Bond Strength Tests: Wire Pull Test and Ball Shear Test


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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.



Fig. 1.  Photos of a wire pull tester stage (left) and a pull hook (right)


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.


Bond shear testing (BST) is another test method for assessing the strength of a ball bond. This test complements, but does not substitute, wire pull testing.  This is because of the existence of failure mechanisms wherein the bond exhibits a high bond shear strength but offers very little resistance to wire pull stresses.


A bond shear tester is needed to perform the bond shear test.  This equipment consists of a sample holder, a shearing arm with a chisel-shaped tool at the end, and an instrument for measuring the shear strength of the bond. 


Initially, the shearing tool is positioned beside the ball bond to be tested. The shearing arm then moves the tool horizontally against the ball, in effect pushing the ball off its bond pad.  The force needed to shear a ball off its pad, known as the bond shear force, is then measured by the ball shear tester. 


The shear force reading of a ball bond must be correlated with its ball diameter for proper assessment of its ball shear strength. On the other hand, the shear force reading of a wedge bond must be correlated with the tensile strength of the wire for proper assessment of its wedge shear strength.


The bond shear failure mode observed during bond shear testing must also be noted. Bond shear failure modes include the following: 1) bond lifting; 2) bond shearing; 3) cratering; 4) bonding surface lifting (separation of the bonding surface from its underlying substrate). Misplacement of the shearing tool produces invalid bond shear failures, the shear force readings of which must be excluded from analysis.


Modern bond strength testers (Fig. 2) incorporate both wire pull and bond shear testing capabilities.


Fig. 2.  Photos of Wire Pull/Bond Shear Testers



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See Also:  Wirebonding Bonding TheoryBond Lifting




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