Organic solderability preservatives, or, if you prefer, organic surface protectants, or OSPs, have been with us for decades. Did you know more than 60% of the world’s boards use OSPs? They are in everything from smartphones to tablets to medical devices, airbags, and engine controls.
Major OEMs like Intel, Apple, Cisco, Continental, Bosch, Denso, and Hitachi Automotive are known to use them. Yet when engineers discuss their preferred finishes, OSPs tend to be on the outside looking in.
A new IPC task group is trying to bring an added layer of credibility to OSPs for high-temperature soldering by developing a standard, along with a series of test methods.
At a glance, OSPs have ample potential. Compared to metallic finishes, they are low-cost and offer much-sought-after surface coplanarity on the coated copper pads. They emerged in the 1980s as a replacement for hot air solder leveling, which was an omnipresent but more expensive, higher maintenance process. Because of their ability to produce thin, even coatings, OSPs seemed superior for assemblers working with advanced packages, and in some cases OSPs cut the cost of the finish up to 50% over HASL and even more versus finishes containing gold or silver. Major OEMs like Lucent adopted OSPs for a large percentage of their boards.
But … back then metallic finishes offered better protection against oxidation, and the solderability of OSPs typically did not hold up following multiple lead-free reflow cycles. Those lingering issues of thermal resistance, copper diffusion suppression and solderability torpedoed consideration of OSPs for many higher rel products. The lack of ability to inspect real product without destructive testing was a limiter, too.
More than 20 years ago, several companies attempted to develop a standard for OSP under the auspices of IPC. In 1997, the Surface Mount Council – remember that? – assembled a white paper that offered a snapshot of the various offerings at the time, but little else. In 2008, the IPC Plating Subcommittee announced plans to release a standard. A key OEM disputed the results of the solderability data, however, and that effort was spiked. In a post-mortem, the subcommittee noted it couldn’t reach consensus on the range of the coating thickness to meet the performance specification.
The latest effort appears to have put that old debate to rest. The committee, which includes members from Continental, Bosch, Sanmina and TTM, has agreed thickness is not a determinant in the performance of the coating. “That was a hang-up several years ago,” recalls task group chairman Michael Carano. “We don’t need one big range.”
Moreover, today’s OSPs are better than their ancestors. The pending spec defines high-temperature OSP as capable of withstanding two IR reflows at a peak temp of 245°-250°C and shows the same wetting balance results at zero reflows as at three, with a maximum 20% drop. And Carano believes the higher-performance OSPs are capable of five reflow cycles. Storage life has also improved, reaching 12 months. OSP is often used in mixed final finish products, such as with hard or soft gold. “It is common in chip packages. The packaging people will love (the standard),” Carano said.
The task group is currently finalizing the data that will support the process control recommendations and testing protocols. It plans to submit the draft of the new spec, known as IPC-4555, to the IPC membership for vote by spring.
This is not to suggest OSPs will broadly replace other finishes. They are reworkable, but electrical testing is generally not recommended, as the coating is hard and can bend the pin probe. Some manufacturers work around this by printing paste on test pads instead.
So why a standard now? Would it help elevate OSP into a greater array of products? Carano believes so. “If there were a workmanship standard, then yes. Right now, every fabricator uses a different protocol because each OEM has a different process.”
Ultimately, I think this is a good way to go. Standards are often accused of constraining innovation, but the opposite is also true: Without standards, it’s difficult for technologies to get implemented. Use of OSPs might not spread much wider – so to speak – in the bare board finish market. But it’s past time for a standard.