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72 PCB007 MAGAZINE I JUNE 2023 bond. Two thicknesses of gold—0.05 µm (2.0 µins) and 0.2 µm (8.0 µins)—were bonded using a 0.25 µm gold wire at three different bonding forces (25, 50, and 75g force). Ultra- sonic 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) were indicative of weak or non-reliable bonds. Break points (B and C) were indicative of a re- liable bond. e data shows that both thicknesses were capable of producing robust joints. However, the thinner (0.05 µm) gold required a higher gram force to make a reliable bond. e thicker gold (0.2 µm) produced reliable bonds at lower gram force, opening the operating window for the bonding parameter. For gold wire bonding applications, design- ers prefer to specify a gold thickness of 3–5 µins. is thickness is beyond the capability of an immersion gold. Increasing dwell time in an immersion gold bath is the leading cause of nickel corrosion. ENEPIG With RAIG ENEPIG forms the most reliable solder joint with lead-free solder and is a gold wire- bondable surface. It is presently specified in cases where both soldering and gold wire bonding are the chosen methods of connectivity. Immersion gold on palladium in EN- EPIG deposits is limited in thickness, due to closeness of the Au and Pd in the EMF series. e driving force of the reaction is much lower than the driv- ing force for the immersion of gold on a nickel surface, as in the case of ENIG. Achieving a thickness in excess of 1.2 µins of gold on Pd will require an ex- tended time in the immersion gold bath which, in turn, would increase the prob- ability of nickel corrosion under the pal- ladium layer. If nickel is corroded under the Pd, the wire bond strength will be compromised and this would lead to bond li failure at the nickel palladium interface. As ENEPIG gains market share, it has be- come evident that the gold wire bonding oper- ating parameters are successful in a relatively narrow range. During assembly, the wire bond- ing parameters (heat, ultrasound frequency, and gram force) must be closely monitored for successful bonds to occur. "Wire pull" stud- ies have shown that increasing the gold thick- ness would widen the window of operation and would statistically improve the yield and reliability of the of the bonds formed. Many designers now specify gold thickness in ex- cess of the IPC-4566A specified range for gold thickness. Nickel corrosion is mainly attributed to im- perfections in the palladium layer coupled with an extended dwell time in a corrosive (low gold concentration) immersion gold elec- trolyte. Under such conditions the immersion gold ions in solution will have access to the underlying nickel and will be reduced to gold metal. e gold metal will deposit on top of the palladium layer and the nickel will corrode. e ENEPIG data set in Figure 2 shows clearly that RAIG eliminated any possibility of Figure 2: ENEPIG data set showing RAIG eliminates any possibility of nickel corrosion and maintains excellent wettability during soldering.