no large impedance matches being introduced by line T's (a two-way split of a trace). For interfaces where multiple packages are receiving from the same line, (for example with a backplane configuration), the line must be split at some point to service all receivers. Some stubs and impedance mismatches are deemed to occur. Multipackage interfaces include BLVDS, DDR2/DDR3/DDR4 C/A bank, RS485 and CAN Bus. There are two main multipackage topologies: Tree and fly-by. Finding signal integrity problems Perform a layout extraction to get the parasitics associated with the layout. Usually worst-case parasitics and best-case parasitics are extracted and used in the simulations. Because of the distributed nature of many of the impairments, electromagnetic simulation is used for extraction. If the PCB or package already exists, the designer can also measure the impairment presented by the connection using high speed instrumentation such as a vector network analyzer. For example, IEEE P802.3ap Task Force uses measured S-parameters as test cases for proposed solutions to the problem of Ethernet over backplanes. Accurate noise modeling is a must. Create a list of expected noise events, including different types of noise, such as coupling and charge sharing. Input Output Buffer Information Specification (IBIS) or circuit models may be used to represent drivers and receivers. For each noise event, decide how to excite the circuit so that the noise event will occur. Create a SPICE (or another circuit simulator) netlist that represents the desired excitation. Run SPICE and record the results. Analyze the simulation results and decide whether any re-design is required. To analyze the results quite often a data eye is generated and a timing budget is calculated. An example video for generating a data eye can be found on YouTube: An Eye is Born. There are special purpose EDA tools that help the engineer perform all these steps on each signal in a design, pointing out problems or verifying the design is ready for manufacture. In selecting which tool is best for a particular task, one must consider characteristics of each such as capacity (how many nodes or elements), performance (simulation speed), accuracy (how good are the models), convergence (how good is the solver), capability (non-linear versus linear, frequency dependent versus frequency independent etc.), and ease of use. Fixing signal integrity problems An IC package or PCB designer removes signal integrity problems through these techniques: Placing a solid reference plane adjacent to the signal