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February 2015 • The PCB Magazine 39 IPC PLATING SUB-COMMITTEE 4-14: SURFACE FINISH SPECIFICATIONS continues George Milad is the national accounts manager of technol- ogy at Uyemura International Corporation. he may be reached by clicking here. feature for soft membrane and steel dome contacts. Ad- ditional applications include use in low inser- tion force (LIF) and zero insertion force (ZIF) edge connectors and for press-fit applications. The electroless palladium layer forms a diffu- sion barrier that impedes nickel diffusion to the gold surface. The immersion gold protects the palladium layer from reacting with contami- nants prior to processing that might otherwise affect joining processes, such as wire bonding and soldering. ENEPIG has a minimum shelf life of 12 months under standard storage condi- tions. Thickness specifications are set to ensure the ability of the finish to meet all the attrib- uted functionality. The ENEPIG IPC-4556 Specification This is the last specification issued by the committee. The document produced is very comprehensive and includes a wealth of infor- mation from the RR studies that were conducted. The Appendix contains a documentation of these studies, each authored by the principle who conducted the testing. It also includes a section on the proper methods of equipment setup for a reliable measurement of very thin layers of metal deposits. The thickness specification for ENEPIG states: • Nickel: 3 to 6 µm [118.1 to 236.2 µin] at ± 4 sigma (standard deviations) from the mean. • Palladium: 0.05 to 0.15 µm [2 to 12 µin] at ± 4 sigma (standard deviations) from the mean. • Gold: minimum 0.025 µm [1.2 µin] at - 4 sigma (standard deviations) below the mean. No upper limit was set for IG. All measurements to be taken on a nominal pad size of 1.5 mm x 1.5 mm [0.060 in x 0.060 in] or equivalent area. Organic Solderabilty Preservative (OSP) IPC Specification (NONE) OSPs are organic coatings that form a com- plex organo-metallic complex with the copper surface of the PWB. This complex preserves the solderability of the copper surface through as- sembly. A wide variety of OSPs have evolved with the increasing complexity of the PWB. Initially, all that was required was a single thermal excur- sion for soldering leads into component holes, Then came surface mount that required at least two thermal excursions (one per side); add on top of that the need to hand solder an occa- sional rework. The biggest relevant evolution is lead-free assembly. LF assembly temperature at 260 ° C is approximately 35 ° C higher than eutec- tic soldering (225 ° C). The manufacturers of OSP have developed new OSPs to meet the market demands. These have a greater ability to with- stand increasing number of thermal excursions and higher temperature, as needed for LF. OSP products include benzotriazoles, imid- azoles, benzimidiazoles and phenyl benzimid- azoles. Some of the newer OSPs have additives occasionally referred to as "oxygen scavengers." These additives can stretch the performance window of the specific OSP. The more ther- mally robust coatings have significantly higher decomposition temperatures than the peak as- sembly reflow temperature. They require con- tact with appropriate flux and/or molten solder to penetrate the coating. They have longer shelf life, survive multiple reflow cycles and are more lead-free assembly compatible. After more than one year of struggling with a specification for OSP, no consensus was reached, and no specification was set forth. This was due to the wide assortment of organic products that were used for solderability preservation for the various applications, each with its own thick- ness recommended values. Acknowledgement To date, all committee activities have been voluntary and acknowledgement is in order for the members and equally important for their respective companies that allow for the time in- vested by their employees. PCB