Introduction to OSP Technology

              OSP is the abbreviation for Organic Solderability Preservatives, which is translated as organic solder film, also known as copper protector, and also known as Preflux in English. Simply put, OSP is an organic film grown chemically on a clean bare copper surface. This film has oxidation resistance, heat shock resistance, and moisture resistance to protect the copper surface from further rusting (oxidation or sulfurization, etc.) in a normal environment; But in the subsequent welding high temperature, this protective film must be easily removed by the flux quickly, so that the exposed clean copper surface can immediately combine with the molten solder to form a solid solder joint in a very short time.

             Actually, OSP is not a new technology. It has actually been around for over 35 years, longer than SMT. OSP has many advantages, such as good flatness, no IMC formation between the copper and the solder pad, allowing direct welding of the solder and copper during welding (good wettability), low-temperature processing technology, low cost (which can be lower than HASL), and low energy consumption during processing. OSP technology was very popular in Japan in the early days,      with about 40% of single panels using this technology, and nearly 30% of double sided panels using it. In the United States,  OSP technology has also surged since 1997, increasing from approximately 10% of usage before 1997 to 35% in 1999.

              OSP has three major categories of materials: rosin, active resin, and azole. Currently, the most widely used is the oxazole OSP. The oxazole OSP has been improved for about 5 generations, named BTA, IA, BIA, SBA, and the latest APA.

The process flow of PCB circuit board OSP:

               Degreasing -->Secondary water washing -->Microetching -->Secondary water washing -->Acid washing -->DI water washing -->Film forming air drying -->DI water washing -->Drying

1. Degreasing

               The effectiveness of oil removal directly affects the quality of film formation. Poor oil removal results in uneven film thickness. On the one hand, the concentration can be controlled within the process range by analyzing the solution. On the other hand, it is also necessary to regularly check whether the oil removal effect is good. If the oil removal effect is not good, the oil removal fluid should be replaced in a timely manner.

2. Micro etching

                The purpose of micro etching is to form a rough copper surface for easy film formation. The thickness of micro etching directly affects the film formation rate, therefore, it is very important to maintain the stability of micro etching thickness in order to form a stable film thickness. It is generally appropriate to control the micro etching thickness between 1.0 and 1.5um. Before each production shift, the micro corrosion rate can be measured and the micro corrosion time can be determined based on the micro corrosion rate.

3. Film-forming

               It is best to use DI water for washing before film forming to prevent contamination of the film forming solution. It is also recommended to use DI water for washing after film formation, and the pH value should be controlled between 4.0 and 7.0 to prevent contamination and damage to the film layer. The key to the OSP process is to control the thickness of the anti oxidation film. The film is too thin and has poor heat resistance and impact resistance. During reflow soldering, the film layer cannot withstand high temperatures (190-200 ° C), ultimately affecting the welding performance. On electronic assembly lines, the film cannot be well dissolved by the flux, affecting the welding performance. It is generally suitable to control the film thickness between 0.2 and 0.5um.

Disadvantages of OSP process for PCB circuit boards

              OSP also has its shortcomings, such as the variety of actual formulas and varying performance. That is to say, the certification and selection of suppliers should be done well enough.

              The drawback of the OSP process is that the protective film formed is extremely thin and prone to scratches (or abrasions), requiring careful operation and operation.

              At the same time, the OSP film (referring to the OSP film on the unwelded connection plate) that has undergone multiple high-temperature welding processes may discolor or crack, affecting solderability and reliability.

               The solder paste printing process needs to be mastered well, as poorly printed boards cannot be cleaned using IPA or other methods, which can damage the OSP layer.

               The thickness of transparent and non-metallic OSP layers is also difficult to measure, and the degree of transparency on the coating coverage is also not easy to see. Therefore, it is difficult to evaluate the quality stability of suppliers in these aspects;

               OSP technology does not have IMC isolation between Cu in the solder pad and Sn in the solder. In lead-free technology, SnCu in solder joints with high Sn content grows rapidly, affecting the reliability of the solder joints.