Rush PCB Inc. offers several types of surface finishes for the PCBs we manufacture. These include Immersion Silver, Immersion Tin, ENEPIG or Electroless Nickel Electroless Palladium Immersion Gold, OSP or Organic Solderability Preservative, ENIG or Electroless Nickel Immersion Gold, and HASL or Hot Air Solder Leveling.
Electrical connection between components and the copper pads on a PCB depends on the conductivity of the copper surface. Being an active metal, copper tends to oxidize from exposure to oxygen and humidity in the environment. An oxidized copper surface has very poor solderability and therefore, significantly affects the electrical connection, reducing the reliability of the assembly drastically.
To prevent oxidation of copper pads, while at the same time preserving their solderability, Rush PCB Inc. recommends using a surface finish. The type of surface finish depends on several factors such as:
- Production volume
- Components that the PCB will use
- Durability requirements
- Environmental Impact
OSP Surface Finish
Of all the surface finishes we provide, customers prefer OSP predominantly because of its low cost and environment friendly attributes. Basically, it is an organic layer that we place over the exposed copper surfaces to stop them from oxidizing. Apart from OSP being moisture repellent and capable of withstanding thermal shocks, flux can easily displace it during the soldering process, allowing molten solder to cover the copper pad fully and make a strong solder joint with the component.
OSP is a water-based chemical compound containing chemicals from the azole family, including benzimidazole, imidazoles, and benzotriazoles, all of which the copper surface absorbs, forming a layer with bonds between the chemicals and the copper atoms. The thickness of the film affects the board finish. We have been offering OSP surface finish for a few decades, even before the advent of SMT components.
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Application Process of OSP
The fundamental requirement for applying an OSP coating is clean and bright copper surface, as this helps achieve a better adhesion. For this, we clean off all organic contaminants from the copper surface, including primarily oxidation films, finger prints, and oil.
An unclean copper surface can impact the build quality of the preservative rather significantly, causing uneven thickness and poor adhesion. Therefore, for ensuring high quality of the finished OSP film, a clean copper surface plays a crucial role. For this, we control the concentration of the cleaning solution, keeping it within a standard range, through analyzing it in our chemical laboratories. We frequently check the cleaning effect, replacing the cleaning solution as its concentration deteriorates. We wash the board with de-ionized water to prevent other ions polluting the cleaned copper surface.
As copper is an active metal and forms an oxide rather quickly, we enhance the topography through micro etching the copper surface. This activity substantially eliminates the oxides formed on the copper surface, allowing a significant improvement in the bonding between the OSP film and copper. As the speed of micro-etching affects the build rate of the film, we control the speed of the micro-etching process to achieve a uniform OSP layer thickness. After application of the OSP, we wash the board with de-ionized water with controlled pH value, to prevent destruction of the preservative layer from polluting ions.
Advantages of OSP
Rush PCB Inc. recommends OSP surface finish as it has several advantages:
Good Wettability—Boards with OSP surface finish offer better solder wetting with flux during the soldering process
Environment Friendly—Being a water-based compound, OSP surface finish does not harm the environment while complying to RoHS requirements.
Fits Double-Sided Reflow Soldering Process—OSP surface finish is suitable for reflow soldering process of both single- and double-sided SMT boards. This property has made its application more popular.
Simple to Apply and Rework—It is easy to rework PCBs with damaged OSP finish. Assemblers can get damaged boards re-coated easily by fabricators.
Extremely Flat Surface—OSP offers a very thin and uniform coating, especially suitable for large close-pitch SMT components like BGAs and QFPs.
Low Cost—As the constituents of OSP are all low-cost chemicals, OSP is the most affordable among all types of surface finishes available.
Storage Duration—If stored properly, it is possible to store PCBs coated with OSP for few months.
Disadvantages of OSP
OSP surface finish has certain disadvantages as well:
Susceptible to Mechanical Damage—The thin coating is susceptible to damage from rubbing, scratching, and rough handling.
Susceptible to Damage from Moisture—OSP being water-based, the presence of water, sweat, and manual handling easily affect it. Operators must use gloves to handle PCBs.
Not ICT Friendly—Aggressive ICT probes can damage the OSP layer.
How to Store PCBs Coated with OSP?
As the OSP coating is so thin, it is susceptible to damage during transportation and storage. Moreover, long periods of high temperature and humidity is likely to damage the OSP surface finish, allowing the copper pad underneath to tarnish, resulting in poor solderability. Therefore, Rush PCB Inc. uses the following practices for proper storage of PCBs with OSP surface finish:
- We separate PCBs from each other by using a release paper between them to stop friction from damaging their surfaces.
- We vacuum pack the boards along with a desiccant, and add a humidity display card.
- For storage, we maintain humidity at 30-70% RH, with temperature at 15-30 °C.
- We recommend storing OSP boards for not more than 12 months.
- During transport, never expose PCBs to direct sunlight.
Problems with OSP During Soldering
The release of flux during soldering must ideally displace the OSP film to allow solder to cover the pad totally. Sometimes there can be problems.
Depending on the thickness of the preservative, the amount of micro-etching, presence of contaminants, and soldering duration, the color of the OSP boards can change. If flux effectively displaces the OSP film and eliminate the oxidation on the copper surface, the soldering process proceeds smoothly.
- For an already-compromised OSP film, flux can no longer reduce the oxidation on the copper surface, and soldering quality decreases drastically.
- For avoiding the above problems, Rush PCB Inc. takes the following measures:
- We control the amount of micro-etching within a certain range
- We eliminate all types of contaminants from all stages during the board fabrication.
Rush PCB Inc. offers quick-turn PCBs of type IPC 1 and standard PCBs of type IPC 2. We offer high quality PCBs with low cost, and this has been our business aim throughout. This has won us a customer satisfaction rate of 99%, and we have managed to extend our customer base consistently.