07 October 2004
The protective coatings generally defined as organic coatings are referred to as OSPs (Organic Solderable Protector). The most common coatings are benzotriazole and imidazole, both of which are organic nitrogen compounds. Benzotriazole has long been recognised as an anti-tarnish coating used in the general metal finishing industry. Inhibitor coatings are extremely thin and essentially invisible on the copper surface.
The coatings protect the copper by chemically bonding to the surface and prevent the reaction between the copper and oxygen. The coating may be applied by dip or spray coating and followed by a rinse operation to remove any residues remaining on the solder mask surface. If required, the coating may be removed and re-applied to rejuvenate a surface that has become solderable. If required, the surfaces would need to be re-cleaned with an acid etch and rinse prior to re-treatment.
The protective coatings have been used for many years by large volume manufacturers for surface mount products, an example of which is IBM. The main limitation of the coating was its general inability to stand up to multiple soldering operations. The coatings are degraded by exposure to high temperature and become unsolderable with mildly activated soldering fluxes. The use of high activity water soluble fluxes have often been used on second side wave soldering processes requiring thorough cleaning.
The coatings have in the past also been susceptible to damage by high humidity storage which can degrade the solderability. Incorrect handling by assembly staff has also been seen to affect the coating due to the introduction of handling soils. A training video covering each of the different solderable finishes is available from the SMART Group to provide guidance on the correct use of these finishes.
The guide to the solderability life of different PCB finishes is based on the correct processing, packing etc:
Tin/Lead reflowed 12 months minimum
Hot air levelled 12 months minimum
Nickel/Gold 12 months minimum
Silver <6 months
Roller tin <1 months
Immersion tin <3-6 months
Protective lacquer <6 months
Copper protective (OSP) <6 -12 months
The new generation of alternative copper protective finishes have been demonstrated to protect the surface during multiple reflow and high temperature storage. They have also been shown to withstand the handling issues during assembly and storage and are destined to provide the best vehicle for the future due to their competitive cost, which is much less than any other finish.
Recent trials have indicated that cooling rates after reflow should be improved to reduce the effects on copper coatings. Cooling the board surface directly after reflow below 80°C can prolong the solderability life of the OSP coating. The use of nitrogen during first side reflow with an oxygen level of 100ppm has also provided improved performance during second-side yields. Generally reflow engineers strive to reduce peak board temperature as it exits the reflow oven to reduce the chance of component misplacement, reduce intermetallic formation and, of course, the board needs to be cool for second pass printing in a high volume operation.
OSP is currently used by divisions of IBM, Siemens, Motorola, AT&T, Olivetti, Compaq and Dell. It is also a common process offered by many printed board suppliers due to its simplicity of processing. OSP coated boards were recently shown by Motorola to provide better joint reliability than gold or tin/lead. If correctly processed by the supplier it can provide a 6 -12 month shelf life.
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