Atmospheric pressure plasma for process improvement in wire bonding
Atmospheric pressure plasma is used in wire bonding as a selective fine cleaning step to remove contamination and residues from the contact surfaces. In general, strong bonds between wire and substrate can only be achieved on clean contact surfaces (bond pads). Thus, plasma treatment has a considerable effect on the quality and consequently the reliability of the entire component. Together with TPT Wire Bonder from Karlsfeld and the joint partner Axend Pte Ltd., relyon plasma has investigated the effects of cold atmospheric pressure plasma on wire bonding with gold wire.
Regensburg / Karlsfeld. Ideally, wire bonding is applied to clean metal surfaces (bond pads) of semiconductor components or carrier materials. In practice, however, there are often contaminations on the surface, which can result in non-stick on pad (NSOP) or so-called “lifts” (elevations of the bonds). Both cases lead to failures, downtimes and quality defects in production. For this reason, atmospheric pressure plasma is used in this process for selective fine cleaning prior to wire bonding, which avoids NSOPs as well as lifts and increases quality.
Investigation of the ball shear strength before and after plasma treatment
A test circuit board of ENEPIG surface (“Electroless Nickel Electroless Palladium Immersion Gold”) was treated on one side with the atmospheric pressure plasma high performance system plasmabrush® PB3 at a distance of 20 mm for 0.5 seconds with a compressed air plasma. Subsequently, 60 bond connections (30 balls and wedges) were applied to the untreated and plasma-treated test circuit board with the TPT HB16 Wedge & Ball Bonder. The plasma pre-treatment is intended to create a stronger connection between the bond pads and the wire.
In visual comparison, the ball bonds on the untreated and plasma treated ENEPIG surfaces seem to be firm and there are no problems with the Bond-Stick during wire bonding process. In order to measure the difference between the plasma pre-treated and the untreated ball bonds, a shear test is performed with the XYZTEC Condor Sigma Bond Tester, which shows a significant difference in quality between the two test series. An average shear strength of 60.89 gf is measured for the 30 samples without plasma pre-treatment before the ball bond is completely detached from the ENEPIG surface. Only a barely visible imprint of the sheared wire contact remains on the bond pad, a typical feature for the failure of the intermetallic bond between gold wire and ENEPIG surface.
In contrast, the bonds on the plasma pre-treated surfaces show a completely different behaviour in the shear tests with an average shear strength of 68.34 gf: The fracture pattern shows a sheared ball bond, leaving wire material on the surface. The shear strength of the bonds on the plasma pre-treated surface is therefore only limited by the shear strength of the wire itself. The results show that the intermetallic bond between the ENEPIG surface and the gold wire can be improved by plasma pretreatment to such an extent that it is significantly stronger than the strength of the gold wire material.
Wire pull test for wedge bonds before and after plasma treatment
For wedge bonds a wire pull test is performed with the XYZTEC Condor Sigma Bond Tester. Inconsistent test results occur with wedges applied without prior plasma treatment, proving that the bond is not ideal although it doesn’t seem critical. All in all, most wedges applied without plasma pre-treatment show so-called lifts, which indicate a weak intermetallic wedge bond.
In case of wedges applied to the plasma pre-treated surface, no failure in the wedge area occurs. All fracture patterns that appear are either within the wire itself or directly in front of the bond point (span or neck break), indicating that the bonds of both the ball and wedge are strong and therefore the wire only breaks in the area between the two bonds.
The results clearly show that surface treatment with atmospheric pressure plasma leads to significant improvements in both ball and wedge bonding, as clearly demonstrated in ball shear and wire pull tests.