Printed circuit boards are now more crucial than ever due to their excellent properties. Today, you cannot think of developing consumer electronics without PCBs at all. However, it is obvious that the PCBs are building blocks of futuristic electronics on all counts.
It means these have to be faultless as well. Or else they will fail to let the electronics perform in their natural state. If you are reading this post, you must be coming from a technical PCB background.
Assuming this, you know that PCBs have several important components that render them different features. One of them is surface finish. It is like a covering that is present between a regular PCB and all of its components.
There are two highly important roles of surface finish for PCBs. Number one is to secure the copper components, and number two is to create enough room for soldering these components with PCBs.
There are different types of PCB surface finish. We are going to have a look at two common types called, OSP and ENIG, in this post.
OSP Finish
An OSP is a natural and fine form of surface finish used for PCBs. The OSP comes in a natural state and does not include toxins, chemicals, and artificial additives. Now, the natural form of OSP makes it important for reducing corrosiveness.
OSP surface finish is also eco-friendly. You can even use OSP with RoHS applications. Keep in mind that OSP includes water and develops a smooth surface for the components assembly of PCBs.
Besides, it is a budget-friendly surface finish, and many manufacturers prefer to rely on it due to its exceptional qualities. Sometimes, even using a HASL cannot yield fruitful results. However, OSP saves you from any sort of hustle because it is a great substitute for HASL for PCB surfaces.
Now that you know well about OSP, its time to hop onto the typical process it follows through this 6-step guide:
- Cleaning: The process begins with cleaning the PCB’s surfaces in case oily residues, solder fragments, toxins, and corrosive chemicals are present on them.
- Topography enhancement: The next step is to etch this surface which will solidify the connection of PCB and OSP itself but make sure to etch the surface with an optimum pace.
- Acid rinse: Now rinse the PCB with sulfuric acid after you have completed the etching step.
- OSP application: Once you have rinsed the PCB with sulfuric acid, you are good to go with applying the OSP to the PCB surface.
- Deionization rinse: The OSP that you have just applied on PCB is ionized, which is perfect for assisting in the soldering step of PCBs.
- Dry: The last step is to completely dry the OSP layer to avoid problems.
Downsides of OSP surface finish:
- It might not overcome dents and impressions
- The OSP can sometimes create a negative impact on soldering
- OSP does not promise longer shelf life as compared to other types
Common uses for OSP include:
- Fine pitch devices: OSP does not involve pad and ridged surfaces, which makes it perfect for utilization in fine pitch applications
- Server boards: OSP is a reliable option to choose when it comes to server boards that work on a higher frequency
- Surface mount technology (SMT): OSP can easily serve well for the SMT PCB components, saves time, and offer quick assembly
Advantages & Disadvantages of OSP Finishes
Advantages
· Lead-free
OSP is an organic type of surface finish. The best part about its organic nature is that it is more eco-friendly than other types of surface finish present, like HASL, which creates an adverse impact on the environment.
· Cost
It is an affordable to go for. OSP does not need high-end and rare materials for processing the PCBs. Due to this, it is accessible to most PCB producers at a much lower price.
· Flat PCB surface finishes.
The texture of the OSP surface is flat and smooth. It means OSP will allow fine regulation of congested and sensitive PCB components. Unlike HASL since it interferes with the performance of PCB parts. HASL interacts with residues, fragments, and air particles and also locks the solder paste within the system.
· Simple process
The process of OSP application is not extensive. It only involves six steps that are easy to carry out. It means technicians can perform OSP easily and quickly without any fear of mishaps and complications.
· Repairable
The OSP is a repairable surface finish. Now what does “repairable” mean here? The repairable nature of OSP means if it chips off from any area, you can fix it. Also, if the finish gets damaged due to any reason or if you want to add more solder with the OSP, it is possible.
Drawbacks
· Limited to RoHS compliance
If you are considering OSP, then keep in mind that it only fits well with RoHS applications. The substances like cadmium and mercury perform perfectly with OSP. Due to this reason, OSP is useless to use for external applications other than RoHS. Therefore completely relying on OSP for other applications is not recommendable.
· Environmental impact
There is no doubt that OSP does not need lead. But it does use some solvents and chemical substances that are not healthy for the environment. The cleaning step of the OSP application process leaves excessive dirty water that can lead to polluting the environment through water-borne diseases as well.
· Sensitive
OSP is not suitable for exposure to extremely hot temperatures for long periods of time. The OSP can completely chip off from the PCBs. It means the boards need to be re-processed, and it will be a lot of time-consuming.
· Short shelf life
All the surface finish types that involve water have a small shelf life. It means it will get expired and be useless after two months. Now, if you want to use OSP, then use it within two months of its accurate shelf life. You cannot leave any amount of OSP open since it can go waste due to environmental changes.
ENIG Fnish for PCB
ENIG surface finish is much more expensive than OSP. However, manufacturers love ENIG for their PCBs despite its rarity and unaffordable nature. It is the qualities of ENIG surface finish that intrigued the producers when it came to making their PCBs flawless.
ENIG has a duo of coverings made of gold and nickel. The role of the nickel is to secure the copper layers and to strongly join the components of PCBs together. Gold is for preventing corrosion and protects the nickel from such damage as well.
There are some major benefits of using ENIG instead of OSP. Let’s find them out below:
- It is better for sustenance during thermal circulation
- It is supportive of the soldering process of PCBs
- ENIG gives an edge for wire binding over other surface finish types
- It is also beneficial to use for high-pitched electronics just similar to OSP
Applications of ENIG:
- ENIG serves best when you need your devices to be more tolerant instead of relying on HASL, as it does not give positive results for this purpose.
The process of ENIG is slightly similar to the OSP. However, we will still learn about it down below:
- The first step is cleaning which is the same as OSP cleaning
- The next is to etch the surface just like OSP
- Pre-dipping is the next step that lays the foundation for using post-dipping substances
- Applying the activator is important to make sure there are no complications with the surface finish
- Post-dipping is for making sure the layers are well in place
- Applying the nickel for solidifying the connection of components
- Using the gold for rendering the PCB potential to fight against corrosion
Standard applications for an ENIG surface finish include:
- Complex surface components: ENIG is mainly for complex PCBs, and it is the main difference that sets it apart from OSP.
- Wire bonding: The wire bonding needed by ENIG is way too less than OSP, especially for aluminum wiring networks.
- High-reliability applications: The PCBs used in the domains of aerospace and medical/healthcare sectors come with ENIG-based surface finish. It is because these domains work on heavy-duty equipment and complex systems that go well with ENIG surface finish.
Advantages & Disadvantages of ENIG
Advantages
· Flat surfaces
ENIG is reliable for flat and smooth surfaces. It allows fine temperature regulation of PCBs and also protects them from catching fires and undergoing abrasive damage by all means possible.
· Long-lasting
ENIG is a long-lasting surface finish and makes PCBs reliable in the long run as well. It promises a much higher degree of prevention against corrosion and abrasive damage. Yes, even more than OSP!!!
It is because ENIG has gold layers that are easy to change and amend as per your needs. Once you have applied ENIG on your PCBs, it will keep solidifying with time naturally without needing any external help at all. For sensitive parts of PCBs, ENIG is suitable.
· Lead-free
ENIG does not need a lead for its purpose. It means you can use it easily for your RoHs projects. Besides, it will also not create a negative impact on the environment as well.
· Good for PTH
There can be no other dependable surface finish than ENIG for PTH purposes. It is sufficient to cover the entire PCB at once without making you use it again and again, which can lead to wastage.
· Maintains solderability during storage
ENIG hates moisture. It means that the PCB parts are less likely to come in contact with moisture that can cause rusting. Due to this, ENIG makes sure that the PCB parts are in optimum condition all the time so that you can focus on other important things meanwhile.
Drawbacks
· Flammable
Now, it’s time to have a look at the dark side of ENIG. If you do not pay close attention to the layouts of your PCBs, ENIG might not like this at all. It can uncover heat-sensitive wires and cause electrical fires. The electrical short circuits can later produce fatal intoxicated emissions that are not healthy to inhale at all.
· Weaker bond for components
Do you know the gold covering present in ENIG is way too thin? The thin gold layer does not give enough room for the solder to distribute completely among the pads. Now, problems can occur when the PCB parts do not adhere well to the boards due to less amount of solder.
· Requires professional installation
ENIG is not something that you can manually use without seeking professional assistance. The ENIG application is only possible when you deeply know the correct method of application.
It only happens when you are a skillful technician. So in case you are not well-familiar with the ENIG application and do it all by yourself. It can lead to wastage, thus increasing the production cost of PCBs at the same time.
· Requires special materials
You cannot replace ENIG frequently. It is because the nickel and gold layers are expensive and sometimes get totally out of sight in the market. Due to this, ENIG is a questionable surface finish because it uses rare materials and equipment for the application.
The Significance Of Your PCB Surface Finish
No PCB can work at its best if its surface is faulty. A fine PCB is one with an appropriate surface finish. The sole purpose of the finish is to protect the PCBs from harsh environmental conditions and temperature changes. Not only this, a good surface finish like ENIG and OSP will prevent corrosion of PCBs and their components as well. However, it is important to go for a suitable surface finish.
But how do you do that? There are some factors that you can analyze in different options for surface finish. These include:
1. Solderability
Soldering is an inevitable factor to take care of when it comes to producing PCBs. The type of surface finish you are choosing can make or break the soldering as well. If a PCB surface is smooth, it will increase the strong bonds between PCB parts with the boards. Make sure your preferred surface finish provides good wire bonding. These wires include different metals, and not every type of surface finish can fit well with these metals.
2. Processing Time
Some surface finish types take a lot of time to process. For instance, HASL has by far the most extensive process of application among all surface finish types. It means the processing time of the surface finish is important to consider. Some types cannot tolerate prolonged exposures, while others can.
3. Reliability
The surface finish needs to be reliable under harsh conditions. Otherwise, there is no point in going for expensive surface finish types. Make sure the surface finish type meets the reliability standards on all counts. Don’t waste your investment in buying an unreliable surface finish that cannot withstand complications nor ensures the protection of PCBs.
Final Words
Now, you know well why the surface finish is an important part of the longevity of PCBs. Also, this post discusses common types that are reliable for surface finish, along with their advantages and disadvantages. You can select the appropriate surface finish based on some factors mentioned in this discussion.
