Understanding the Wire Bond Pull Test
By Lee Levine Presented by Ray Cathcart
? It may appear that the bonder/tool/wire is producing weak wire bonds ? It’s important to understand the data and the test conditions ? The bonds may be weak, or the specs may need to be corrected after reevaluating the process/test methods
? The maximum force that the pull test can detect is limited by the strength of the wire. ? The Heat Affected Zone (HAZ) above the ball should be the weakest part of a wirebond. ? Both the ball and the wedge bond welded areas are several times larger than the wire cross section. If the bond lifts or fractures at the interface the bond was defective. ? Preferred failure modes are neck, midspan, heel of second bond, in that order. ? The breaking force measured at the hook is not the same as the force exerted on the wire.
Pull Test Resolution of Forces
Simplified Equations for Pull Test
= F* M1
= F* M2
Pull Test R.O.F. Simplified
F1 = M1*F F2 = M2*F where F = Pull Test Measurement F1, F2 = Force on the wire at 1st or 2nd Bond
Low Pull Test Values?
? Do 1st and 2nd bond pull tests. If the values are close to the strength of the wire (80%) and the failure mode is good (no lifts) then the problem is loop geometry. ? If 1st or 2nd bond pull tests are weak, or if lifts occur, it’s a parameter or material problem. DOEs or plasma cleaning may help.
Low Loop Heights (TSOP Packages)
? M1 & M2 are > 1; Range 1-3 ? Pull Test readings are lower than the actual force in the wire ? Perception that bonds are weak MAY be incorrect
F1 = M1*F F2 = M2*F
High or Standard Loop Height
? M1 & M2 < 1 ? Range for M1 & M2 .5 to 1.0 ? Force in Wire is lower than the Test Force ? If the Wire Break Load is 10 grams then the Pull Test results are higher than 10 grams.
F1 = M1*F F2 = M2*F
? Mid span means that the angle of the wire at 1st and second bond is equal, resulting in the same force in each leg. ? It is NOT the middle of the loop length when the die is higher than 2nd bond. ? A 10-20% deviation from mid span doesn’t have a significant effect on the results.
Pull Test Excel Template
Pull Test Force [grams]
6 5 4
H d h lhp ts
10 210 6 0 7.5
Pull-Test Mid-Span e hl 40% 84%
3 2 1 0
5 10 15 20 25 30 35 40 45 50 55 65 70 75 80 85 90 95 100 Hook Location [% of d from 1st bond]
1st Bond Break Force 2nd Bond Break Force
Pull-Test Mid-Span Point
? Understand the geometry. ? The goal is strong bonds, not high numbers. ? Be honest, it’s easy to deceive yourself and the customer by pulling in a location that masks a weak bond. ? The pull test is limited to the strength of the wire. Shear strength determines the strength of the weld interface. ? Very low loops may have low pull strength values for strong welds because the geometry multiplier is >1. ? The Pull strength template is a useful tool to explain the variables of the pull test. Especially when there is a communications problem with a customer. Be sure you have the facts on your side.