Wire Pull Testing (WPT), or bond pull testing, is one of several available time-zero tests for wire bond strength and quality. It consists of applying an upward force under the wire to be tested, effectively pulling the wire away from the die.
Wire pull testing requires a special equipment commonly referred to as a wire pull tester (or bond pull tester), which consists of two major parts: 1) a mechanism for applying the upward pulling force on the wire using a tool known as a pull hook; and 2) a calibrated instrument for measuring the force at which the wire eventually breaks. This breaking force is usually expressed in grams-force.
There exist many variants or test conditions for performing the WPT, but the most widely used test condition for conventional wirebonded devices is the double-bond wire pull test. This procedure consists of applying the pull hook under a wire that is attached at both ends (say, one end to a bond pad on the die, and the other end to the bonding finger or bonding post of the package). The pull hook is usually positioned at the highest point along the loop of the wire, and the pulling force is usually applied perpendicular to the die surface (vertically if the die surface is horizontal).
The wire pull tester measures the pulling force at which the wire or bond fails. The measured force is then recorded in grams-force. Aside from the bond strength reading, the operator must also record the bond failure mode. Failure mode in this context refers to one of the following: 1) first bond (ball bond) lifting; 2) neck break; 3) midspan wire break; 4) heel break; and 5) second bond (wedge bond) lifting. First or second bond lifting is unacceptable and should prompt the process owner to investigate why such a failure mode occurred.
