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Uyemura has introduced TWX-40, an important technology development that allows gold deposits up to 8μin in ENIG and ENEPIG processing, in a single step.
The standard deposit for ENEPIG has been 1−2 µin of gold, a thickness now considered insufficient for certain OEM applications. Designers are now encountering applications demanding a minimum 3−5 µins gold in order to widen their operating window, add process flexibility, and assure the reliability of their wire bonding or pressure pin applications.
With standard immersion gold processes, heavier deposits on nickel or palladium in particular are problematic. To produce thicker deposits, shops are often forced to increase dwell time in the immersion gold, but with even minor porosity in the palladium layer, gold attacks the underlying nickel, leading to corrosion byproducts and poor reliability. The inevitable outcome of this nickel corrosion is two-fold: either wire bonds lift, or press pin connections become unreliable at the palladium-nickel interface.
One answer to this challenge has been to deposit autocatalytic gold over immersion gold—an additional step, requiring costly bath make-up for what is inevitably a limited bath life.
For ENIG/ ENEPIG processors serving a diverse group of customers and applications, TWX-40 is a better, proven solution with higher reliability. It saves dollars and processing time, and produces highly consistent results.
TWX-40 is a reduction-assisted immersion gold bath that deposits gold using both immersion and autocatalytic (electroless) reactions. The autocatalytic feature allows the gold to build without attacking the underlying electroless nickel layer. Deposit uniformity is independent of pad sizes and PCB surface geometry, or residual capacitance potential.
The demand for thicker gold is increasingly becoming a requirement for OEM specs, particularly those in the medical, aerospace and “high reliability/harsh environment” sectors. TWX-40 is the first chemistry to enable fabricators to achieve these substantially thicker deposits in one step, at low cost, while preventing nickel corrosion.
TWX-40 gold electrolyte plates directly on electroless palladium or nickel without intermediate activation. Plating speed is 0.12 μm/15 min. at 78°C; the bath exhibits excellent stability and fine geometry edge resolution.
TWX-40 has substantially widened the operating window for wire bonding by facilitating the deposition of thicker gold deposits, while maintaining the integrity of the underlying palladium and nickel. It is broadly recommended for shops working with specifications outside IPC’s gold thickness requirements. Best results are achieved when the bath is used with UIC’s Talon electroless palladium baths as the underlayer for PWBs and IC packages.
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