Wire bond behavior during molding operations of electronic packages

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Abstract
This paper presents some modeling work on the behavior of wire bonds during a typical transfer molding operation. In order to fill up the small recesses in a production size mold, transfer pressures as high as 1.000 psi are commonly required. The large driving pressure is potentially harmful to the fragile wire bonds. Lit the worst case scenario, the highly viscous melt front may cause enough shearing action to lift off the wire bonds completely. More often, however, wire sweeping is reported as one of the major causes of shorts in devices where long interconnect wires are looped between bond pads. Several interdependent factors govern the extent of wire sweep in a mold cavity of arbitrary geometry. The dimensions of the cavity, for instance, dictate the velocity of the melt front profile with the corresponding flow stresses and shear rates. The location of the chip to be encapsulated similarly influences the plastic filing pattern. So does the configuration of the particular chip design (e.g. planar vs recessed lead frames). Naturally, the severity of the problem is also controlled by the nature of the wires used during the bonding. For example, wire sweep may be controlled to a certain degree by tailoring the wire modulus to offer resiliency and spring‐ back after the melt front has swept across, but prior to curing. Furthermore, a low drag coefficient would also be obtained if some measure of control on the shape of the wire (e.g. circular vs rectangular cross‐section) or the pro‐ file exposed (e.g. angle of orientation of the wires with respect to the flow) is exerted.