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 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 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 SMT 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 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.
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