The Chinese name is OSP, the full name is Organic Solderability, and the English name is Preflux, which features a very short time.

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 Solderability Preservatives in Chinese, also known as Copper Protector, and Preflux in English. Simply put, OSP is to chemically grow a layer of organic film on a clean bare copper surface. This layer of film has anti-oxidation, thermal shock resistance, and moisture resistance, and is used to protect the copper surface from rusting (oxidation or sulfidation, etc.) in a normal environment; but in the subsequent welding high temperature, this protective film must be very It is easy to be quickly removed by the flux, so that the exposed clean copper surface can be immediately combined with the molten solder into a strong solder joint in a very short time.

With the development of electronic products in the direction of light, thin, short, miniaturized, and multi-functional, printed circuit boards are developing in the direction of high precision, thinness, multilayer, and small holes, especially the rapid development of SMT As a result, high-density thin boards for SMT (such as printed boards such as IC cards, mobile phones, notebook computers, tuners, etc.) continue to develop, making the hot air leveling process less and more unsuitable for the above requirements. At the same time, the Sn-Pb solder used in the hot air leveling process does not meet the environmental protection requirements. With the formal implementation of the EU RoHS Directive on July 1, 2006, the industry urgently needs to find a lead-free alternative to PCB surface treatment. The most common one is organic solder. Protection (OSP), electroless nickel-gold immersion (ENIG), silver immersion and tin immersion.

 

Processing method

The following figure shows the performance comparison of several common PCB surface treatment methods, hot air leveling (Sn-Pb HASL), Ag immersion, Sn immersion, OSP, and electroless nickel immersion gold (ENIG). The latter four are suitable for lead-free processes . It can be seen that OSP has simple process and low cost, so it is more and more popular in the industry.

Physical properties Sn-Pb HASL immersion Ag immersion Sn OSP ENIG

Shelf life (months) 12 12 12 12 6

Can experience reflow times 4 5 5 》4 4

Cost Medium Medium Medium Low High

Process complexity high medium medium low high

Process temperature 240°C 50°C 70°C 40°C 80°C

Thickness range, micron 1-25 0.05-0.20 0.8-1.2 0.2-0.5 0.05-0.2Au 3-5Ni

Flux compatibility Good Good Good Fair Good

OSP is the abbreviation of Organic Solderability Preservatives, which is translated as Organic Solderability Preservatives in Chinese, also known as Copper Protector, or Preflux in English. Simply put, OSP is to chemically grow a layer of organic film on the clean bare copper surface. This film has anti-oxidation, thermal shock resistance, and moisture resistance to protect the copper surface from normal conditions. Rust (oxidation or sulfide, etc.); but in the subsequent high temperature welding, this protective film must be easily removed by the flux quickly, so that the exposed clean copper surface can be combined with molten solder in a very short time Immediately combine to form a strong solder joint.

In fact, OSP is not a new technology, it has actually been more than 35 years, longer than the history of SMT. OSP has many advantages, such as good flat surface, no IMC formation between the copper and the pad, allowing direct solder and copper soldering during soldering (good wettability), low-temperature processing technology, low cost (lower than HASL), Less energy use during processing, etc. OSP technology was very popular in Japan in the early days. About 40% of single panels used this technology, and nearly 30% of double panels used it. In the United States, OSP technology has also surged since 1997, from about 10% of its usage before 1997 to 35% in 1999.

 

material

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 5 generations, which are named BTA, IA, BIA, SBA and the latest APA.

 

Process flow

Degreasing-->Secondary Washing-->Micro-etching-->Secondary Washing-->Pickling-->DI Washing-->Film Air Drying-->DI Washing-->Drying

1. 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 oil removal effect is good. If the oil removal effect is not good, replace the oil removal liquid in time.

2. Micro erosion

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. Therefore, to form a stable film thickness, it is very important to keep the thickness of the microetching stable. Generally, it is more appropriate to control the micro-etching thickness to 1.0-1.5um. Before each shift, the micro-etching rate can be measured, and the micro-etching time can be determined according to the micro-etching rate.

3. Film formation

It is better to use DI water for the 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 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 the thermal shock resistance is poor. During reflow soldering, the film will not be resistant to high temperatures (190-200°C), which will ultimately affect the soldering performance. On the electronic assembly line, the film cannot be well dissolved by the flux. , Affect welding performance. It is generally appropriate to control the film thickness between 0.2-0.5um.

 

Process disadvantages

Of course, OSP has its shortcomings. For example, there are many types of actual formulas and different performance. In other words, the certification and selection of suppliers must be done well enough.

The disadvantage of the OSP process is that the protective film formed is extremely thin, easy to scratch (or scratch), and must be carefully operated and operational. At the same time, the OSP film (referring to the OSP film on the unsoldered connection pad) that has undergone multiple high-temperature welding processes will undergo discoloration or cracks, which affects solderability and reliability.

Figure 1 Acceptance standards and physical pictures of OSP oxidation discoloration

Application guide editor

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

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

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

Packaging and storage

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

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

(b) Do not expose to direct sunlight and maintain 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, you must check the humidity display card and go online within 12 hours. Never open many packages at once. It will take a long time to solve the problem in case you can’t finish the package or if something goes wrong with the equipment , It is easy to go wrong. Go through the furnace as soon as possible after printing and don't stay, because the flux in the solder paste corrodes the OSP film strongly. Maintain a good workshop environment: relative humidity 40~60%, temperature: 22~27℃). During the production process, avoid touching the PCB surface directly with your hands to prevent its surface from being contaminated by sweat and oxidized.

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

(e) After completing SMT, complete the DIP manual plug-in in the shortest possible time (up to 36 hours).

(f) OSP PCB cannot be baked. High-temperature baking can easily cause discoloration and deterioration of OSP. If the empty board exceeds the service life, it can be returned to the manufacturer for OSP rework.

Steel plate design

OSP has a slightly larger opening area than the ordinary tin-sprayed steel mesh, so when the PCB is changed from tin-sprayed to OSP, the steel mesh is best opened again to ensure that the solder can cover the entire pad. Basically, you can use the principle of spraying tin plate for steel engraving. Considering that OSP is flat and beneficial to solder paste formation, and PAD cannot provide part of the solder, the opening can be increased appropriately, but it is better to eat the tin. Too much. After the opening is enlarged, in order to solve the problem of tin beads, tombstones and OSP PCB exposed copper in SMT CHIP parts, the stencil opening design method of the solder paste printer is changed to a concave design.

(a) When designing the solder paste printed steel plate, try to make the solder all cover the pad as much as possible. According to the IPC 610-D version of the PCBA solder quality visual inspection standard, the exposure of a small part of the edge of the pad can be judged to be acceptable, but the coverage must reach at least 80% of the pad area.

(b) If the parts are not placed on the PCB for some reason, the solder paste should also cover the pads as much as possible.

(c) In order to prevent OSP PCB from waiting too long in the SMT process to cause the through holes to oxidize, which may cause solderability and reliability problems, you can consider printing all ICT test points and DIP through holes with solder paste at the solder paste printing station. To protect the through holes from oxidation and rust.

Bad heavy industry

(a) Try to avoid printing errors, because cleaning will damage the OSP protective layer.

(b) When the PCB printing solder paste is bad, because the OSP protective film is easily corroded by organic solvents, all OSP PCBs cannot be soaked or cleaned with high volatile solvents. It is recommended to wipe off the solder paste with a non-woven cloth dipped in 75% alcohol.

(c) After the heavy industry is completed, the SMT soldering operation on the PCB surface of the current heavy industry should be completed within 2 hours.

Temperature curve

The peak temperature setting during reflow soldering should not be too high (240-245℃), and the time in the furnace should be controlled well, otherwise the soldering problem of the pad may occur when the second side is made. Of course, this situation also indicates the board Not high temperature resistance.

For double-sided assembly, the first reflow requires a nitrogen atmosphere to maintain the solderability of the second side. The current OSP will also disappear when there is flux and heat, but the protective agent on the second side remains intact until it is printed with solder paste or wave soldering. At this time, reflow or wave soldering does not necessarily require an inert gas environment .

In the case of the first aerobic heating, the OSP (not heat-resistant) in the through hole will be partially or completely decomposed as on the pad, so that it may leak out of the substrate. This can be observed by the degree of discoloration of the OSP However, the solubility and fluidity of the decomposed and oxidized OSP residues will be significantly reduced. Non-original flux can deal with, the main welding area of ​​the through hole is the inner hole, and the weldable area of ​​the inner hole will be decomposed and oxidized OSP The impact of residues.

test

With OSP surface treatment, if the test point is not covered by solder, it will cause contact problems with the needle bed fixture during ICT testing. There are many human factors that can affect the ICT test results, some of which are: OSP provider type, number of passes in the reflow oven, whether the wave process, nitrogen reflow or air reflow, and the type of simulation test during ICT. Merely changing to a sharper probe type to penetrate the OSP layer will only cause damage and puncture the PCA test via or test pad. Therefore, it is strongly recommended not to directly probe the exposed copper pads, and it is required to consider tinning all test points when opening the steel mesh.