Detailed explanation of OSP process

Detailed explanation of OSP process

OSP is a process for surface treatment of printed circuit board (PCB) copper foil that meets the requirements of the RoHS directive. OSP is the abbreviation of Organic Solderability Preservatives, which is translated as organic solder protection film in Chinese, also known as copper protection agent, and also known as Preflux in English. Simply put, OSP is to chemically grow an organic film on the clean bare copper surface. This layer of film has anti-oxidation, thermal shock resistance, moisture resistance, and is used to protect the copper surface from rusting (oxidation or vulcanization, etc.) in the normal environment; but in the subsequent high temperature welding, this protective film must be very It is easily and quickly removed by the flux, so that the exposed clean copper surface can be combined with the molten solder to form a strong solder joint in a very short time.

OSP has three categories of materials: rosin (Rosin), active resin (Active Resin) and azole (Azole). The most widely used is the azole OSP. The azole OSPs have been improved for about 5 generations, named BTA, IA, BIA, SBA and the latest APA.

The process of OSP is: degreasing-->secondary washing-->micro-etching-->secondary washing-->pickling-->DI washing-->film-forming air-drying-->DI washing-->drying, Compared with other surface treatment processes, it is relatively easy. The purpose of micro-etching is to form a rough copper surface to facilitate film formation. The thickness of the micro-etching directly affects the film formation rate. Therefore, to form a stable film thickness, it is very important to maintain the stability of the micro-etching thickness.

1. Degreasing

The quality of 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, it is also necessary to frequently check whether the degreasing effect is good. If the degreasing effect is not good, the degreasing fluid should be replaced in time.

2. Micro-etching

The purpose of micro-etching is to form a rough copper surface to facilitate film formation. The thickness of the micro-etching directly affects the film formation rate. Therefore, to form a stable film thickness, it is very important to maintain the stability of the micro-etching thickness. Generally, it is more appropriate to control the thickness of micro-etching at 1.0-1.5um. Before each shift of production, the micro-erosion rate can be measured, and the micro-erosion time can be determined according to the micro-erosion rate.

3. Film formation

It is best to use DI water for water washing before film formation to prevent the film formation liquid from being contaminated. It is also best to use DI water for washing after film formation, and the pH value should be controlled between 4.0-7.0 to prevent the film layer from being polluted and damaged. The key to the OSP process is to control the thickness of the anti-oxidation film. The film is too thin and has poor thermal shock resistance. During reflow soldering, the film cannot withstand high temperature (190-200°C), which will ultimately affect the soldering performance. On the electronic assembly line, the film cannot be well dissolved by flux. , affecting the welding performance. Generally, it is more appropriate to control the film thickness between 0.2-0.5um.

Disadvantages of OSP process:

Of course OSP also has its shortcomings, for example, there are many types of actual formulas and different performances. That is to say, the certification and selection of suppliers should be done well enough.

The disadvantage of the OSP process is that the protective film formed is extremely thin and easily scratched (or scratched), and must be carefully manipulated and op-amped. At the same time, the OSP film (referring to the OSP film on the unsoldered connection pad) that has undergone multiple high-temperature soldering processes will have discoloration or cracks, which will affect the solderability and reliability.

test

With the OSP finish, if the test points are not covered with solder, it will cause contact problems with bed of needles fixtures during ICT testing. There are many human factors that affect ICT test results, some of which are: OSP provider type, number of passes in the reflow oven, whether it is a wave process, nitrogen reflow or air reflow, and the type of simulation test at ICT. Simply changing to a sharper probe type to go through the OSP layer will only cause damage and puncture the PCA test vias or test pads. Therefore, it is strongly recommended not to probe the exposed copper pads directly, and it is required to consider tinning all test points when opening the stencil.

Application Guide

The solder paste printing process should be mastered well, because the poorly printed board cannot be cleaned with IPA, etc., which will damage the OSP layer.

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

OSP technology has no IMC isolation of other materials between Cu of the pad and Sn of the solder. In lead-free technology, the SnCu in the solder joint with high Sn content grows rapidly, which affects the reliability of the solder joint.

Packaging and storage

The organic coating on the surface of OSP PCB is extremely thin. If exposed to high temperature and high humidity for a long time, the surface of the PCB will be oxidized and the solderability will be deteriorated. After the reflow soldering process, the organic coating on the surface of the PCB will also become thinner, resulting in PCB copper. Foil is easily oxidized. Therefore, the storage method and use of OSP PCB and SMT semi-finished board should comply with the following principles:

(a) The incoming materials of OSP PCB should be vacuum packed with desiccant and humidity display card attached. When transporting and storing, use release paper between PCBs with OSP to prevent friction from damaging the OSP surface.

(b) Do not expose to direct sunlight, keep a good warehouse storage environment, relative humidity: 30~70%, temperature: 15~30℃, and the shelf life is less than 6 months.

(c) When unpacking at the SMT site, the humidity display card must be checked and put online within 12 hours. Never unpack many packages at one time. , it is prone to problems. After printing, do not stay in the oven as soon as possible, because the flux in the solder paste is very corrosive to the OSP film. Maintain a good workshop environment: relative humidity 40~60%, temperature: 22~27℃). During the production process, it is necessary to avoid touching the surface of the PCB with your hands directly to avoid oxidation on the surface due to sweat contamination.

(d) After the SMT single-sided patch is completed, the second-side SMT component patch assembly must be completed within 24 hours.

(e) After completing the SMT, complete the DIP hand insert in the shortest possible time (up to 36 hours).

(f) OSP PCB cannot be baked, and high temperature baking is easy to cause discoloration and deterioration of OSP. If the empty board exceeds the expiration date, it can be returned to the manufacturer for OSP rework.