PCB007 Magazine

PCB-Jun2018

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28 PCB007 MAGAZINE I JUNE 2018 So far, the systems under discussion have been established systems incorporating nickel as a diffusion barrier. Some more novel univer- sal final finish systems no longer include nick- el to achieve more technologically challeng- ing functions. An example is electroless pal- ladium/autocatalytic gold (EPAG). This novel finish was designed with autocatalytic gold as the process of preference. This process is gain- ing traction by meeting stringent requirements globally. However, most fabricators would prefer to handle an immersion process to circumnavi- gate the risks of costly plate-outs that can only be suppressed by continual cyanide dosing and typically higher gold concentrations in the bath. The conundrum is, as ever, performance versus handling—which is usually perceived as cost. The term 'reduction assisted' is being in- creasingly cited when referring to immersion gold baths in the electronics industry. The term 'reduction assisted' suggests that the pro- cess has autocatalytic characteristics. This type of electroless bath is justifiably regarded as a low-corrosion immersion gold bath as is there- fore considered as a potential solution for thick gold plating. This article will cover the following topics: • A method to establish the autocatalytic characteristics of electroless gold baths • An example of the beneficial impact of autocatalytic in a production relevant scenario • Basic assembly data Determining the Autocatalytic Properties of an Electroless Gold Bath Two established electroless gold baths, a re- duction-assisted immersion gold bath (RA-Au) and an electroless gold bath designed to be at least partially autocatalytic (SA-Au), were eval- uated regarding their respective autocatalytic characteristics. Platinum-sputtered quartz crystals were used as the working electrodes for a quartz mi- cro balance (QMB). The electrodes were plat- ed as follows: • Plated with nickel and palladium (SET 1) • Plated with electrolytic gold only (SET 2) Both sets of electrodes are required to give a numeric value for the autocatalytic character- istics in %. SET 1 simulated an ENEPIG finish and there- fore immersion process whilst SET 2 ensured that only an autocatalytic reaction was possi- ble. Both sets were plated for 12 minutes in a reduction-assisted immersion gold bath (RA- Au) and a semi-autocatalytic electroless gold bath (SA-Au). In the case of SET 1, the QMB generated a value that represents the total gold plated and the nickel taken into solution. The nickel tak- en into solution is an indication for the immer- sion process. To isolate the gold, a gold strip- ping process was carried out. The solute was analyzed for gold by ICP-MS (Figure 2). SET 1 can only verify whether the process is a partial displacement process or 100% au- tocatalytic. SET 2 coupons can only instigate Figure 2: A summary of the calculations and interpretation for SET 1.

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