USB cables find usage in a variety of electronic systems today. Numerous physical configurations and pin numbers are proof of their wide breadth of applications. One of the most popular is the USB-C, which was formerly referred to as USB Type C to differentiate it from types A and B. Identifiable by their symmetrical shape and dual layer pinout, these connectors may function quite differently depending on their application. Accordingly, and considering their prevalence in high-speed digital communications systems, understanding the ins and outs of USB-C is important for creating optimal PCB USB designs.
USB-C: Cables and Connectors
Power delivery in USB-C coexists with SuperSpeed digital data. To reach the potential of this interface, you must first understand how the cables and connectors can influence PCB layout requirements.
USB-C Cables - The fundamentals
There is no single standardized USB-C cable. Each cable is tailored for its specific application and electronically identified. High current applications require thicker gauge copper. Cable length also limits application due to power loss and signal attenuation at high data rates. The figure below illustrates these effects on a USB-C cable.
USB-C cable conductor arrangement
A USB-C cable has 8 coax or 4 twinax high speed data connections. The lines are differential, providing 4 lanes of digital signals with 2 in each direction. Each lane can carry up to 5Gbps or more of data. These high data rates require length-matching and impedance control. A legacy USB 2.0 interface is also available, and there are two wires for a sideband. The power wire VBUS can carry up to 3 amps of current and 20 volts.
Configuration occurs on the configuration channel. In earlier versions of the specification, there were two configuration wires. Newer versions removed one of these and provided VCONN instead, which can provide more power. There are two dedicated ground wires, and ground current also flows in the coaxial shields.
USB specifications are freely available from usb.org. The website includes many types of cables, including USB 2.0, HDMI, high-speed data, and power-only.
Micro-coax cross section
|Micro-coax cables, shown in the figure on the left, could easily be mistaken for ordinary wires. They have an insulating jacket, and inside is a braided shield that hides another insulated wire. This inner wire carries the data. Two matched micro-coax cables create a single high-speed differential data line. One coax carries the signal and the other carries a negative copy of the same signal.|
||Another implementation of the pair of data lines, shown on the left, is referred to as twinaxial (twinax) cabling where both the positive and negative copies of the signal share the same shield. It’s easier to match lengths in this type of wire, which may lead to increased implementation of this alternative.
The cables also include ground connections, buried components such as bypass capacitors, and cable ID circuits. All help to avoid device damage and reduce EMI emissions and EMC issues.
USB-C Connectors - PCBA Mounting Considerations
The connector design takes into account any EMI requirements for high-power and high-speed applications. It has a grounded shell, for example, and may have shell tabs that solder to the board which requires slots. These slots add complexity to PCB fabrication.
Connector types include surface mount, mixed surface-mount, through-hole, rugged, and mid-mounted.
As shown above, the mid-mounted connector sits inside a board cutout. Be sure to consider board panelization before using this type of connector.
The connector pins ride inside grooves on a wafer made of plastic. The pins have different lengths that enable sufficient power delivery before signal connection.
USB-C connector pins
Connector housings are available in ruggedized forms. A mixed through-hole and surface-mount connector can provide additional ruggedness while also improving the layout of two-layer designs. Even more rugged connectors are cast from aluminum and can be watertight.
By design, the strength of the cable is limited. The intent is for the cable to break before the connector. Stress on the cable connection typically results from bending. For small amounts of bending, the cable is undamaged as the graph below shows.
Range of USB-C connector bend
Up to a torque of 0.75 Newton meters (Nm), a USB-C cable can bend without damage to the cable or the connector. At some torque between 0.75 Nm and 2 Nm, the USB plug will weaken and eventually break with continued stress (as the curve in the graph above illustrates). To save the more expensive devices from breaking, the mechanical strength of the board can be simulated using finite element analysis. Structural integrity is just one of the many considerations that should guide your USB PCB layout when utilizing USB-C connectors.
DFM for High-Speed Digital PCBs
USB-C PCB Layout Guidelines
When creating PCBA layouts with a USB-C connector, using a paddle (or paddle card) can be beneficial.
The Paddle Card
The paddle is a prototyping board and reference design that establishes SuperSpeed data lines, the USB 2.0 interface, and provides power delivery. The symmetry of the top and bottom sides is not a coincidence; USB Type-C is a reversible plug with no preferred bottom or top. Power, grounds, and signals appear on both sides so that reversing the connector can be unwound by the circuitry on the board. The pinout below illustrates this symmetry.
USB-C connector pinout
As shown above, most of the pins are duplicated so that the connector works in both orientations. For SuperSpeed digital connections, the signal lanes are swapped. For applications where software can't untangle the bits, IC switching circuits can swap the signals again.
An older version of the specification featured two configuration pins, config1 and config2. Alternate operation modes, including USB 3.0/3.1, only utilize a single configuration pin and VCONN serves a variety of other functions such as providing more power.
USB-C Power Delivery
The power delivery process for the USB-C connector is shown below.
USB-C power delivery process
As the progression above indicates, when a device first powers up, it receives a 5V supply with 500mA of current. This is an increase in default power over previous USB generations. For devices that require more current or voltage, a combination of power delivery hardware and software negotiation processes can convince the host system to increase the voltage and current.
Available power is granted in increments of current at a fixed voltage, as shown by the graph below.
USB-C incremental power delivery
When the current reaches the limit of 3 amps, activation of a higher voltage occurs in addition to a reduction of the maximum current to provide the same amount of power.
Routing SuperSpeed + Traces on USB PC
SuperSpeed 5GHz differential signal pairs require matched length routing and impedance control. The maximum skew between the positive and negative signals in the cable is 100ps, and the printed circuit board should not add much to this. The FUSB340 is a 10Gbps SuperSpeed switch, and it has a typical skew of 6ps. Keeping the routing skew down to about 6ps requires lengths matched to around 1mm.
It’s also important to keep the impedance between 75 ohms and 105 ohms and trace length as short as possible as inexpensive printed circuit board materials are lossy at high frequencies. It’s also recommended to keep the traces on a single layer. For multiple layers, the length on each layer needs to be matched. This accounts for the differences in signal propagation velocity between layers. Vias routed to inner layers, especially on thick boards, create transmission line stubs that will harm signal integrity, but they can be back-drilled to remove the stub.
Through-hole connector pins should not stick through the bottom of the board, since this also creates a transmission line stub. The connector pins need to be about the same length as the thickness of the board. If the leads are too long, they will require manual lead trimming. And leads that aren’t long enough to reach the bottom side of the circuit board will have soldering problems.
Calculating the trace width and spacing needed for high-speed differential lines goes beyond simple rules of thumb or design curves. Signal integrity analysis tools include a 2D field solver that can help design the traces and spaces. Analysis is limited by the reality of board fabrication, which doesn't always conform to ideal geometries and dielectrics.
|Tempo's Custom PCB Manufacturing Service
It’s always a good idea to work with your contract manufacturer to find a stackup that meets your needs. Differential test coupons added to PCB panels enable bare-board manufacturers to verify if boards can meet the requirements. At Tempo Automation, the industry leader for fast, high-quality PCBA prototyping and on-demand production, we prioritize maintaining your design intent throughout the entire PCBA manufacturing process.
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.
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