Probe cards are commonly used in the testing of integrated circuit devices, including memory chips. An exemplary probe card available from Kulicke and Soffa Industries Inc. of Gilbert, Ariz. has, on one side, an array of metal probes that are arranged to make contact with external electrical contacts, usually in the form of pads or bumps, on the chip being tested. The probes may be mounted, for example, within a probe head. Other probe cards have probes mounted directly to a substrate such as a space transformer. Probe cards also typically include has a printed circuit board (PCB) with traces that can be connected to a test circuit. The spacing of the traces on a PCB is typically substantially wider than the spacing of the pads on modern integrated circuits. In order to electrically connect the probes to the PCB traces, a probe card typically includes a space transformer. For example, in probe cards including a probe head, a space transformer (e.g., an MLO space transformer, an MLC space transformer, etc.) may be disposed between the probe head and the PCB. In probe cards where the probes are attached directly at one end to a substrate, the substrate may be the space transformer.
A space transformer is typically a generally planar structure. One side of the space transformer includes pads for contacting with the probes. The locations of the contact pads generally matches the positions of the tips of the probes. On its opposite face, the space transformer has contact pads to connect to the PCB (e.g., through a direct connection, through an interposer, etc). The contact pads for the PCB can be more widely spaced than the contact pads for the probes. For example, the PCB pads may have a pitch of 0.050″ (1.25 mm). In order to connect the two sets of contact pads, one form of space transformer consists of a stack of ceramic layers with metallized through holes (conductive vias) that extend through the layers and metallized tracks or traces between the layers. The vias and tracks or traces provide conductive paths from the probe pads to respective PCB pads. By following a path through and between the layers, the conductive paths can spread out from the probe pad spacing to the PCB pad spacing.
One form of existing space transformer includes a multi-layer ceramic in which metallization is applied to the layers of ceramic prior to firing. The layers are then stacked and the combination is fired. However, ceramic materials tend to shrink and distort when they are fired. Even with low temperature co-fired ceramic (LTCC) materials, which are typically fired by sintering at temperatures below 1,000° C., the distortion can be appreciable, resulting in distortion of the probe pads. This has become an increasing problem as the sizes of integrated circuits have decreased.
Thus, it would be desirable to provide an improved space transformer overcoming one or more deficiencies of conventional space transformers.