PCB Surface Finish Solutions Pros & Cons
PCB Surface finish form the critical interface between the board and the components. In recent years, their widespread availability has overwhelmed some electronic designers. This post hopes to shed some light on the pros and cons of the four most dominant PCB surface finish solutions on the market: Organic Solderability Preservative (OSP), Electrolysis Nickel Immersion Gold (ENIG), Electroplated Nickel Gold and Immersion Tin or Silver. The following post applies to Rigid Printed Circuits Boards (PCB) and Flexible Printed Circuit (FPC).
Note: PCBs are generally made of rigid materials and will not bend during their application. FPCs are usually thin and made of materials capable of bending and / or movement during application. Processing and application requirements dictate whether 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 electrical and mechanical component attachment.
- Surface bondability for chip mounted components using gold and aluminum wire.
- Any combinations of the above.
- Mechanical applications (eg stress, strains etc.).
- Environmental conditions (eg temperature, relative humidity etc.).
- Mechanical contacts requiring 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 during the process, thus no additional removal processes are needed.
Caution: once removed, the bare copper is exposed and subject to oxidation. When multiple finishes are needed on the same PCB, OSP can be applied over other types of surface finish (eg wire bonding and soldering, mechanical contact surfaces and soldering, etc.).
Electrolysis Nickel Immersion Gold (ENIG)
ENIG is a widely used surface finish for soldering, aluminum wire wedge bonding and mechanical contact points (connector pads, test points, etc.). The copper surface has an electrolysis nickel layer deposited (150 micro inches minimum) to seal the copper. A layer of gold is then deposited to protect the nickel from oxidation and provide a solderable surface to the nickel. The gold is absorbed and dispersed into the solder. The gold is an immersion process and the thickness is self-limiting (2 to 3 micro inches max).
The nickel layer is very brittle and cannot be subjected to stress or strains in the Z axis without cracking. Flexible PCBs are especially susceptible to this with all areas subject to potential bending supported with rigidizing materials.
Caution: Improperly controlled ENIG processing can result in weak solder connections which may not be visible and / or result in failure. A typical sign of failure is a flat black copper pad after the attached component has been forcibly removed.
Electroplated Nickel Gold
In today's complex circuits, this surface finish is very limited because it requires that all surfaces to be plated have to be electrically connected (ie an electrical charge must be present for plating). These interconnections must then be broken to make the circuit functional. The plated nickel is very solderable and not subject to the solderability issues of ENIG. The plated gold has no limits on thickness and can support wire-bonding processes like Thermo Compression Bonding (ie ball bonding).
Caution: Thicker gold can result in solder joints being too brittle when using lead based solders.
Immersion Tin and Immersion Silver
These processes provide solderable surfaces but tend to have oxidation and tarnish issues that impact solderability. They are not widely used or available.