Meaning
Physical layer applied to a circuit board pad prevents the intermixing of copper atoms with the tin based solder alloy. High reliability electronics use a substrate diffusion barrier to maintain the strength of the connection over long periods of operation. This layer stops the formation of excessive intermetallic compounds that would otherwise make the joint brittle and prone to cracking.
Layer Integrity
Electroplated nickel is the most common material used to separate the copper pad from the solder. Effective substrate diffusion barrier performance depends on the thickness and the purity of this nickel layer. If the plating is too thin or contains pores, copper atoms will migrate through the gaps and react with the tin, creating a weak bond interface.
Migration Block
Atomic movement is accelerated by the high temperatures found inside a working device or during the reflow process. A functional substrate diffusion barrier acts as a wall that limits this diffusion to a few nanometers. This control is necessary for fine pitch components where the volume of solder is small and even minor chemical changes can alter the mechanical properties of the joint.
Material Selection
Alternatives to nickel, such as cobalt or palladium, are sometimes used in specialized high temperature applications. Choice of a substrate diffusion barrier must also consider the compatibility with the final surface finish like gold or silver. Manufacturers must balance the cost of these materials against the required lifetime of the product in the field.
