September 2023 Continued from previous page


tion indicating its tensile ulti- mate strength, which allows for the calculation of the theoretical pull strength based on the bond and loop geometry. During pull testing, the test


specimen is securely fastened to the base of the pull tester. A hook is positioned beneath the bond wire, and a pull force is applied until the wire fails under tensile load. The maximum tensile value obtained from this test is collect- ed and serves as an objective data point for analyzing the over- all strength and employing sta- tistical process methods.


Failure Modes Failure modes are also docu-


mented to ensure a comprehen- sive understanding of aspects like heel formation and wire strength. When joining metals, it is essential for the weld joint to be stronger than the components being joined. Therefore, in - stances where the wire lifts off the bond pad or the plating (if present) separates from the bond pad should not occur. The weakest element of the


interconnect should always be the wire’s heel formation, making heel break the most frequent failure mode. Once the product is released for production, pull test- ing becomes a crucial indicator of bond tool wear for manufacturing engineering. It is important to note that the overall pull strength of the bond loop is influenced by the loop geometry, not solely the tensile strength of the wire. The bond angle determines


the required pull force to reach the wire’s tensile strength. Conseq - uently, if the loop formation lacks tight control, the results of the pull test can be perplexing in some cases. Once the wire bond process has reached its full development stage, undergone thorough docu- mentation, and been released for manufacturing, the manufactur- ing engineering teams shoulder the responsibility of maintaining control over the process. It is impractical to test every


bond destructively, yet develop- ing confidence in the process necessitates testing. Therefore, process monitoring and control are typically achieved through the implementation of statistical process control (SPC) methods. A testing plan is devised, specify- ing the number of wire bonds to be pulled and sheared per mate- rial lot, and the collected data is recorded on a running chart. This approach to statistical


process control minimizes the extent of testing while ensuring product quality remains within the defined process parameters. Furthermore, it serves as an alert mechanism for the manu- facturing engineering team, noti- fying them if the process deviates


www.us - tech.com Destructive Wirebond Testing...


from control. Certain manufacturers rely


on Mil Std 883 as a reference for guidance on wire bond pull strength and testing. SMART Microsystems utilizes published


wire strength and analytical data to establish minimum wire bond tensile and shear strength. The company employs shear testing as the primary source of objective data for assessing wire bond weld


strength, while pull testing is employed to evaluate loop forma- tion, loop geometry, and the over- all health of the formation process. When products are ready for


production, rigorous in-process SPC shear and pull testing becomes indispensable. No sub- stitute exists for this meticulous testing approach to ensure prod- uct quality and process integrity. Contact: SMART


Wirebond (WB1 and WB2) SPC pull test data.


Microsystems, Ltd., 141 Innova - tion Drive, Elyria, OH 44035 % 440-366-4203 E-mail: info@smartmicrosystems.com Web: www.smartmicrosystems.com r


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