PCB Technical - OSP surface treatment PCB improvement measures

1.Choose the right OSP potion

OSP has three types of materials: Rosin, Active Resin and Azole. Currently the most widely used is the azole OSP. The azole OSP has been improved for about 6 generations, and its decomposition temperature can be as high as 354.9°C, which is suitable for lead-free processes and multiple reflow soldering. Before PCB production, it is necessary to select the appropriate potion according to the production process of the product.

2.The thickness and uniformity of the OSP film must be strictly controlled during the PCB production process

The key to the OSP process is to control the thickness of the protective film. The film thickness is too thin, and the thermal shock resistance is poor. During reflow soldering, the film cannot withstand high temperatures, cracking and thinning, which will easily cause the pad to oxidize and affect the solderability; if the film thickness is too thick, it cannot be very good during soldering. The dissolution and removal of the flux by the flux will also lead to poor soldering.

3.The production process of OSP board

Placing the board - degreasing - washing - micro-etching - washing - pre-soaking - DI washing - blotting - upper protective film (OSP) - blotting - DI washing - drying - drying - drying - drying - drying - unwinding

4.The main factors affecting OSP film thickness

a.Degreasing. The degreasing effect directly affects the quality of film formation. Poor degreasing 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, always check whether the degreasing effect is good. If the degreasing effect is not good, the degreasing fluid should be replaced in time.

b.Micro-eclipse. The purpose of micro-etching is to form a rough copper surface to facilitate film formation. The thickness of the microetching directly affects the film formation rate. To form a stable film thickness, the thickness of the microetching must be kept stable. Generally, it is more appropriate to control the micro-etching thickness at 1.0~1.5um. Before each shift, it is necessary to measure the micro-corrosion rate, and determine the micro-corrosion time according to the micro-corrosion rate.

c.Pre-soaking. Pre-soaking can prevent harmful ions such as chloride ions from damaging the OSP tank solution. The main function of the OSP prepreg cylinder is to accelerate the formation of the OSP film thickness and deal with the impact of other harmful ions on the OSP cylinder. There is an appropriate amount of copper ions in the prepreg solution, which can promote the formation of complex protective film and shorten the dip coating time. It is generally believed that due to the presence of copper ions, the alkylbenzimidazole and copper ions have been complexed to a certain extent in the pre-flux solution. When this kind of complex with a certain degree of aggregation is deposited on the copper surface to form a complex film, a thicker protective layer can be formed in a short time, thus acting as a complexing accelerator. For example, the content of alkylbenzimidazole or similar components (imidazoles) in the prepreg is very small. When the copper ion is excessive, the prepreg solution will age prematurely and need to be replaced. Therefore, it is necessary to focus on controlling the concentration of the prepreg and the prepreg time.

d.The concentration of the main components of OSP. Alkylbenzimidazole or similar components (imidazoles) are the main components in the OSP solution, and the concentration is the key to determining the OSP film thickness. During the production process, it is necessary to focus on monitoring the concentration of OSP potion.

e.The pH value of the solution. The stability of the pH value has a greater impact on the film formation rate. In order to maintain the stability of the pH value, a certain amount of buffering agent is added to the solution tank. Generally, the PH value is controlled at 2.9~3.1, and a dense, uniform OSP film with moderate thickness can be obtained. When the PH value is high and PH>5, the solubility of alkylbenzimidazole will decrease and oily matter will precipitate; when the PH value is low and PH<2, the formed film will be partially dissolved. Therefore, it is necessary to focus on monitoring the PH value.

f.The temperature of the solution. The change of temperature also has a greater influence on the film formation rate. The higher the temperature, the faster the film formation rate. Therefore, the temperature of the OSP tank needs to be controlled.

g.Film formation time (dip coating time). Under certain conditions of OSP bath composition, temperature and pH value, the longer the film formation time, the thicker the film formation. Therefore, it is necessary to control the film formation time.

5.OSP film thickness detection  

At present, most PCB factories use UV ultraviolet spectrometers to measure the OSP film thickness. The principle is mainly to use the imidazole compounds in the OSP film to have strong absorption characteristics in the ultraviolet region,and then measure the absorbance at the maximum time. This method is simple and easy to calculate the film thickness of OSP, but the test error is relatively large. Another method is to use FIB technology to measure the actual thickness of the OSP film. PCB factories need to use appropriate methods to detect and control the thickness of the OSP film during production to ensure that the thickness of the OSP film meets the standard requirements.

6. OSP board packaging and storage requirements

Because the OSP film is extremely thin, if exposed to high temperature and humidity for a long time, the PCB surface will be oxidized and the solderability will deteriorate. After the reflow soldering process, the OSP on the PCB surface will also crack and become thinner, which will easily lead to PCB copper foil Oxidizes and deteriorates solderability.

6.1 OSP board packaging requirements

The incoming materials of the OSP board should be vacuum packed, with desiccant and humidity display card attached. Separate the PCB board from the board to avoid scratches or friction damage to the OSP film.

6.2 OSP board storage requirements

It cannot be directly exposed to sunlight. It should be stored in an environment with relative humidity: 30~70% and temperature: 15~30 degree Celsius. The shelf life is less than 6 months. It is recommended to use a special moisture-proof cabinet for storage. If the PCB is damp or expired, it cannot be baked and can only be returned to the PCB factory for OSP rework.

7.The use and precautions of the OSP board in the SMT section

a. Before opening the PCB, check whether the PCB packaging is damaged and whether the humidity indicator card is discolored. If it is damaged or discolored, it cannot be used. It needs to be on-line production within 8 hours after opening. It is recommended to use as many openings as possible, and vacuum packaging should be used in time for PCBs that have not been produced or the last number of PCBs.

b. It is necessary to control the temperature and humidity of the SMT workshop. It is recommended that the workshop temperature: 25±3 degree Celsius, humidity: 50±10%. During the production process, it is forbidden to directly touch the surface of the PCB pad with bare hands to prevent sweat pollution, causing oxidation and leading to poor soldering.

c.The PCB for printing solder paste should be mounted as soon as possible to complete the components and pass the furnace, try to avoid printing errors or mounting problems leading to washing, because washing will damage the OSP film. For cleaning, it is recommended to wipe off the solder paste with a non-woven cloth dipped in 75% alcohol. The PCB after cleaning must be soldered within 2 hours.

d.After the SMT single-sided placement is completed, the placement of the second side SMT components needs to be completed within 24 hours, and the selective soldering or wave soldering of the DIP (plug-in) components must be completed within 36 hours.

e.Since the solder paste of OSP-treated PCB has poorer fluidity than other surface-treated PCBs, the solder joints are likely to expose copper. When designing the stencil opening, you can increase it appropriately. It is recommended to open the hole according to the pad 1:1.05 or 1:1.1, but you need to pay attention to the anti-tin bead treatment of the CHIP component.

f.The peak temperature and reflow time of the OSP board during reflow soldering is recommended to be as close to the lower limit of the process window as possible when the soldering quality is met, and the peak temperature and reflow time should be as low as possible; when producing double-sided panels, it is recommended to produce the first side (small The temperature of the component side) should be lowered appropriately, and the temperature on both sides should be set separately to reduce the damage of high temperature to the OSP film. If possible,it is recommended to use nitrogen production, which can effectively improve the problem of poor oxidation and welding of the second side of the double-sided OSP board.