High Bandwidth Memory Interface on Organic Substrate: Challenges to Electrical Design

Several designs of High-Bandwidth Memory (HBM) interface have been reported so far, all on silicon interposer. With the promise of organic interposer to become a lower-cost alternative, complete understanding of electrical performance of such interface is required. HBM interface connects SoC (System on Chip) and HBM dies that are placed next to each other on a common substrate; therefore, it is only a few millimeters long. With all eight channels routed, it counts close to 1700 signals that run on three layers, as limited by today's process technology. As the HBM die and, subsequently, the interface width is only 6 millimeters, these signals have to be routed with very high density. This results in high crosstalk. Specific to the organic interface, the short HMB signal lines, in combination with the driver complex-valued output impedance and the capacitive input of the receiver, creates under-dampened LC(R) tank resonators circuit with natural frequency of oscillation around 3-4 GHz. Even a weak crosstalk excitation from an adjacent aggressor signals causes a quiet victim signal to undergo resonant oscillation, or ringing. The coupling between adjacent signals even within the breakout area is severe enough to reduce noise margins to zero. The resistive loss in signal traces must be sufficient to dampen this ringing. We consider various technology options to increase the loss and compare their relative efficacy. Two distinct types of crosstalk are identified and their respective effect on HBM2 timing and noise margin is discussed. Effects of the meshed (a.k.a. perforated) reference plane on intra -and interlayer crosstalk is studied. With the trace cross-sectional dimensions at 2x2um and Nyquist frequency of 1GHz, the signals operate at the onset of skin effect, with per-unit-length resistance and inductance undergoing severe dispersion. This differs from signals routed as wider traces on an organic package, where the skin effect develops at much lower frequencies. It is also in sharp contrast to on-die signal routing, where RC is an adequate model of the signal interconnect.