LCD Display Inverter

“Electrically, this will provide good equipotential bonding for all ground planes. The exposed pad can even be replicated on the bottom layer (see Figure 1), which can be used as a decoupling thermal ground point and where the bottom-side heat sink is mounted.

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Today we are going to talk about how to use the exposed pad to make the best connection, hope it will be helpful for your next high speed design project.

01. Know the exposed pad

Exposed Pad (EPAD) is sometimes overlooked, but it is very important to get the most out of the signal chain and to dissipate heat from the device.

The exposed pad, which Analog Devices calls pin 0, is the pad underneath most devices today. It is an important connection through which all internal grounds of the chip are connected to the center point under the device.

If you’ve noticed, many converters and amplifiers today lack ground pins because of the exposed pad.

The key is to properly secure (ie solder) this pin to the PCB for a solid electrical and thermal connection. If this connection is not secure, confusion will occur, in other words, the design may not work.

02. How to achieve the best connection

There are three steps to optimal electrical and thermal connections with exposed pads –

1. Where possible, exposed pads should be replicated on each PCB layer

The purpose of this is to create a dense thermal connection to all grounds and ground planes for rapid heat dissipation. This step is relevant for high power devices and applications with high channel counts.

Electrically, this will provide good equipotential bonding for all ground planes. The exposed pad can even be replicated on the bottom layer (see Figure 1), which can be used as a decoupling thermal ground point and where the bottom-side heat sink is mounted.

Figure 1 Example of Exposed Pad Layout

2. Divide the exposed pad into multiple identical parts, like a chessboard

Use a silkscreen crossover grid on the open exposed pad, or use a solder mask. This step ensures a solid connection between the device and the PCB.

During reflow assembly, there is no way to determine how the solder paste will flow and ultimately connect the device to the PCB. Connections may exist, but are unevenly distributed. Probably only get one connection, and the connection is small, or worse, in the corner.

Dividing the exposed pad into smaller sections ensures that each area has a connection point for a more robust, evenly connected exposed pad (see Figures 2 and 3).

Figure 2 Example of improper EPAD placement

Figure 3 Example of a preferred EPAD layout

3. It should be ensured that all parts have vias connected to the ground

Areas are usually large enough to place multiple vias. Be sure to fill each via with solder paste or epoxy prior to assembly.

This step is very important to ensure that the exposed pad solder paste does not reflow into these via voids, preventing proper connections.