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IPC Task Group Edges Toward Finish of OSP Spec
BANNOCKBURN, IL – An IPC task group is making progress on a new standard for organic surface protectants for printed circuit boards.

The committee is developing a series of test methods to enhance the assessment of an OSP’s ability to meet performance requirements of high-temperature soldering. The latest document is in draft form and should be ready for a membership vote by spring, the task group chairman said.

IPC-4555, Performance Specification for High Temperature Organic Solderability Preservatives (OSP) for Printed Boards, is a reboot of an effort begun more than a decade ago. In 2008, the task group attempted a similar standard, but was undone when a key OEM disputed the results of the solderability data. Unable to reach consensus, the task group disbanded the effort.

Last year, a new task group made up of major companies, including Continental, Bosch, Raytheon, TTM and Sanmina, embarked on a new draft. IPC tapped Michael Carano, vice president of technology and business development at RBP Chemical, to chair the task group.

OSPs are commonplace, which makes the lack of a performance standard notable. According to Carano, 62% of world’s boards use OSPs. “They are in medical devices, tablets, airbags, engine controls, and telecom,” he said, and used in combination with other final finishes in advanced packages.

The goal is to develop performance specifications for high-temperature OSPs, defined as capable of withstanding up to two IR reflows in conjunction with tin-silver-copper (SAC) or tin-bismuth (SnBi) alloys at a peak temperature of 245°-250°C and showing the same wetting balance results at three reflows as zero, with a maximum 20% drop.

The new spec calls for all classes of high-temperature OSP to have a minimum shelf-life of 12 months when left in original packaging and following recommendations of IPC-1601. Solderability must meet J-STD-003 category B. SIR will be measured per IPC-TM-650, Method, GR78-Core, and electrolytic corrosion per IPC-TM-650, Method

Multiple round-robin studies were conducted to generate data to support the requirements in the pending specification, the task group said.

The issue that killed the spec a decade ago has been resolved. Back then, the task group could not agree on the allowable range of coating thickness. According to Carano, the current task group has agreed that coating thickness is not a determinant in the performance of the coating. “We don’t need one big range,” he said. Instead, the thickness requirement will be per the chemical supplier data sheet.

“OSP is difficult to test through the coating because the coating is hard, and, depending on the pin probe, you might bend the probe,” Carano allowed. “Suppliers are working on ways around that.”

“The thicknesses of most finishes can be measured with XRF. OSP uses a spectrophotometer. Or, we can verify the empirical data from a focused ion beam. The task group compared thickness with spectrophotometer with FIB cut, and they correlated very well. It’s like accelerated corrosion testing.”

For rework, the task group notes multiple methods exist for removing the immersion gold deposit to evaluate for the presence of hyper-corrosion of the nickel. Cognizant of the possibility of false positives for hyper-corrosion, the committee is working on the release of a specific test method to cover appropriate methods for gold stripping, which will be incorporated as an amendment upon release. (MB)