MC33772B's EPAD

[JakeJung]

On the bottom side of the MC33772B, there is a 5.4 mm × 5.4 mm exposed metal area.

However, the PCB design guide recommends a solder mask opening of only 3.7–3.8 mm, which suggests an EPAD size of about 3.7 mm.

In actual inspection, the exposed metal area on the IC bottom is 5.4 mm, which is larger than the recommended PCB opening.

According to the datasheet, the region from 3.7 mm (min) to 5.4 mm (max) is specified as a PCB routing keep-out area.

If 3.7 mm is the recommended EPAD size and the 3.7–5.4 mm region is defined as a PCB routing keep-out area, what is the reason that this whole region is designed as exposed metal on the bottom side of the package?

[guoweisun]

You can read more PCB design for exposed pad from page 12 below link file.

AN4388: Quad Flat Package (QFP) – Application Note

5.2 Solder stencil and solder paste

[JakeJung]

I already know your answer, but what I really want to understand is why the opened metal area on the bottom side of the MC33772 is 5.4 mm.

According to the application note, the solder opening area is 3.7 mm, but the device’s bottom-side metal area is 5.4 mm. Why is there a difference?

According to the datasheet, the 3.7 mm to 5.4 mm region is specified as a routing keep-out zone.

Given that, I would like to understand why this same 3.7–5.4 mm area is exposed as open metal on the bottom side of the device.