Hot part rise and via count reported at the first tab-net pad, not the exposed pad

Overview Files Skills Discover Packages Releases Dependencies Issues3 Breadcrumbs Pull Requests Activity

Closed general

John Lauer · 25d ago ·closed by John Lauer

What happens

For a hot part whose tab net has several pads (an HTSSOP-28 with an exposed pad: GND on pins 6, 22 and the exposed pad 29), adom-fields analyze spreads the watts over all those pads by area (good), but reports the part's riseC, riseMovingC, vias and the tab-rise / thermal-vias issues at cells[0], the first tab-net pad in board order: here pin 6, 2.9 mm from the exposed pad.

So a board with a 2 x 5 via grid in the exposed pad gets vias: 3 (the vias within 3 mm of pin 6) and a thermal-vias issue asking for 12, and the reported rise is pin 6's, not the die pad's.

Where

src/fields.rs: chip_pos.push((reference.clone(), cells[0].0, cells[0].1, ...)) after pads is filtered by p.net == hot.tabNet.

Suggestion

Report at the largest tab-net pad (the exposed pad by area), or at the area-weighted centroid of the tab-net pads; count vias within the radius of every tab-net pad (or of the largest). Optionally a spec field hot.<ref>.tabPad to name it.

Seen with adom-fields 0.9.5 on a TMC2226 driver molecule (KiCad 10.0.5 board, footprint Package_SO:TSSOP-28-1EP_4.4x9.7mm_P0.65mm_EP2.85x6.7mm). Moving pad 29 first in the footprint file does not help: KiCad stores pads by number.

1 Reply

John Lauer · 21d ago

Fixed in adom-fields 0.9.8.

A hot part with a tab net is now read at its tab: the pad you name in hot.<ref>.tabPad (for your TMC2226, "29"), otherwise the largest pad on the tab net. The exposed pad now wins over the corner pins that share GND. riseC, riseMovingC and the tab-rise / thermal-vias issues all use that point.
vias counts the tab net's vias near every tab-net pad, including a footprint's own thermal-via holes. Your 2 x 5 grid in the exposed pad will count in full.
The watts are still spread over all tab-net pads by area, as before.

On a QFN-48 with an exposed pad (0.5 W in it), the part now reads +15.2 °C at the exposed pad (it read +10.4 °C at a corner pin before), with its 9 thermal vias counted (it read 0).

Update with adom-wiki pkg install adom/adom-fields, then check adom-fields --version shows 0.9.8. Thanks for the precise report.

John Lauer closed this · 21d ago