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Large plane SMT routing questions (2 replies)

Dave.Bell@LMCO.com
1 month ago
Dave.Bell@LMCO.com 1 month ago

I am prototyping a small 4-layer board with an RJ-45 with POE wiring and a simple buck regulator for device power.

The regulator components will be mounted on the bottom side, and per design guides, require large areas of traces or planes to connect the components (SOIC-8, inductor, 2 large caps.) My plan is to implement these all as SMT, with bottom-layer plane interconnects, adding pads/vias to connect to the buried power and ground planes.

With ExpressPCB, what's the best way to accomplish the connections? If I try to use Rectangles, I am warned that they are covered by solder mask, so presumably, an SMT pad placed on a Rectangle does not connect to it. Correct?

If I use Filled Planes instead, the planes actively avoid connecting to SMT pads and routed traces. I can complete connections with traces, but have to insert (through-hole) pads to connect, and the Plane still routes around the traces, leaving gaps and hanging threads of plane.

Any experienced suggestions very much appreciated!!

Thank you,

Dave

ExpressPCB
1 month ago
ExpressPCB 1 month ago

Rectangles and Planes are covered with solder mask. However you can draw on the solder mask to expose the plated copper. To draw on one of these layers, it must first be displayed by checking View top solder mask layer or View bottom solder mask layer in the Options dialog box. The solder mask layers are displayed in reverse (solder mask is applied everywhere on the board, except where something is drawn).
For example, if trace segments or rectangles are used as the fingers of an edge connector, they must be un-masked to expose them. In this case you will want to place a large rectangle over the entire edge connector on the top and bottom solder mask layers to prevent the fingers from being covered.

Dave.Bell@LMCO.com
1 month ago
Dave.Bell@LMCO.com 1 month ago

Thanks for the response from ExpressPCB!

I want to be sure I understand it correctly. (I recall now reading this, in regards to edge connectors and similar.)

When I view the BSM, are the highlighted areas the gaps in solder mask coverage?

If so, I believe my problem is solved. I'm attaching a JPEG of two screenshots below.

The top portion is while viewing the BSM. The bottom is the "normal" view, of silkscreen, top and bottom layers of close to the same area, with device pads selected.

If I read it correctly, the light-colored areas of the BSM correspond to the bottom-layer pads. The darker areas are the bottom-layer rectangles I'm using for interconnects...

Does that look correct?

 

Thank you!

Dave

 

https://www.expresspcb.com/wp-content/uploads/2019/02/Solder-Masking.jpg
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