PCB Trace Technologies Inc offers many types of surface finishes for the PCBs we manufacture. These include HASL or Hot Air Solder Leveling, ENIG or Electroless Nickel Immersion Gold, OSP or Organic Solderability Preservative, ENEPIG or Electroless Nickel Electroless Palladium Immersion Gold, Immersion Tin, and Immersion Silver.
Conductivity of the surface on the copper pad on a PCB decides the quality of the electrical connection between it and the component. Exposure to humidity and oxygen in the atmosphere makes the copper oxidize readily, as it is an active metal. However, the surface of the oxidized copper has very poor solderability. This not only affects the electrical connection significantly, but also reduces the reliability of the connection drastically.
Manufacturers recommend using surface finishes for preventing oxidation of copper pads, while preserving their solderability at the same time. Several factors affect the specific surface finish that the manufacturer will use:
- Components on the board
- Durability requirements
- Production volume
- Environmental impact
- Cost
Why OSP Surface Finish
Of all the surface finishes we offer, the OSP or Organic Solderability Preservative is the most preferred by our customers. This is on account of its environment friendly attributes and low cost. Being an organic layer, it covers the exposed copper surface and prevents it from oxidization. Although the OSP layer is moisture repellant and capable of defying thermal shocks, it is easily possible for flux to displace it during the soldering process. This allows molten solder to cover the copper pad entirely and form a strong solder joint with the component.
Being a water-based chemical compound, OSP contains chemicals belonging to the azole family. This includes benzotriazoles, imidazoles, and benzimidazoles. The copper surface can absorb all of them, and they form a film layer that bonds the copper atoms with the chemicals. The board finish depends on the thickness of the film.
OSP Surface Finish Application
Before application of the OSP finish, it is necessary to make sure the copper surface on the board is clean and bright, as this is the prerequisite for achieving a good adhesion. This requires cleaning off all organic contaminants from the board surface, including primarily oil and fingerprint smudges and oxidation films.
If the copper surface remains unclean, it can significantly impact the build quality of the preservative. This can result in poor adhesion and uneven thickness. Therefore, a clean copper surface is essential for ensuring a high quality of the finished OSP film. For this, the PCB fabricator must control the concentration of the cleaning solution and keep it within a specific range by continuously analyzing it in their chemical laboratories. The cleaning effect needs a frequent check, and as its concentration deteriorates, they must replace the cleaning solution. The fabricator needs to wash the board with de-ionized water so that external ions do not pollute the clean copper surface.
Copper, being an active metal, easily and quickly establishes an oxide layer. The operator must enhance the surface topography by micro-etching the copper surface. Micro-etching substantially eliminates the oxides that form on the copper surface. This process also allows a significant bonding improvement between the copper surface and the OSP film.
The fabricator must control the speed of the micro-etching process, as it affects the build rate of the film. They control the speed of the process for achieving a uniform OSP layer thickness. Once the OSP application is completed, the board undergoes a wash with de-ionized water that has a controlled pH value. This prevents polluting ions from damaging the preservative layer.
Advantages of OSP Surface Finish
As an eminent PCB manufacturer, we recommend OSP surface finish to our customers as it offers several advantages:
Highly flat surface—OSP surface finish offers a uniform but very thin coating. This is essential for large fine-pitch SMT components like QFPs and BGAs.
Suitable for Double-Sided Reflow—The OSP surface finish is suitable for reflow soldering both single- and double-sided boards with SMT components.
Simple to Apply and Rework—It is easy for the fabricators to apply the OSP surface finish to PCBs. Reworking PCBs with damaged OSP finish is not difficult, as the OSP surface finish is easy to re-coat.
Environment Friendly—The components making up the OSP surface finish do not harm the environment. They also comply with the RoHS directives and requirements.
Excellent Wettability—During the soldering process, better solder wetting with flux is possible with boards using OSP surface finish.
Lower Cost—With the constituents of OSP being all low-cost chemicals, it makes OSP the most affordable among all other types of surface finishes available in the market.
Storage—when stored properly, PCBs with the OSP surface finish can last for several months.
Disadvantages of OSP Surface Finish
Although the OSP surface finish has several advantages as listed above, it has certain disadvantages as well:
Not ICT Friendly—It is possible to damage the OSP layer by aggressive probing during in-circuit testing.
Moisture Damage—Being water-based, OSP surface finish is susceptible to the presence of manual handling, sweat, and water, all of which can easily affect it. Operators must use gloves when handling such PCBs.
Mechanical Damage—As the OSP surface finish is a thin coating, it is susceptible to rough handling, scratching, and rubbing.
Storing PCBs with OSP Surface Finish
The OSP coating, being a thin coating, is susceptible to damage during transportation and storage. As the OSP coating is water-based, storing for long periods under high temperature and humidity can damage the surface finish, exposing the underlying copper, allowing it to tarnish and oxidize, leading to poor solderability. Therefore, we suggest following good practices while storing PCBs using OSP surface finish:
Use a Release Paper—the boards must be separated from one another by using a release paper between adjacent boards. This stops the surfaces from rubbing and damaging. The recommendation is to vacuum pack the boards along with a desiccant. Adding a humidity display card is an additional help.
Humidity Control—The recommendation is to maintain humidity between 30 and 70 % RH, with temperatures between 15 and 30 °C.
Limited Storage Time—The recommendation is to store OSP boards for not more than a year. Boards must not face exposure to direct sunlight during transport.
OSP Surface Finish and Soldering
During the soldering process, flux ideally displaces the OSP film and allows molten solder to cover the pad entirely. However, this may not happen for certain reasons:
Thick Coating—If the preservative is thick, micro-etching is not thorough, contaminants are present, or soldering duration is long, the flux may not ideally displace the OSP layer and the soldering process may not proceed smoothly.
Compromised Coating—If the surface coating is already compromised, flux may no longer be able to reduce the oxidation on the copper surface. This may lead to a drastic reduction of the soldering quality.
Improper Micro-Etching—Unless the amount of micro-etching is within a certain range, it is not possible to eliminate all types of contaminants, resulting in a drastic reduction of the soldering quality.
Conclusion
As an eminent PCB manufacturer, PCB Trace Technologies Inc offers all types of high-quality PCBs with quick-turn and at low costs. As the industry focuses on environment protection and pollution reduction, the advent of the OSP surface finish has lead to reducing PCBs with HASL and lead as harmful surface finishes.
