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Soldering surfaces for printed circuit boards, Part 5: Organic Solderability Preservative (OSP)

KSG Leiterpaltt

Organic surface protection (OSP, or organic solderability preservative) is a cost-effective alternative to metallic surface finishes for printed circuit boards. An organic layer is formed on the exposed copper areas, which protects against oxidation by oxygen. This maintains the solderability of the PCB for a defined period of time without the need for an additional metallic protective layer.

The shelf lives of printed circuit boards coated with OSP ENTEK Plus HT are at least six months at a maximum of 30°C and a maximum of 70% RH. A new coating can then be applied to renew the shelf life. Alternatives where the shelf life can be increased to over 12 months are currently being investigated. KSG applies an OSP layer thickness that permits multiple thermal stresses during further processing of the PCBs. The layer thickness produced is regularly checked by means of spectrophotometric analysis.

Advantages and disadvantages of OSP

Compared to the hot air leveling (tinning) process, organic surface protection (OSP) is characterized by a very thin and homogeneous deposition. This makes it ideal for flat SMD pads and simplifies solder paste printing. Due to the deposition at 35°C, there is no thermal stress on the printed circuit board and there is no distortion or disintegration of the base material. OSP is a preferred surface finish for radio frequency applications because the electrical transmission properties are not adversely affected by the organic coating, compared to a metallic surface finish such as electroless nickel immersion gold. OSP is suitable for press-fit technology and can also be combined with other surface finishes (e.g. electroplated nickel gold).

It should be noted that the OSP layer is attacked by solvents, acids and alkalis. As a result, reworking during solder paste printing is only possible to a limited extent, as the flux in the solder paste attacks the OSP layer and the copper can be exposed after removing the solder paste and cleaning the PCB. When packaging printed circuit boards with an OSP coating, the intermediate paper should be sulfur-free and pH-neutral, and the desiccant in the package should never come into direct contact with the OSP layer, as desiccants can contain sulfur compounds and thus damage the organic protective layer.

Recommendations for processability

To achieve optimum soldering results, the soldering parameters must be adapted to the new surface. For all ENTEK Plus HT PCBs, particular attention must be paid to the choice of flux and solder paste used, as well as the temperature effects on the surface in soldering systems without shielding gas.

The OSP layer is damaged by chlorides and sulfur compounds from the ambient air. This means that once the delivery packaging has been opened, unprocessed PCBs must be repackaged as quickly as possible and stored under the specified conditions. Furthermore, the use of adhesive tapes, adhesive labels, stamping ink, markers and rubber bands in direct contact with the OSP layer should be avoided, as these may also contain sulfur compounds.

Care with the surface temperature

The OSP coating is affected by heat and oxygen. If printed circuit boards are exposed to particularly high temperatures, oxygen diffuses through the OSP layer and the thickness of the copper-oxide layer at the boundary between the copper of the PCB and the organic layer increases (by a few nanometers). The use of low-temperature reflow profiles and nitrogen as a shielding gas produces less copper oxide than high-temperature reflow profiles and processing in an air atmosphere. This aspect is not only important for the component placement itself, but also for baking before assembly. This means that extended baking above 100°C is not possible without a shielding gas. The process supplier specifies 125°C for three hours as the limit for ENTEK Plus HT. Temperature has another effect on the OSP layer: the OSP becomes harder and more compact, which is enhanced by the use of nitrogen. The aspects explained above mean that the OSP layer is difficult to remove and the choice of flux and solder paste used plays an important role.

Selecting the right flux and solder paste

What happens to the OSP layer during soldering? The components of the flux or solder paste—specifically organic solvents and organic acids—ensure that the OSP layer is dissolved away. Other components, mainly activators, remove the very thin copper oxide layer that is present between the OSP layer and the copper of the circuit boards. The intermetallic phase can then form between the copper and tin during soldering, i.e. defect-free wetting can take place.

It has been shown that certain solvents and organic acids remove the ENTEK Plus HT layer more effectively and that the copper-oxide layer is better dissolved by certain activators. In general, combinations of the above-mentioned ingredients achieve better results. This is particularly important if the OSP layer has already undergone high-temperature steps, such as with multiple reflow soldering. Specially adapted fluxes for selective and wave soldering can clean the copper surface better; in the reflow process, the temperature-stressed OSP layer can be removed more effectively with the choice of solder paste.

Qualification test

Thermal aging, which is used to simulate long-term storage for tinned PCBs, cannot be used to test the reliability of ENTEK Plus HT PCBs. The climate test (five days, 40°C, 92% relative humidity) is a well-established test for the organic surface. This clearly shows whether the quality of the applied surface coating is flawless or impaired. Defects or pores are very easy to detect after the test, as unprotected copper areas oxidize and turn dark during the climate test. In addition to the climate test, the KSG Group also regularly checks the wetting ability using the solder-spread test and solder-float test as part of the reliability testing.

Key takeaways

Overall, OSP is extremely versatile and offers a number of advantages over metallic surface finishes, in addition to cost and environmental aspects. To maximize the durability and performance of the OSP surface, it is important to carefully follow the recommendations for further processing. Just recently, the KSG Group expanded its capacity in the field of organic surface processing and commissioned a new OSP line. You can find all the information on this in this press release.

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