0
\$\begingroup\$

i am designing the 4 layer pcb for the first time. I got typical 4 layer stackup - signal on the top, GND, PWR, signal on the bottom Most of the signals are routed on top, few are on bottom, and everything is connected to its respective GNDs (i got two gnds since there's USB isolator used in the design) and 4 different voltage levels (5V for Hall sensor, 3.3V digital for powering ADC of the STM32 and another 3.3VA for reference and etc) The question is more about manufacturing - since most of the signals are routed on top, the bottom has pretty few copper. I've red about the warping and mechanical PCBs (and especially multilayer PCBs) can be affected by in case of significant variation of copper density on opposite layers. It seems obvious to create huge poly on the bottom and connect it to GND, but i am a bit afraid of solderability - the board has 35um of copper thickness on all conductive layers.

Is it OK to create hatched poly on bottom and leave it floating?

P.S. all pads and vias are connected directly to the GND and PWRs, no thermal reliefs are used. The board is typical HASL, no conformal coating or such stuff Thanks in advance bottom side top side

\$\endgroup\$
0

1 Answer 1

Reset to default
0
\$\begingroup\$

You generally do not want to leave copper floating. It scatters radio waves and can make EMI worse. But copper balancing isn't something we typically have to be concerned about with modern PCBs.

Your concerns about solderability are kind of pointless. You already have a ground plane and power planes. Not having a plane on the fourth layer isn't going to save you. Besides, that's what thermal relief on pads is for.

Besides, it seems like a huge waste of a 4-layer board. You can't fit it on a two layer PCB? Or at least make it signal, GND, 5V, and 3.3V.

\$\endgroup\$
4
  • \$\begingroup\$ thanks you for your answer. The board is pretty dense, and it has limitations on mounting components on the bottom. In first version of the pcb, it had just 2 layers - bit there were no usb isolator IC and dc/dc required for isolation. i've attached some pics \$\endgroup\$
    – silkyre6xtenz
    Commented Sep 14, 2021 at 4:37
  • \$\begingroup\$ @silkyre6xtenz I would just make that area a power plane to make routing easier to be honest. \$\endgroup\$
    – DKNguyen
    Commented Sep 14, 2021 at 4:40
  • \$\begingroup\$ power planes are already in layer 3. \$\endgroup\$
    – silkyre6xtenz
    Commented Sep 14, 2021 at 4:41
  • \$\begingroup\$ @silkyre6xtenz You seem to have two voltages though. So one plane for each should make things easier. \$\endgroup\$
    – DKNguyen
    Commented Sep 14, 2021 at 4:42

Not the answer you're looking for? Browse other questions tagged or ask your own question.