Process windows for low-temperature Au wire bonding

Abstract

The process windows are presented for low-temperature Au wire bonding on Au/Ni/Cu bond pads of varying Au-layer thicknesses metallized on an organic FR-4 printed circuit board (PCB). Three different plating techniques were used to deposit the Au layers: electrolytic plating, immersion plating, and immersion plating followed by electrolytic plating. Wide ranges of wire bond force, bond power, and bond-pad temperature were used to identify the combination of these processing parameters that can produce good wire bonds, allowing the construction of process windows. The criterion for successful bonds is no peel off for all 20 wires tested. The wire pull strengths and wire deformation ratios are measured to evaluate the bond quality after a successful wire bond. Elemental and surface characterization techniques were used to evaluate the bond-pad surfaces and are correlated to wire bondability and wire pull strength. Based on the process windows along with the pull strength data, the bond-pad metallization and bonding conditions can be further optimized for improved wire bondability and product yields. The wire bondability of the electrolytic bond pad increased with Au-layer thickness. The bond pad with an Au-layer thickness of 0.7 µm displayed the highest bondability for all bonding conditions used. The bondability of immersion bond pads was comparable to electrolytic bond pads with a similar Au thickness. Although a high temperature was beneficial to wire bondability with a wide process window, it did not improve the bond quality as measured by wire pull strength.

Effect of plasma treatment of Au-Ni-Cu bond pads on process windows of Au wire bonding

“…This observation is in spite the fact that a high bond pad temperature allowed successful bonds to be made using a low bond power. As expected, the higher the bond power, the lower the pull strength, consistent with our previous results [3]. With increasing the bond power above the threshold level required to make successful bonds, the extra energy arising from the high bond power is consumed to deform the wire, weakening the neck and eventually causing the pull strength to decrease.…”

“…In recent years, techniques based on immersion Au plating over electroless Ni plating have been developed to replace the conventional expensive electrolytic Au plating [2]. In our previous paper [3], process windows were successfully established and the corresponding wire bond strengths were evaluated for several different Au-Ni bond pads that were produced using different plating methods. Important properties of the bonds pads, including Au layer thickness, surface morphology, surface roughness, and elemental composition were characterized to correlate these parameters with the wire bonding quality.…”

“…The PCB substrate and wire bond pads used in this paper were selected among the six different Au-Ni-Cu metallizations with different Au layer thicknesses studied in our previous work [3]. They were Samples 2, 4, and 5: the first two bond pads consisted of electrolytic Au plates with different Au layer thicknesses, and the last bond pad consisted of a very thin immersion Au layer, as summarized in Table I.…”

“…The thermosonic wire bonding process and the wire pull tests used were essentially the same as those applied in the previous [3]. Au wire wedge bonding was preformed on a wedge bonder (ASM model AB559A) equipped with a 61.2-kHz transducer (Uthle model) and a wedge tool (SPT model, FP308-TI-1820-L-CGM).…”

3D Multi Scale Modeling of Wire Bonding Induced Peeling in Cu/Low-k Interconnects: Application of an Energy Based Criteria and Correlations with Experiments

“…However, thermosonic bonding mechanism is complicated and the physical motion of the capillary and free air ball (FAB) is difficult to be modeled precisely. Some assumptions need to be made, particularly in the aspect of Au-Al intermetallic formation [11,12], the interaction of this with the bonding parameters [13,14] and its effects on pad peeling failure rate. Despite the fact that some technological barriers remain in analyzing the whole mechanism by simulation, some promising works have been published recently proposing various approaches for investigating, although often separately, the various factors of wire bonding physics [3,15,16,17].…”