Introduction
On printed circuit boards, the surface finish applied to the exposed copper traces and pads is a critical parameter impacting solderability, shelf life, and assembly performance.
One surface finish option is OSP – Organic Solderability Preservative. In this article, we’ll provide a comprehensive overview of PCB OSP coating covering:
- What OSP is and how it is applied
- Benefits and downsides compared to other finishes
- How OSP protects copper from oxidation
- Impact on soldering and solderability
- Shelf life and storage considerations
- Uses and applications where OSP makes sense
- How to specify OSP on fabrication drawings
- FAQs about OSP use and capabilities
After reading this guide, you’ll understand what the OSP surface finish is, where it shines, and how to effectively leverage it on your PCBs. Let’s get started!
What is PCB OSP Surface Finish?
OSP stands for Organic Solderability Preservative. As the name indicates, it is an organic coating applied to the copper traces of a PCB to protect them from oxidation and maintain solderability.
The OSP finish contains an organic acid that reacts with the copper surface to form a protective layer only a few atoms thick. This prevents air from reaching the copper and oxidizing it.
Key properties of OSP include:
- Thin organic coating measured in angstroms
- Applied through an aqueous chemistry process
- Prevents copper oxidation for a fixed shelf life
- Provides good wetting and solderability
- Lower cost than many other finishes
- Environmentally friendly process
With its ability to protect copper, maintain solderability, and low cost, OSP offers some major benefits but also must be applied judiciously based on design needs.
Benefits of OSP Finish
Here are some of the major advantages of using an OSP surface finish on PCBs:
Solderability
- Preserves good wetting and solderability of copper traces
- Allows soldering after months of shelf life
Lower Cost
- OSP chemistry is low cost compared to many finishes
- Process does not require significant equipment investment
Environmental Friendliness
- Aqueous process with minimal chemical waste
- Easy rinsing and lower toxicity effluents
Repairability
- Unlike some finishes, OSP allows rework and repair of solder joints
- Fresh solder will still wet OSP coated pads
Testability
- Allows good contact resistance for ICT testing
- Probe pins do not damage soft OSP coating
Flexibility
- Can coat a wide range of trace geometries and pitches
- Conformal, thin coating withstands board flexing
Downsides of OSP
While useful in many cases, OSP does have some limitations to consider:
Limited Shelf Life
- OSP only protects copper for 6-12 months typically
- Oxidation protection declines over time
Reduced Abrasion Resistance
- Soft organic coating wears during handling
- Pads may lose solderability if contacts scrape OSP off
Difficult Inspection
- Clear coating blends with copper color
- Hard to identify lifting, patches, or inadequate application
Lower Copper Adhesion
- Does not bond to copper as tenaciously as some finishes
- Soldering can pull OSP completely off trace
Moisture Sensitivity
- Absorbs moisture at high humidity levels
- Requires good moisture control in storage
Limited Reflow Cycles
- Each reflow “consumes” some of the OSP thickness
- Pads eventually oxidize after too many reflows
With tighter process controls, these limitations can be managed. But they do require awareness when applying OSP.
How OSP Protects Copper from Oxidation
The OSP organic compound reacts with the top few atomic layers of copper to form a protective coating of copper-OSP complexes.
This thin barrier prevents oxygen and humidity from reaching the copper traces underneath. So the copper is preserved from oxidizing and remains solderable.
However, over time the protection diminishes as the OSP gradually gets consumed through:
- Exposure to atmosphere
- Traces rubbing and abrading
- Thermal cycling and reflow depleting the OSP layer
OSP shelf life until oxidation occurs depends on coating thickness and storage conditions. But expect 6-12 months maximum before issues arise.
OSP Impact on Soldering and Solderability
A key benefit of OSP is maintaining good solderability of the copper pads and traces. This allows successful hand and reflow soldering.
The thin OSP coating does not function as a thermal barrier during soldering. The organic acids vaporize locally as the pad heats up, exposing the fresh copper underneath to melt and alloy with the solder.
However, each soldering process does consume a bit of the OSP thickness. So solderability degrades after repeated reflows as the copper begins showing through.
OSP lasts through around 4-6 reflow cycles typically. For boards that undergo lots of rework, OSP may not be the optimal finish choice.
Shelf Life and Storage Considerations
Since OSP only protects copper for a limited time, proper storage is critical. Here are some guidelines:
- Store in low humidity environment < 30% RH
- Use moisture barrier bags and desiccant packs
- Avoid direct sunlight exposure
- Control storage temperature stable around 20°C
- First-in-first-out (FIFO) inventory management
- Test solderability if stored over 6 months
With careful moisture control and storage tenure tracking, OSP coated boards can last 9-12 months before substantial oxidation appears.
Long term storage or harsh conditions shorten usable life. But good controls allow tapping the benefits of OSP when appropriate.
Uses and Applications Suited for OSP
Here are some typical uses and applications where OSP is a strong surface finish choice:
Low Volume Prototypes
- Provides solderability for prototype runs
- Avoids higher cost finishes for short runs
Double-Sided Boards
- Good match to lower complexity boards
- Reduces cost versus immersion finishes
Quick-Turn Fabrication
- Fast application suits quick turns around
Automated Assembly
- Solderability survives pick-and-place
- Finish withstands conveyor handling
Manual Soldering
- Maintains pad solderability for hand assembly
- Lower cost than noble metal finishes
Engineering Testing
- Allows functional board testing before production
- Avoids cost of final finish for eval boards
Short Shelf Life Designs
- Acceptable if product ships quickly
- Mitigates the short OSP lifespan
How to Specify OSP on Fabrication Drawings
Here are some tips for effectively specifying OSP finish on fabrication documentation:
- Call out OSP finish for the appropriate trace layers
- Indicate maximum allowed shelf life e.g. 9 months
- Define expected storage conditions if high humidity
- Specify maximum reflow cycles e.g. 4 passes
- Require solderability testing after certain duration
- Allow ENIG or immersion tin as alternates
- Call out IPC spec conformance e.g. IPC-4558
Clearly communicating needs and performance expectations ensures the OSP finish reliably meets design goals.
Frequently Asked Questions
Here are some common FAQs about OSP finish:
Q: Is OSP finish lead-free and RoHS compliant?
A: Yes, OSP processes are designed to be lead-free and conform to RoHS guidelines.
Q: Can components be mixed on boards with OSP and ENIG finishes?
A: Generally this is okay but beware ENIG may outlast OSP and require matching lifecycles.
Q: Can OSP withstand multiple reflow cycles?
A: OSP can survive around 4-6 reflows before pads oxidize. So minimize rework.
Q: What checks ensure an effective OSP coating?
A: Suppliers should test thickness, solderability after thermal shock, and other parameters per IPC specs.
Q: Does OSP have to be removed for hot air solder leveling?
A: No, OSP can remain during HASL application and enhances solderability.
Conclusion
In summary, OSP finish provides a lower cost solderable coating for PCBs when properly applied. It excellently protects copper traces from oxidation for months when well controlled.
However, OSP does have shelf life limitations and diminishes after multiple reflows. This requires matching OSP to appropriate short shelf life or low assembly complexity designs.
With its environmental friendliness, cost savings, and ease of application, OSP strikes an excellent balance for many less demanding uses. Just be aware of the tradeoffs versus more robust finishes.
Collaborating with your PCB manufacturer to apply OSP selectively based on design needs ensures gaining maximum benefit from this useful surface finish where it fits best.
In the making process of printed circuit boards the surface of the board is coated with copper. All electric connections depend upon the conductivity of the copper. On the other hand, the copper is also highly chemically reactive, when it exposed to the moisture of atmosphere it oxidizes abruptly. As a result high temperature requirement for soldering and ultimately it affect the end product reliability. Therefore there is need aroused for surface finishing of boards. The employment of surface finish coating serve two purposes, the first is to protect copper from oxidizing and another is to provide a surface which can maintain it quality after soldering and during assembly of various components with printed circuit boards.
There are various types of surface finishes available which involves various chemical substances, like: Hot air soldering leveling, Immersion in Tin/silver, OSP and ENIG. Among all these processes of surface finishing OSP has turned out to be a low cost process and environment friendly.
Introduction: OSP abbreviated as “Organic Solderability Preservative” . At the time of surface finishing of board it refers to a organic layer, which adhere with the copper by means of adsorption. Being organic it is permanent wall to prevent oxidation of copper, thermal shocks moisture. This organic coating also gives easy removal or less deposition of flux during soldering and ultimately helps to reduce the soldering time of assembly process of printed circuit board.
The chemical compound associated with this process belong to azole family compounds like benzotriazoles, imidazoles and benzimidazoles. All these chemicals by means of coordination bonding adsorbed on the copper surface, and a film is created over the entire board. The thickness of surface finish coating depends upon what chemical compound is used to proceed. For example the film made through benzotriazeoles is thin, on the contrary that through imidazoles are thick. Therefore according to requirement the chemical compound is being selected for surface finishing.
In this process topographical enhancement are also implemented. Micro etching usually eliminates the oxidation of copper and also strengthens the bonds between copper and the pcb OSP solution. The speed of micro etching is also an important parameter for surface finishing. Usually the speed of etching is 1 to 1.5µm per minute.
Advantages of OSP
The major point is the low cost and easy processing makes this process of surface finishing more popular across the printed circuit board industry. Few advantages are enlisted below:
1. Simple PCB Manufacturing process: Printed circuit boards coated with OSP are easy to rework and maintain. Therefore, it is an advantage for PCB manufacturers to repair the surface finish coating with less time and cost, once coating found damaged.
2. OSP coated boards gives good performance in terms of solder wetting and joining between flux, vias and pads.
3. Because of application of water based compound in OSP surface finishing, makes it environment friendly. Therefore this can be termed as a green electronic product which is catering to the green regulations.
4. Implementation of simple chemical compound and less complexity of process it is of low cost. With most of the chemicals of OSP there is no need of solder mask ink. But some chemicals require a little proportion of solder mask ink in some special cases.
5. Storage time is long for the printed circuit board if coated with OSP. It is implemented with single-side SMT assembly as well as double sided prototype pcb assembly.
Storage Requirement: The coating generated by OSP surface finish is quite thin. Therefore, care must be taken, when printed circuit in operation or is being transported. If board with OSP surface finish is exposed to the open atmosphere and humidity, then there are chances that oxidation will possibly damage the surface of printed circuit board, and then it leads to further consequences ahead in the process of assembly and its functioning. There are certain principle which are required to be followed during storage and transportation of OSP finished printed circuit board:
1. Vacuum packaging could be a solution. Also there is a requirement to save the printed circuit board from friction. In order to achieve that a release paper must be provided between boards. Since the assembly is very delicate, then minor negligence would affect the cost and other parts.
2. OSP surface finished printed circuit boards are not directly exposed to sunlight. The standard storage requirement for boards as such: relative humidity(30-60%RH), temperature (15to28oC)and storage time must be less than 12 hours.
In some cases the OSP surface finishing changes its color when various electronic components are assembled with the printed circuit board. This ultimately affect the thickness of the preservative over the board surface, micro etching quantity, enhanced soldering time and dispersion of unwanted particles over the board surface. This problem can be inspected visually.
There are two possibility:
1. In the process of soldering , flux is capable enough to eliminate oxidation of the coating. Then it cannot influence the soldering performance. Therefore, there are no such measures required for prevention.
2. Because of inability of flux the oxidation of the surface finish coating takes place, and this ultimately result decline in soldering performance.
Therefore, following measures can be taken to ensure the performance and appearance of OSP surface finish in the printed circuit board manufacturing process.
3. Thickness of the OSP layer is very critical. Therefore, it is require to keep it in a specified range.
4. Similarly amount of micro etching also required to be controlled within the standard range.
5. During the fabrication process of printed circuit board, it must be taken into consideration that no residual deposits remain over the OSP coating. If such residual deposited over the board then it affect the performance of soldering ability.
