Lead-free Compatible OSPs: What Does This Really Mean?

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Introduction

The setting: Early 2008. Many fabricators and EMS providers are continuing to push programs for RoHs compliance. This, of course, requires electronic assemblies to be, among other things, free of lead-bearing materials. To adopt such a program, the surface finish, component finish and the solder used in assembly must be lead-free. While the questions surrounding these issues are many, one in particular appears more frequently: How do we know that a particular finish is truly lead-free soldering compatible? This particular question is being addressed by one of the IPC standards writing committees responsible for developing an industry standard specification of organic solderability preservatives (OSP).

The Nature of the Beast

The IPC committee made up of a cross-section of suppliers, PWB fabricators, OEMs and EMS companies is working to complete the development of IPC-4555 (standard for OSP). This committee is developing a protocol to test OSP coatings for lead-free (high-temperature) compatibility. The protocol requires that copper coupons coated with the OSP would be subjected to three lead-free reflows, then tested for wettability with an appropriate wetting balance instrument. It should be noted that after the coupons are processed through the OSP solution, the coupons shall be rinsed with DI water and dried at 105°C. Following the protocol of the ANSI-JSTD-003B, the samples shall be fluxed using the standard 0.5% activated flux and immersed in SAC305 at a temperature of 255°C for 10 seconds. Further:

• Ten samples per test shall be run.
• The mean and standard deviation of the maximum force achieved shall be documented.

Interestingly, the committee has also recommended that a second set of test coupons be processed under the same pretreatment conditions. This time, however, without the drying step. Again, the ANSI-JSTD-003B, wetting balance procedure to measure wettability, will be utilized.

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Editor's Note: This column originally appeared in the November 2014 issue of The PCB Magazine.

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