<p>The manufacturing of lithium-ion batteries requires numerous precise joining processes (e.g., welding) of the different components to ensure optimal performance and reliability. Ultrasonic spot welding (USW) is one such technology that is primarily used to join battery tabs to cell current collectors (foil-to-tab welds). Several different design and manufacturing considerations (such as surface preparation, material compatibility, and tooling) need to be optimized to ensure that a high-quality bond is achieved. Beginning in mid-2016, fielded Samsung Note7 devices experienced battery failures across cells from both cell suppliers. Some failures manifested as an energetic failure outcome known as thermal runaway, often resulting in a fire. For one supplier, the inadequate quality of the USW foil-to-tab welds was found to be a primary contributor to field failures. This paper discusses the theory and design/manufacturing considerations of USW in lithium-ion cells (such as surface preparation, material compatibility, and tooling), and the failure analysis performed on the Note7 to assess the role of USW in the root cause of the failures.</p>