The present invention generally relates to the testing of IC devices such as packaged semiconductor chips (also referred to as packaged dies), and more particularly relates to device testers configured to conform to the shape of integrated circuit (IC) devices under test (DUTs).
Conventional integrated circuit devices include a die, incorporating the IC, attached a substrate. The die is bonded electrically (e.g. solder) and physically (e.g. epoxy) to the top of the substrate, at an elevated temperature sufficient to melt solder and to cure the epoxy.
Initially, before they are bonded to each other, both the substrate and the die are flat. However as illustrated by the simplified and exaggerated cross-sectional view (not to scale) of FIG. 12A, during the cooling process after heated bonding process, device 1280A becomes slightly curved (slightly bowed like the top of a mushroom) because of a mismatch of expansion and contraction coefficients of the die and the substrate.
The curvature of the device at room temperature (after cooling) should not be a problem because during the assembly of the device to a motherboard, at the elevated reflow temperature (sufficient to melt solder paste) inside a surface mount technology (SMT) reflow oven, the reheated device should become substantially flat again, thereby ensuring satisfactory electrical bonds between the device pads and the motherboard contacts.
Ideally, the curvature of the device should be preserved prior to assembly to the motherboard. However, these devices need to be tested for proper functionality at different temperatures prior to assembly to the motherboard. Typical device testers are designed with the assumption that the devices under test (DUTs) are flat. As a consequence, the flat profiles of the pedestal, the substrate pusher and the test socket result in undue pressure being exerted on the curved DUT, especially on the die, during testing.
This undue pressure problem is exacerbated by the existence of other components, in addition to the die, on the same substrate. So in a typical tester, the pedestal and the substrate pusher only contact the die and the perimeter of the substrate, respectively, leaving the remaining surface of the substrate, where the other components reside, unsupported.
As a result, after testing the surface of the device is somewhat flattened due to the undue pressure from the pedestal and the pusher, and often uneven, due to the uneven pressure between the supported and unsupported surfaces of the DUT. FIG. 12B is a simplified and exaggerated cross-section view (not to scale) of one such exemplary post-testing uneven, e.g., wavy, device 1280B.
Hence there is an urgent need for improved device tester designs that do not unduly deform the DUTs, especially for devices with thinner substrates needed for manufacturing compact portable electronic devices such as smart phones and tablets.