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46 PCB007 MAGAZINE I MARCH 2021 Two thicknesses of gold, 0.05 µm (2.0 µins) and 0.2 µm (8.0 µins) were bonded using a 0.25 mm gold wire at three different bonding forces (25, 50 and 75g force). Ultrasonic power (mW) and time (sec) were held constant. e wires were then pulled and the break point in the wire was recorded. Bond lis (E) and brakes at the heel of the bond (D) indicated weak or non-reliable bonds. Break points (B and C) were indicative of a reliable bond. e data shows that both thicknesses could produce robust joints. However, the thinner (0.05 mm) gold required a higher gram force to form a reliable bond. e thicker gold (0.2 µm) produced reliable bonds at lower gram force, opening the operating window for the bond- ing parameter. For gold wire bonding applications, design- ers prefer to specify a gold thickness of 3–5 mins, which is beyond the capability of im- mersion gold. Increasing dwell time in an im- mersion gold bath is the most common cause of nickel corrosion. RAIG Applications RAIG gold is an immersion gold replace- ment for ENIG, ENEPIG and EPIG. It meets the design requirement for thicker gold with no nickel corrosion, in a single plating step. ENIG ENIG is a common gold surface finish that is solderable and aluminum wire bondable. It is also used as a contacting surface. ENIG is non-electrolytic and does not require bussing or connectivity during plating. It is ideal for SMT pads. e choice of chemicals and the conditions of plating are important to avoid corrosion of the nickel. Nickel corrosion, also referred to as "black pad," occurs in the gold bath. Immersion gold deposition is, in es- sence, a corrosion or displacement reaction. If conditions are not controlled, nickel may go into solution at the expense of hydrogen ions without gold deposition, creating nick- el corrosion. Under controlled plating conditions, nickel corrosion does not occur. It can occur, howev- er, if the nickel deposit is uneven, the gold bath pH is too low, the gold concentration is below spec or the dwell time in the bath is too long. Such conditions are usually avoidable, except when the design requires thicker immersion gold (in excess of 2.5 µins). Some manufacturers attempt to achieve this thickness by extending the dwell time in the gold bath, however, immersion gold baths are not designed to deposit more than 2.5 µins. If higher gold (3–5 µins) is a design requirement, another gold deposition system must be used. Two alternatives are electroless gold and reduction-assisted immersion gold (RAIG). Electroless gold is not common at PWB manufacturing sites. Electroless gold requires an immersion gold strike prior to electro- less deposition. Adding another gold bath is a costly proposition. RAIG gold is a single bath that can deposit 3–5 µins of gold without corrosion. ENEPIG is finish is gaining a lot of traction. EN- EPIG forms the most reliable solder joint with lead-free solder and is also a gold-wire-bond- able surface. In a previous column, I discussed the possibility of nickel corrosion beneath the palladium layer under certain condition, one being extended dwell time in an immersion gold bath to achieve higher gold thickness. e use of RAIG gold allows for a thicker deposit without nickel corrosion. Increasing dwell time in an immersion gold bath is the most common cause of nickel corrosion.