Semiconductor layout with different trace sizes
It is often said that beauty is in the eyes of the beholder. This statement could be easily extended to affirm that beauty can be found in almost anything, even things that may seem to be polar opposites. For example, the precision and uniformity of a marching band with everyone wearing identical attire is certainly a beautiful sight. On the other hand, can it be argued that a garden or greenhouse filled with various sized, multicolored flowers is any less beautiful? It can be argued then that both homogeneity and heterogeneity can be embodiments of the best qualities of the same thing.
This duality also exists for the routing of circuit boards. In some cases, the use of uniform trace sizes or copper weights that make it easier to determine the controlled impedance. However, there are other situations where varying current requirements make a single copper weight impossible, or at least impractical to implement. Evaluation boards [LINK to: “Tempo Blog | Semiconductor | PCB Evaluation vs Development Board: What is the Difference?”] are an example of when heterogeneous routing yields the best PCB layout as designing traces for the varying current in semiconductors is a primary objective. Let’s take a look at these boards and how to best route them for their intended function.
Semiconductor Board Current Levels
The development of evaluation boards can rival the most difficult or complex circuit board processes. One reason for this is the design intent is often not well-defined for a specific component or set of outputs as most boards are. Instead, the objective is to “evaluate” various design options to ascertain the best option for an application or to develop a multi-purpose board design that can be utilized for several different types of applications. Therefore, the PCB layout for the semiconductor, which is typically the primary object of analysis, may include various PCB layout options. As an example, the schematic below is of a general-purpose controller for a Brushless DC (BLDC) or Permanent Magnet Synchronous Motor (PMSM) controller with an output of up to 800W.
Semiconductor layout with different trace sizes1
1 Image is from NXP at https://www.nxp.com/design/designs/s32k142-24-v-motor-control-reference-design:S32K142-MC24.Accessed on 4/26/20.
The semiconductor shown is the NXP S32K142; although, another MCU can be used. As illustrated, the types of inputs and outputs require that varying current traces, which may include the list below, be available.
Current Trace Types for Semiconductor Evaluation Boards
- Power distribution network (PDN)
- CAN bus
- Sensor inputs
- Ground connections
- Multiplexer signals
- Other control signals
The current weight requirements for the above signals will range from nominal (1 oz) to high (3+ oz) depending on the application. Now, let’s see how to best design the board to accommodate this type of variance.
Optimal Routing of Current on Semiconductor Boards
Just as for other PCBAs, you should seek to optimize your design by following the best PCB layout and stackup tips. However, different current requirements present challenges that must be addressed specifically, by following guidelines as listed below.
Guidelines for Optimal Routing of the Varying Current in Semiconductors
- If possible, use wider traces for higher currents
Generally, you will have some flexibility in spacing traces for evaluation boards; therefore, it may be possible to use a single copper weight and wider traces to accommodate high currents.
- If not, select the highest copper weight for surface and internal layers
If selecting a single copper weight is not possible, then you should opt for the greatest weight required as excess copper can be etched or trimmed for smaller traces.
- Increase the number of vias and distribute traces
For high currents that must be run through the board to other layers, it can be advantageous to distribute current among multiple vias.
|Tempo‘s Custom Demo and Evaluation Board PCBA Manufacturing Service
Designing the PCB layout and stackup to accommodate the varying current in semiconductors for evaluation boards can be challenging. However, by following the guidelines above, they can be addressed successfully. Tempo Automation, the leading prototype and low-volume PCBA manufacturer in the industry, employs advanced equipment and custom smart manufacturing techniques to ensure that your boards are high-quality and built fast to enable you to spend more time on evaluations.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the varying current in semiconductor requirements for evaluation boards, contact us.