OSP Process and SMT Application Guide

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OSP Process and SMT Application Guide [Copy link]

OSP Process and SMT Application Guide

湖州生力电子有限公司 沈新海

Abstract: This article briefly introduces the production process, advantages and disadvantages of OSP PCB, and provides guidance for our company's application in OSP PCB SMT process.

Keywords: OSP PCB SMT lead-free process

As people are developing electronic products in the direction of light, thin, short, miniaturized and multifunctional, printed circuit boards are developing towards high precision, thinness, multi-layer and small holes, especially the rapid development of SMT, which makes the high-density thin boards for SMT (such as IC cards, mobile phones, laptops, tuners and other printed boards) continue to develop, making the hot air leveling process increasingly unsuitable for the above requirements. At the same time, the Sn-Pb solder used in the hot air leveling process does not meet environmental protection requirements. With the formal implementation of the EU RoHS directive on July 1, 2006, the industry urgently needs to seek lead-free alternatives for PCB surface treatment. The most common ones are organic solder protection (OSP), electroless nickel gold immersion (ENIG), silver immersion and tin immersion.

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

Physical properties	Sn-Pb HASL	Ag Immersion	Sn	Volunteer Fire Department	AGREE
Shelf life (months)	12	12	12	12	6
Number of refluxes	4	5	5	3 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 , micrometer	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	generally	good

OSP is the abbreviation of Organic Solderability Preservatives, which is translated into organic solderability preservatives in Chinese, also known as copper protection agent, and also known as Preflux in English. Simply put, OSP is to grow a layer of organic film on the clean bare copper surface by chemical methods. This film has anti-oxidation, heat shock resistance, and moisture resistance, and is used to protect the copper surface from rusting (oxidation or sulfide, etc.) in a normal environment; but in the subsequent high temperature of welding, this protective film must be easily removed by the flux quickly, so that the exposed clean copper surface can be immediately combined with the molten solder in a very short time to form a solid solder joint.

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

There are three major categories of OSP materials: rosin, active resin and azole. Currently, the most widely used is the azole OSP. Azole OSP has gone through about five generations of improvements, namely BTA, IA, BIA, SBA and the latest APA.

OSP process flow:

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

1. Degreasing

The quality of the degreasing effect directly affects the film quality. If the degreasing is not good, the film thickness will be uneven. On the one hand, the concentration can be controlled within the process range by analyzing the solution. On the other hand, the degreasing effect should be checked frequently. If the degreasing effect is not good, the degreasing solution 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 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 micro-etching thickness at 1.0-1.5um. Before each shift of production, the micro-etching rate can be measured and the micro-etching time can be determined based on the micro-etching rate.

3. Film formation

It is best to use DI water for washing before film formation to prevent the film-forming 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 contaminated and damaged. The key to the OSP process is to control the thickness of the anti-oxidation film. If the film is too thin, the thermal shock resistance is poor. During reflow soldering, the film layer cannot withstand high temperatures (190-200°C), which ultimately affects the welding performance. On the electronic assembly line, the film cannot be well dissolved by the flux, affecting the welding performance. Generally, it is more appropriate to control the film thickness between 0.2-0.5um.

Disadvantages of OSP Process

OSP certainly has its shortcomings, such as the large variety of actual formulas and different performances. This means that 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 and easily scratched (or abraded), and must be carefully operated and operated.

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 discolor or crack, affecting solderability and reliability.

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

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

OSP technology has no IMC isolation of other materials between the Cu of the pad and the Sn of the solder. In lead-free technology, the SnCu in the solder joints with high Sn content grows very fast, affecting the reliability of the solder joints.

SMT Application Guide for OSP PCB:

1. OSP PCB packaging, storage, and use:

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

(a) OSP PCB 來料应采用真空包装，并附上干燥剂及湿度显示卡。运输和保存时，带有OSP的PCB之间要使用隔离纸以防止摩擦损害OSP表面。

(b) Do not expose to direct sunlight . Maintain a good warehouse storage environment with relative humidity of 30 ~ 70% and temperature of 15 ~ 30 ℃ . 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 once. If you can't finish printing, or if there is a problem with the equipment that takes a long time to solve, it is easy to cause problems. After printing, pass the furnace as soon as possible and don't stay, 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 ℃ ). Avoid direct contact with the PCB surface with your hands during the production process to prevent its surface from being contaminated by sweat and oxidized.

(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 SMT , DIP hand plug-in should be completed in the shortest possible time (maximum 36 hours) .

(f) OSP PCB cannot be baked. High temperature baking can easily cause OSP to change color and deteriorate. If the blank board exceeds the service life, it can be returned to the manufacturer for OSP rework.

2. Design of SMT solder paste printed steel plate for OSP PCB

OSP steel mesh opening area is slightly larger than that of ordinary tin-spraying plate, so when PCB is changed from tin-spraying to OSP, it is best to reopen the steel mesh to ensure that the solder can cover the entire pad. The principle of tin-spraying plate can basically be used for steel plate engraving. Considering that OSP is flat, it is beneficial to solder paste forming, and PAD cannot provide part of the solder, so the opening can be appropriately enlarged, but it is better to be full of tin, not too much. After the opening is enlarged, in order to solve the problems of solder beads , monuments and OSP PCB exposed copper on SMT CHIP parts , the solder paste printer steel mesh opening design method is changed to a concave design.

(a) When designing solder paste printed steel plates , try to make the solder cover the pads as much as possible. According to IPC 610-D version of PCBA solder quality visual inspection standard , a small amount of exposed copper on the edge of the pad can be judged acceptable, but the coverage rate must be at least 80% of the pad area .

(b) If the component position on the PCB is not placed for some reason , the solder paste should also cover the pad as much as possible.

(c) In order to prevent the OSP PCB from waiting too long in the SMT process and causing oxidation of the through holes, which may lead to solderability and reliability issues, it is possible to consider printing solder paste on all ICT test points and DIP through holes at the solder paste printing station to protect the through holes from oxidation and rust.

3. Rework of OSP PCB printing solder paste

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

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

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

4. OSP PCB reflow oven temperature curve

During reflow soldering, the peak temperature should not be set too high ( 240-245℃), and the time in the furnace should be controlled well, otherwise the solder pad may eat tin when doing the second side. Of course, this situation also means that the board is not resistant to high temperatures.

For double-sided assembly, the first reflow requires a nitrogen environment 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 solder paste is printed or wave soldering is performed, at which time an inert gas environment is not necessarily required during reflow or wave soldering.

在首次的有氧加热情况下通孔里的OSP（不耐热的品种）会与焊盘上一样产生部分乃至全部的分解，以至于有漏出基材的可能，这可通过OSP的变色程度观察到，而分解和氧化的OSP残留物溶解性和流动性都会显著的下降，非原焊剂可对付的，通孔的主要焊接面积在内孔，内孔的可焊面积会受到分解和氧化的OSP残留物的影响。

5. ICT test of OSP PCB

With OSP surface treatment, if the test points are not covered by solder, it will cause contact problems with the bed of nails fixture during ICT testing. There are many process factors that will affect the ICT test results, some of which are: OSP provider type, number of times in the reflow oven, whether it is a wave process, nitrogen reflow or air reflow, and the type of simulation test during ICT. Simply changing to a sharper probe type to penetrate the OSP layer will only cause damage and pierce the PCA test vias or test pads. It is therefore 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.