A PCB’s electrical connection depends on copper conductivity. However, whenever you expose copper to humidity, it tends to undergo oxidation. This causes problems in soldering, requiring high temperature that threatens the solid fixation of components on the printed circuit board. It also reduces the end product’s reliability.
Concerning PCB performance, surface finish undergoes two major responsibilities. First, protects the copper from oxidation. Second, offering a surface that delivers high solderability whenever you are set to assemble the components on the printed circuit board.
You can classify board finishes based on the type of technologies. Also, you can do this according to the chemical substances they are involved in. These include ENEPIG, ENIG, OSP, Immersion Silver/Tin, Hot air soldering leveling (HASL), and more. In comparison with other types of finishes, OSP is becoming prevalent due to its environment-friendly and low cost attributes. This is one good reason why we must understand it better. This is what this article is focuses on.
What is OSP PCB?
OSP or organic solderability preservatives is also anti-tarnish. It is known as a technique for protecting the surface of printed circuit boards. This refers to an organic finish layer that is generated on a bear and clean copper by adsorption. Also, the organic finish can stop copper from moisture, thermal shock, or being oxide.
Also, using flux, you can get rid of it with ease later in the soldering process. This ensures that you can join the clean and exposed copper with melting solder. By this, you are sure of generating these solder joints within a very short time.
The application of this chemical compound (water-based) is categorized with the azole family like benzimidazoles, imidazoles, and benzotriazoles. All of these get adsorbed while on the copper surface. Also, there is coordination within the copper atoms and these chemical compounds leading to film production.
Concerning film thickness, any film that is made via benzotriazoles is usually thin. Those made through imidazoles are relatively thick. This difference in thickness brings a unique impact to the board finish.
What Benefits does OSP PCB Offer?
Printed circuit boards that have OSP coating usually do well during the soldering process. OSP coating PCB boards are usually very flexible and you can repair it with ease whenever damage comes to it.
Manufacturers of OSP coating PCB can rework these circuit boards without any issue. This allows assemblers to generate a new coating anytime they need to.
Also OSP is also environmentally-friendly. It makes use of a water-based complex in generating coating on the copper, which doesn’t threaten the surrounding.
Furthermore, OSP is great for electronics meeting the standards set for use in the environment. Also, the development process, as well as applying simple elements helps in reducing the cost necessary in the manufacturing and the fabrication of printed circuit boards.
Another great benefit of OSP PCB is that you can store it for a long time. However, you must do this in the right storage condition. These conditions are relative humidity and vacuum storage.
Manufacturing Steps of OSP PCB
The very first step when manufacturing OSP PCB is cleaning. This has to do with removing contaminants such as oxidation, fingerprints, or oil remains. This is very significant as it ensures that the build comes with a similar thickness throughout the copper surface.
The next step has to do with forming a rudimentary surface on the copper. This ensures that the film generation comes easy. This modification may remove the oxidation on the copper. In the end, this may prevent the attachment of the OSP solution to copper.
Next in the manufacturing process is the deionization washing. This helps in removing other ions that may have somehow gotten the copper surface contaminated.
The final step here is film formation. Here, you will repeat the deionization rinse in order to get rid of any contamination of ion after the preservative build having a pH value that falls within 0.4 – 7.0.
Things to Consider when Choosing an OSP Surface Finish for Your PCB
To choose an OSP finish, there are several aspects you should rely on. This includes the plan of the PCB, finishing procedure, and your last outcome’s value. Below are some aspects you should consider.
Solderability: This is necessary to prevent soldering issues that may surface and prevent the osp PCB from functioning effectively.
Reflect on whether the osp finish will solder effectively to copper. This is because copper makes OSP serve as the right finish for cases like this.
Also, consider the duration for the processing. This is because some finishes may be short or long. Finishes like HASL feature longer processing duration. Therefore, you must consider this when selecting an OSP surface finish.
Consider the reliability. This has to do with measuring how well that osp finish you have chosen will help in withstanding the setting if you consider using it.
Corrosion: Here you should consider the material’s vulnerability to corrosion before choosing an OSP surface finish. This is because some types of surface finishing like silver, oxidize easily in contrast to others.
Materials Used for OSP PCB Finish
You can manufacture osp pcb finishes from different materials. Let’s consider these materials.
ENIG – Electroless Nickel Immersion Gold
ENIG is a useful material for osp pcb finish. Also, ENIG is gotten from two coats that are made from metal covers. This covering is a gold coating shielding a nickel coat. The gold helps in shielding the nickel.
Electrolytic Hard Gold
Just like ENIG, you get the electrolytic hard gold from nickel coat. This also has a gold coat as shelter. This coat is made of tough gold. Also, it helps in shielding the nickel during severe-wear settings.
Immersion Silver
Immersion silver is another important material necessary for OSP PCB finish. These materials are known to be mutually organic and metallic.
OSP – Organic Solderability Preservative
Ordinarily, you can prepare OSP from any organic stabilizer. This inhibits a PCB’s copper plate from oxidizing. You place the OSP’s tiny coat above the printed circuit board. This helps to shield it before you solder it to another device.
Differences between OSP Finish and HASL Finish in Printed Circuit Boards
Organic Solderability Preservative (OSP) is made up of an organic surface finish (water-based). This is usually useful in copper padding.
This preservative attaches selectively to copper. It also helps in guarding your copper padding before soldering. Also, OSP is known to be ecologically-friendly. Furthermore, it provides a kind of coplanar surface. It is free of lead, and requires minimal tools.
However, in comparison to HASL, it is not that strong and could be delicate. This is why you must be very careful when handling it.
For some time, HASL was included among surface finishes that are widely used. However, companies have seen its restrictions recently. Mostly, surface finishes for PCB may be strong and cheap. However, significant modifications during the manufacturing of PCB have revealed areas where it is deficient.
HASL causes rough surfaces and this is why it isn’t great for elements with good pitch. Although its design makes it lead-free, in terms of reliability, OSP is better.
OSP Coating PCB Problems
After soldering, your boards may have a different color. This may be from brown color to something dark brown. This simply means that there’s a compromise in your OSP coat.
Also, tarnish may occur. This happens due to exposure to air for long periods after the supplier has manufactured and packaged it.
Another possible osp coating PCB problem is that the boards may develop some oxidation spots. This may be caused by the improper application onto the board’s surface. Any oxidation sign has to be handled immediately before applying the coating.
To remove any oxidation on the copper, use micro etching significantly. Doing oxidation after the coating means improper application.
OSP coating PCB boards can also form a uniform color or a darker color. This happens when you expose it to high temperature and relative humidity.
Preventing OSP Coating PCB Problems
You can prevent osp coating PCB problems through flux while soldering. This gets rid of oxidation ensuring that the soldering ability will not be affected.
You must regulate your OSP coating PCB thickness to fall within a specific level. This is for effective and proper coating and preventing any coating issues.
In addition, regulate the micro-etching quantity to fall within a specified range. This helps in removing oxidation present on copper extensively before coating. Also, remove any oxide that develops on copper via micro-etching. You can achieve this through a speed that impacts the build rate of films.
Also, you can prevent OSP coating PCB problems by applying the OSP properly and using regulated thickness.
Ensure that you store properly at relative temperature and humidity. Also take note of the duration of shelf life that doesn’t last up to twelve months.
Do deionization before the coating process and after it. This helps in getting rid of any ion contaminant that may be present.
OSP Thickness in PCB
OSP is known as a shielding film that won’t be noticed easily just with a glance. In addition, no large variation exists between the completed sheet with OSP surface and the copper plate (naked).
If you see an opening unluckily on the OSP, then its copper film will start corroding from that open part. This becomes another issue that will affect SMT mounting. This is why the OSP must be thicker. This makes sure that the copper film is better safeguarded.
The Importance of Micro Etching in the OSP PCB Finish Process
While improvements in topography are on, the conduction of micro etching is done to get rid of the oxidation present on the copper foil.
With micro-etching, you can enhance the forces of connection between the OSP compound and the copper coating. The micro etching’s quickness affects the speed of coat generation.
To get a uniform and even thickness for your coat, you must maintain how steady the quickness of the micro etching is. Also, you must regulate the speed of micro etching to fall within the range of 1.0 µm – 1.5 µm every minute.
Make sure that you use the deionization solution before the stabilizers form. This is to take caution in case some other ions contaminate the OSP component. This contamination may lead to discoloration after the reflow soldering process.
Correspondingly, use the deionization solution after the stabilizers form using a pH value ranging between 4.0 and 7.0.
Storage Requirements for OSP Coating PCB
PCBs having their surface finish as OSP are usually exposed to a high humidity and temperature for a very long time. This causes oxidation on the PCB surfaces. This eventually causes low solderability. Therefore, before you store OSP coatings PCBs, here are some principles to work with.
Use the vacuum package with humidity indicator card and desiccant. Also, make sure you place release paper in-between printed circuit boards to prevent friction from destroying the surface of your PCB.
Also, don’t expose these pcbs to sunlight. Below are the requirements for optimal storage.
- Relative humidity should be between 30 to 70% RH
- Temperature should fall between 15 and 30 degrees
- Storage time should be less than twelve months
Conclusion
The focus here is to protect the environment, as well as developing society rapidly. This is why the printed circuit board industry has done well to reduce pollution. Some unique surface finishes like HASL are the major causes of most corruption. Before 2005, lead was the focus because it was the major source of the majority of PCBs. Moreover, water and air was affected by a harmful and negative substance.
After this period, manufacturers have been experiencing some pressure from some governments and institutions to get rid of the HASL to use lead-free HASL to replace it. Unfortunately, using this new process will lead to increases in costs involved in the manufacturing of PCB. This is why the application of OSP will always serve as the mainstream in protecting the environment, as well as reducing costs.
