I. Five Common Types of HDMI Connectors

HDMI signals do not require any D/A or A/D conversion before transmission, maintaining exceptional signal purity. When selecting components and routing your layout, it is essential to understand the five common connector types:

1. HDMI Type A
   The most widely used standard specification. It features 19 pins with a width of 13.90 mm and a thickness of 4.45 mm. Similar to DVI Single‑Link transmission, the vast majority of audiovisual equipment we encounter daily uses this interface.
2. HDMI Type B
   This type is quite rare and was primarily intended for early professional‑grade, ultra‑high‑resolution applications. It uses 29 pins with a width of 21 mm. Type B offers dual‑link bandwidth, equivalent to DVI Dual‑Link, providing twice the data transmission capacity of Type A.
3. HDMI Type C (Mini HDMI)
   Designed mainly for compact devices. It also has 19 pins but is significantly scaled down to a width of 10.42 mm and a thickness of 2.4 mm. Its small footprint makes it ideal for portable equipment like DSLR cameras and camcorders.
4. HDMI Type D (Micro HDMI)
   A further miniaturisation based on Mini HDMI. It retains the 19‑pin configuration with a width of just 6.4 mm and a thickness of 2.8 mm. Similar in size to the older Mini USB interface, it is commonly used in high‑end mobile peripherals.
5. HDMI Type E
   This interface is specifically designed for automotive entertainment systems. Built to withstand unstable in‑cabin environments, Type E features ruggedised designs with strong shock resistance, moisture protection, and high tolerance for extreme temperature fluctuations.

💡 Engineer's Tip: These HDMI interfaces are “not fully physically compatible” (e.g., Type A cannot plug into Type B). However, because Type A and Type C (Mini) differ only in physical dimensions while sharing identical pin definitions, they are perfectly compatible via passive adapter cables.

II. HDMI Pin Definitions and Hardware Handshake Principle

Taking the most common HDMI Type A as an example:

When a Source (e.g., a PC) and a Sink (e.g., a monitor) are connected via an HDMI cable, the hardware follows a clear handshake protocol:

1. First, pins 1 ‑ 17 and pin 19 make contact. Finally, pin 18 (+5V Power) connects.
2. Once the Sink receives the +5 V from the Source on pin 18, it pulls pin 19, the HPD (Hot Plug Detect) signal, to a logic high.
3. Upon detecting the HPD high signal, the Source knows the external device is connected and ready. It then initiates the DDC (Display Data Channel) to read the Sink’s E‑EDID (Enhanced Extended Display Identification Data) to obtain information such as supported resolutions and timings.
4. After initialisation, the high‑speed data channels are established. Whether the devices automatically switch to the HDMI A/V transceiving state is then handled by the software drivers of the respective devices.

III. HDMI PCB Layout Guidelines

To ensure the flawless transmission of high‑speed TMDS differential signals and prevent severe EMI issues, layout engineers must strictly adhere to the following guidelines:

1. Impedance and Skew Control
   The PCB characteristic impedance standard for differential pairs is 100 Ω (±10%). To prevent phase shift, the length mismatch among the four differential pairs (Inter‑pair skew for Data x3, Clock) must be controlled within 10 mil. The length mismatch between the two traces of a single differential pair (Intra‑pair skew) must be less than 5 mil. The spacing (Air gap) between the four differential pairs should be kept at 15 mil or more to minimise crosstalk.

2. Reference Ground and Vias
   High‑speed signals must be routed adjacent to a solid main GND plane, and crossing split planes is strictly prohibited. If layer transitions (vias) are necessary, Stitching Vias must be placed as close to the signal vias as possible to ensure an uninterrupted return path.

3. ESD Protection Component Placement
   ESD protection devices must be placed as physically close to the HDMI connector as possible. It is imperative to use small‑package pull‑up resistors and ESD components, because smaller pads introduce less parasitic capacitance and lower impedance degradation.

4. Eliminate Stubs
   Fine‑tuning matching resistors for signal lines should be placed near the receptacle. The traces should route directly through the pads of the ESD components and resistors to keep any resulting branch “Stubs” as short as possible.

IV. The Key to High Yield: Design for Manufacturability (DFM) Checks

A perfect layout is useless if the PCB fabrication house cannot manufacture it, or if the yield is unacceptably low. Before releasing your board for manufacturing, you must verify the following DFM items for HDMI interfaces:

1. Impedance Trace Process Margin
   During fabrication, the manufacturing tolerance for impedance traces is tightly controlled at ±10% (standard traces are usually ±20%). Therefore, your designed width for impedance traces should ideally be larger than the manufacturer’s “minimum process limit” to leave an adequate safety margin, preventing impedance failures due to over‑etching.

2. Solder Mask Openings and Solder Mask Dams
   The pin pitch of HDMI connectors is very dense. Whether using SMD or Through‑Hole (DIP) pins, absolutely no solder mask openings should be missed. More importantly, the “Solder Mask Dam” between adjacent pads must be greater than 4 mil. If the dam is too narrow, the manufacturer cannot print the green solder mask web between the pads, which highly increases the risk of solder bridging (short circuits) during the SMT reflow process.

3. Missing Slot Holes for DIP Pins
   This is a fatal error! HDMI DIP mounting shields or signal pins frequently utilise slotted holes. When exporting Gerbers or manufacturing files from CAD software, engineers sometimes miss these slots because they might be drawn on a separate Mechanical Layer instead of the Drill Layer. If the slot holes are omitted, the connectors simply will not fit into the bare boards, resulting in the entire batch being scrapped.