PCB Surface Finish Possibilities Pros & Cons
PCB Surface finish form the critical program between the board as well as the components. In recent years, their availability that is widespread has some electronic designers. This post hopes to shed some light regarding the advantages and disadvantages associated with four most principal PCB area complete solutions available on the market: Organic Solderability Preservative (OSP), Electrolysis Nickel Immersion Gold (ENIG), Electroplated Nickel Gold and Immersion Tin or Silver. The after post pertains to Rigid Printed Circuits Boards (PCB) and Flexible Printed Circuit (FPC).
Note: PCBs are generally manufactured from rigid materials and certainly will not bend in their application. FPCs are thin making of materials effective at bending movement that is and/or application. Processing and application requirements dictate whether or not the PCB surface finish is electroplated, electrolysis, immersion or deposited.
Conditions that influence PCB Surface Finish Selection:
Oxidation protection of PCBs metal traces (usually copper).
Surface solderability for electric and component attachment that is mechanical.
Surface bondability for chip mounted components using gold and aluminum wire.
Any combinations associated with the above.
Mechanical applications (e.g. stress, strains etc.).
Environmental conditions (e.g. heat, general moisture etc.).
Mechanical associates needing abrasion resistance and oxidation protection.
General Discussion of Available Surface finishes
Organic Solderability Preservative (OSP)
OSP has a limited shelf life. Its most frequent use is soldering when the protectant is dissipated throughout the procedure, thus no additional treatment processes are essential.
Care: as soon as removed, the bare copper is exposed and subject to oxidation. Whenever numerous finishes are expected on the exact same PCB, OSP may be applied over other kinds of area finish (age.g. cable bonding and soldering, mechanical contact surfaces and soldering, etc.).
Electrolysis Nickel Immersion Gold (ENIG)
ENIG is a commonly used surface finish for soldering, aluminum cable wedge bonding and contact that is mechanical (connector pads, test points, etc.). The copper area has an electrolysis nickel layer deposited (150 inches that are micro) to seal the copper. A layer of silver is then deposited to protect the nickel from oxidation and offer a solderable surface to the nickel. The silver is dispersed and absorbed into the solder. The gold is an immersion process and the thickness is self-limiting (2 to 3 inches that are micro).
The nickel layer is quite brittle and cannot be exposed to stress or strains into the Z axis without cracking. Versatile PCBs are specifically vunerable to this with all areas at the mercy of possible bending supported with rigidizing materials.
Caution: Improperly controlled ENIG processing can lead to weak solder connections which may not be visible and/or end in failure. An average sign of failure is an appartment black colored copper pad after the connected component has been forcibly eliminated.
Electroplated Nickel Gold
This surface finish is very limited because it requires that all surfaces to be plated have to be electrically connected (i.e. an electrical charge must be present for plating) in today’s complex circuits. These interconnections must then be broken to make the circuit practical. The plated nickel is quite solderable and not at the mercy of the solderability dilemmas of ENIG. The plated gold has no restrictions on thickness and can support processes that are wire-bonding Thermo Compression Bonding (i.e. ball bonding).
Caution: Thicker silver can lead to solder joints being too brittle whenever making use of lead based solders.
Immersion Tin and Immersion Silver
These processes provide solderable areas but tend to have oxidation and tarnish conditions that effect solderability. They’re not commonly utilized or available.