Organic surface protection (OSP, Organic Solderability Preservative or Organic Surface Protection) is a cost-effective alternative to metallic end surfaces on 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 life of printed circuit boards coated with OSP ENTEK Plus HT is at least six months at a maximum of 30 °C and a maximum of 70 % RH. After this, a new coating can be applied to renew the shelf life. Alternatives with which the shelf life can be increased to over 12 months are currently being investigated. KSG applies an OSP layer thickness that enables multiple thermal loads 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 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 conductor pad and there is no distortion or shattering of the base material. OSP is a preferred end surface for high-frequency applications because the electrical transmission properties are not adversely affected by the organic coating compared to a metallic end surface such as electroless nickel-gold. OSP is suitable for press-fit technology and can also be combined with other end surfaces (e.g. electroplated nickel-gold).
It should be noted that the OSP layer is attacked by solvents, acids and alkalis. As a result, reworking 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 PCBs with an OSP coating, the paper should be sulphur-free and pH-neutral and the desiccant in the package should never come into direct contact with the OSP layer, as desiccants can contain sulphur compounds and thus damage the organic protective layer.
Recommendations for workability
In order 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 and the temperature effects on the surface for soldering systems without shielding gas.
The OSP layer is damaged by chlorides and sulphur 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 aforementioned 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 sulphur compounds.
Observing the surface temperature
The OSP coating is affected by heat and oxygen. If the printed circuit boards are exposed to particularly high temperatures, oxygen diffuses through the OSP layer and the copper oxide layer at the boundary between the copper of the printed circuit board and the organic layer increases (in the range of a few nanometers). By using low-temperature reflow profiles and nitrogen as a shielding gas, less copper oxide is produced than with high-temperature reflow profiles and processing in an air atmosphere. This aspect should not only be taken into account for placement itself, but also for annealing before placement. This means that extended tempering above 100 °C is not possible without inert gas. The process supplier specifies 125 °C for 3 hours as the limit for ENTEK Plus HT. When temperature is applied to the OSP layer, there is a further effect: the OSP becomes harder and more compact, which is reinforced by the use of nitrogen. The aspects explained above mean that the OSP layer is difficult to remove and the selection of the 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. Other components, mainly the activators, remove the very thin copper oxide layer between the OSP layer and the copper of the circuit boards. The intermetallic phase between copper and tin can then form during soldering, i.e. wetting can take place without defects.
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 triggered by certain activators. In general, combinations of the components mentioned achieve better results. This is particularly important if the OSP layer has already undergone temperature steps, e.g. in the case of 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 ageing, 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 (5 days, 40 °C, 92 % relative humidity) is a tried and tested 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 during the climate test and turn dark. 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 test.
Key Take Aways
Overall, OSP is extremely versatile and offers a number of advantages compared to metallic end surfaces, 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. The KSG Group has recently expanded its capacities in the field of organic surface treatment and commissioned a new OSP plant. You can find all information about this in this press release.
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