protect the leads against corrosion; 2) protect the leads against abrasion; 3) improve the solderability of the leads; and 4) improve the appearance of the leads. There are two widely used leadfinish techniques in the semiconductor industry, namely, plating and coating. Further, there are two types of plating, i.e., pure metal plating such as tin plating and alloy plating such as tin/lead plating.

Coating is the process of depositing a filler metal (usually solder) over a surface, achieving metallurgical bonding through surface wetting. The filler metal should have a melting temperature below 315 degrees Celsius for the process to be classified as coating. The driving force for a solder coating process is surface tension, i.e., wetting of the surface to be coated by the solder must be achieved. A solder diffusion layer grows at the surface-solder interface as solder spreads through the surface during the coating process.

Tin plating is a form of pure metal electroplating, which is the process of depositing a coating of metal on a surface by passing a current through a conductive medium, or electrolyte. An electroplating system has four (4) components: 1) the cathode, which is the surface to be coated; 2) the anode, which is the source of coating metal; 3) the electrolyte, the aqueous medium through which the metal ions from the anode transfer to the cathode; and 4) the power source, which supplies the current or energy needed for the plating process.

The cathode, which is the material to be plated, is the electrode where electrons are consumed or where 'reduction' occurs. The anode, which serves as source material for the plating, is the electrode where oxidation occurs, i.e., where electrons and metal ions are released.

Example of reduction: Sn⁺² + 2e^- => Sn⁰

Example of oxidation: Sn⁰ => Sn⁺² + 2e^-

The energy needed for the plating process to occur is known as the electrochemical potential or voltage. This voltage is the total of three voltages: the reversible potential, the overpotential, and the ohmic potential. If the applied voltage is less than the electrochemical potential, the process of plating will not occur.

Solder plating is a form of alloy plating. An alloy is composed of at least two elements, at least one of which is a metal. An alloy has better properties than its component metals: it is harder, more corrosion-resistant, has better solderability and better appearance.

Fig. 1. Example of an Electroplating Machine for Lead Finish

Common Lead Finish-related Failure Mechanisms/Attributes:

Lead Corrosion - corrosion of the leads due to imperfections in the lead finish

Poor Solderability - insufficient wetting of the solder often caused by contaminants, excess additives, and inadequate plate thickness

Tin Whiskers - formation of very thin extrusions of tin material from the lead finish that can result in electrical shorts between adjacent pins; observed in pure tin plating or alloy systems with a high content of tin

Other Lead Finish Failure Attributes: Solder Dullness, Solder Roughness, Pitting, Tarnishing, Blistering, Dendrites, Nodules, Graininess, Deposits, Burns

Front-End Assembly Links: Wafer Backgrind; Die Preparation; Die Attach; Wirebonding; Die Overcoat

Back-End Assembly Links: Molding; Sealing; Marking; DTFS; Leadfinish

HOME