Stitch Bond Process of Pd-Coated Cu Wire: Experimental and Numerical Studies of Process Parameters and Materials
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Cost reduction is the main driver in the recent transition to Cu wire bonding from predominate Au wire bonding. Other cost reduction in packaging comes from new developments in substrates and lead frames, for example, Pre-Plated Frames (PPF) and uPPF for QFP and QFN reduce the plating and material cost. However, 2nd bonds (stitch bonds) can be more challenging on some of the new leadframe types due to the rough surface finish and thin plating thickness. Pd-coated Cu (PCC) wire has been recently introduced to improve the wire bonding process with bare CU wire, mainly to improve reliability and enhance the stitch bond process. More fundamental studies are required to understand the influences of bonding parameters and bonding tools to improve stitch bondability. The stitch bond process of 0.7 mil diameter PCC wire on Au/Ni/Pd-plated quad flat-no lead (QFN) PPF substrate is investigated in this study. Two capillaries with the same geometry but different surface finishes are used to investigate the effect of capillary surface finish on the stitch bond process. The two capillary types are a polished finish type which is commonly used for Au wire bonding, and a granular finish capillary that has a much rougher surface finish. Process window between no stick on lead (NSOL) and short tail is compared. The effect of process parameters including bond force and table scrub amplitude is studied. The process window test results revealed that the granular capillary has larger process window and a lower chance of short tail occurrence. It has been shown that a higher scrub amplitude increases the chance of successful stitch bond formation. To further compare the capillary surface finishes, 3 sets of parameter settings with different bond force and scrub amplitude are tested. For all three parameter sets tested, the granular capillary showed better quality in bond strength. The granular capillary resulted in higher stitch pull strength compared to the polished type. A finite element model (FEM) of the process was developed to better understand the experimental observations. The amount of surface expansion (plastic deformation) of the wire at the wire and substrate interface was extracted from the model and attributed to the degree of adhesion (bonding). The model was used to confirm the experimental observation of adhesion (bonding) with different surface finish.