Sporadic Voiding Effects in Ni - Sn Solder Joints: Understanding, Control, and Prevention

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An increasing number of microelectronics applications involve multiple reflows and/or extended operations at high temperatures where intermetallic growth in solder joints, particularly on Ni surfaces, becomes a concern. The slower growth rate of Ni3Sn4 intermetallic compound (IMC) compared to Cu has garnered attention, especially for packaging assemblies exposed often to high temperatures. However, one effect that has largely been overlooked in Ni solder joints is the evolution of large voids between the Ni3Sn4 IMC and the solder once the IMC thickness has grown to more than 5µm. These voids can lead to various issues in solder joints, such as compromised electrical and thermal conductivity and deteriorating reliability. The relatively limited studies of this matter show the voiding to be independent of the solder alloy and annealing temperature, report observations of voids on a rolled Ni foil as well as on both electrolytic and electroless Ni and conclude with the suggestion that the voiding as such is ‘inevitable’. We do, however, show that under specific set of treatment conditions the voiding propensity is virtually nonexistent in solder joins with high-purity Ni pads, whereas joints with electroplated or electroless Ni films feature substantially higher voiding propensity. This suggests that the void nucleation likely depends on impurities in the Ni associated with the plating process and is thus preventable either by pretreatment of this or by control of the reflow process.The work we present in this report reveals detailed differences in voiding trends between high-purity Ni joints and electroplated Ni. We demonstrate that voiding between Ni3Sn4 and solder in plated Ni is influenced by interactions between plating design and subsequent process parameters. Systematic studies have shown that voids only appear at higher reflow temperatures, regardless of the additives used in Ni plating. Our study explores the propensity for void formation by adjusting Ni electrodeposition parameters, deoxygenating, and deliberately altering the plating bath composition. In pursuit of the objective to identify the root cause of void formation, our mechanistic findings suggest that those impurities, likely in the form of Ni(OH)2 or NiOOH, along with factors such as Kirkendall vacancy formation, lead to enhanced void formation and growth. We estimated impurity content using void volume as an indicator and conducted composition analysis to examine the electrodeposited Ni for the presence of O2 and fractions of ionized Ni species. We also present thermogravimetry and X-ray photoelectron spectroscopy that confirm the presence of nickel oxo-hydroxide inclusions. At the end of this report, we also discuss different voiding mitigation and/or elimination approaches. The discussion revolves around strategies for either control of the Ni electrodeposition process to minimize impurity incorporation or post-deposition treatments to reactively eliminate already incorporated impurities in the Ni deposits.