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32 PCB007 MAGAZINE I JUNE 2018 tistical difference between the performance of the electroless gold bath that has autocatalyt- ic characteristics (SA-Au) and the reduction-as- sisted immersion gold bath (RA-Au). This ob- servation is true independently of bath age and represented by MTOs. As previously mentioned, novel high-perfor- mance finishes do not include nickel as a dif- fusion barrier and employ autocatalytic elec- troless golds as a state-of-the-art solution. An example is electroless palladium/auto- catalytic gold (EPAG). To evaluate gold wire bonding per- formance, EPAG, employing a semi-au- tocatalytic electroless gold, was com- pared to an ENEPIG finish, employing a reduction-assisted immersion gold. In both cases 23 µm Tanaka gold wire with a breaking strain of 9.9 g was used. To maintain a level playing field the wire bondable gold finishes were plated at 100nm. The evaluation meth- od was cold ball pull testing (CBP). Figure 9 demonstrates that both gold finishes achieved the required pull strength of 4.95 g which is 50% of the breaking strain of the wire used for the test. This satisfies the pass criteria ac- cording to DVS 2811. Conclusion Existing electroless gold baths with autocatalytic characteristics are a drop- in solution to replace immersion gold baths in ENIG, ENEPIG, whilst being state of the art for the EPAG process. It has been demonstrated that a gold fi- nal finish with autocatalytic character- istics has no performance drawbacks but can be shown to have the bene- fit of reducing or eliminating corrosion when higher gold thicknesses are re- quired. It has also been shown that re- duction-assisted immersion golds do behave autocatalytically. For process sequences or more information regarding semi-autocat- alytic gold baths or the latest in the development of a fully autocatalyt- ic gold bath that can eliminate corrosion at thickness of >100 nm please contact the author. PCB007 Rick Nichols is global product manager, surface finishing, with Atotech Deutschland GmbH. Figure 9: A comparison of the pull strength results for RA-Au and SA-Au after ageing at 150°C for four hours.

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