Copper plays a conductive role in a printed circuit board (PCB). As an active chemical substance, copper tends to be oxidized when being exposed to atmospheric humidity, thereafter leading to issues that will possibly take place in high-temperature soldering, detrimental to components mounting on PCBs and reliability of end products. Surface finish has two key functions: to protect copper from being oxidized and to provide a surface for soldering when components are being assembled on PCBs.
Board finishes can be classified into different classifications based on different technologies and involved chemical substances: HASL (hot air soldering leveling), Immersion Tin/Silver, OSP, ENIG and ENEPIG, etc. Among all the finishes, OSP is becoming increasingly prevalent owing to its low cost and environment-friendly attributes, which adds more necessity for us to better understand it. That's what this article aims to tell you.
Brief Introduction of OSP (Organic Solderability Preservatives)
OSP is short for "organic solderability preservatives", and it's also called anti-tarnish. It refers to a layer of organic finish generated on clean and bare copper by adsorption. On the one hand, this organic finish is capable of stopping copper from oxidation, thermal shock and humidity. On the other hand, it has to be easily eliminated by flux in the later process of soldering so that the exposed clean copper can be jointed with melting solder so that solder joints can be generated in extremely short time.
The applied water-based chemical compound belongs to azole family such as benzotriazoles, imidazoles, and benzimidazoles, all of which get adsorbed on the copper surface with coordination formed between them and copper atoms, leading to the production of film. In terms of film thickness, film made through benzotriazoles is thin while that made through imidazoles is relatively thick. The differentiation on thickness will bring distinct impact to effect of board finish which will be discussed in the later part of this article.
Manufacturing Process of OSP
As a matter of fact, OSP has a decade's history even longer than that of SMT (Surface Mount Technology). Here's the manufacturing process of OSP.
Note: DI refers to deionization.
The function of "Clean" is to clean off the organic contaminants such as oil, finger prints, oxidation film etc. so as to get copper foil surface stay clean and bright. This step plays a crucial role in preservatives build quality. Bad cleaning will tend to cause uneven thickness of preservatives build. In order to ensure the high quality of finished OSP film, on the one hand, concentration of cleaning solution should be controlled within a standard range through chemical laboratory analysis. On the other hand, cleaning effect is advised to be checked as often as possible and once the effect doesn't reach the standard, cleaning solution should be replaced in time.
In the process of Topography enhancement, micro etching is usually applied to substantially eliminate the oxidation generated on copper foil so that the bonding forces can be improved between copper foil and OSP solution. The speed of micro etching directly influences the film build rate. Thus, in order to obtain smooth and even film thickness, it's critical to keep the stability of micro etching speed. Generally speaking, it's suitable to control the micro etching speed in the range from 1.0 to 1.5μm per minute.
It's best that DI rinse is utilized before preservatives build in case OSP solution will be polluted by other ions, which leads to tarnish after reflow solder. Similarly, it's best that DI rinse should be utilized after preservatives build with the PH value between 4.0 and 7.0 in case preservatives would be destroyed as a result of pollution.
Advantages of OSP
The advantages of OSP can be summarized into:
• Simple process and Reworkable: Circuits boards coated with OSP can be easily reworked by PCB fabricators so that PCB assemblers are allowed to have fresh coatings once its coating is found damaged.
• Good wetting: OSP-coated boards perform better in terms of solder wetting when flux meets vias and pads.
• Environment friendly: Since water-based compound is applied in the process of the generation of OSP, it does no harm to our environment, just falling into people's expectations for green world. As a result, OSP is an optimal selection for electronic products catering to green regulations such as RoHS.
• Cost effective: Due to the simple chemical compounds applied in OSP creation and its easy manufacturing process, OSP stands out in terms of cost among all types of surface finishes. It costs less, leading to a lower cost of circuit boards at the end.
• Fit for reflow soldering in double-side SMT assembly: Along with the constant development and progress of OSP, it has been accepted from single-sided SMT assembly to double-sided SMT assembly, dramatically widening its application fields.
• Low requirement for solder mask ink
• Long storage time
Storage Requirement of PCBs with OSP
Since the preservative generated by OSP technology is so thin and easy to be cut, much care must be taken in the process of operation and transportation. PCBs with OSP as surface finish are exposed to high temperature and humidity for such a long time that oxidation will possibly take place on the surface of PCBs, which tends to cause low solderability. Therefore, storing methods must stick to these principles:
a. Vacuum package should be utilized with desiccant and humidity display card. Put release paper between PCBs to stop friction destroying PCB surface.
b. These PCBs can't be directly exposed to the sunlight. The requirements of good storage environment include: relative humidity (30-70%RH), temperature (15-30°C) and storage time (fewer than 12 months).
c. In the process of fabrication, it's not allowed to directly touch OSP PCBs with hands so as to avoid the oxidation as a result of the contact of sweat.
Possible Problems of OSP after Soldering and Solutions
Sometimes, the color of OSP boards changes after soldering, which mainly has something to do with thickness of preservatives, micro etching quantity, soldering times and even abnormal contaminants. Luckily, this problem can be observed just from the appearance. Usually, there are two circumstances:
For Circumstance#1, in the process of soldering, flux is capable of helping eliminating oxidations so that soldering performance won't be influenced. Accordingly, no more measurements have to be taken. On the contrary, Circumstance#2 takes place because OSP integrity has been destroyed so that flux can't eliminate oxidations, which will greatly decrease the soldering performance.
Therefore, some improvements and measurements must be taken in order to ensure the appearance and performance, including:
• The thickness of OSP must be controlled within a certain range.
• The amount of micro-etching must be controlled within a certain range.
• During PCB fabrication, contaminants (gel residue, ink etc.) must be 100% eliminated in case partial abnormity and bad solderability take place.
Chemical Mechanism of OSP Thickness Formation
Just as what is mentioned previously, thickness of film differs with applications of different ions.
In the OSP solution, there are a few copper ions with charge of plus three and OSP solution can be dissociated in acid solution. After cleaning and micro etching, PCBs are then put into OSP tank liquor when copper ions with charge of plus one is generated around bare copper on PCBs. Then complexation takes place on PCB surface copper by empty electrons of copper ions with charge of two in dissociated solution so that dipolar bond can be formed with 2d10 of copper. Finally, a type of complicated reticular structure preservatives will be formed.
The key of OSP technology lies in the control of OSP thickness. Basically, there's a misunderstanding that the thicker the preservative is, the more protection it'll provide to soldering tin. As a matter of fact, if the preservative is too thick, it's difficult for flux to eliminate preservative at the moment of soldering. A large amount of research indicates that when OSP preservative is too thick, the spreading rate of tin paste will be decreased, copper exposure will be caused from time to time, and contact resistance will go up and even solder joints are difficult to form. However, if the film is too thin, the capacity of thermal shock resistance will be decreased so that in the process of reflow, it won't resist high temperature so that soldering performance will be influenced. Therefore, generally speaking, it's suitable that the preservative thickness should be controlled in the range from 0.2 to 0.5μm.
Naturally, it's not the job of electronics designers like you to carefully inspect the thickness of film when receiving circuit boards. During board production, thickness of surface finish has to be rigorously controlled before they leave workshop. For example, PCB fabrication by PCBCart conforms to guidelines and regulations of Standard IPC 2. Plus, more than 10 years' experience and times' experiments allow engineers there to obtain optimal solution concentration and moving speed of conveyor belt, which further leads to perfect OSP thickness.