I’m not the expert on this, and I’ve read a couple of conflicting points to solder to, but I’ve got my B+ soldered to PP27 and B- soldered to a USB grounding solder point between PP27 and PP51 and I’ve had no problems with power. Here’s list of Testing points on a Raspberry Pi 3B and 3B+:
PP1 VCC +5V (from micro USB receptacle)
PP2 VCC +5V (from micro USB receptacle)
PP3 GND
PP4 GND
PP5 GND
PP6 GND
PP7 5V after polyfuse
PP8 3V3
PP9 1V8
PP10 Power LED
PP11
PP12
PP13 Status ACT LED
PP20 H5V (5V at HDMI connector)
PP21 RUN (reset signal)
PP22 Ethernet Activity LED
PP23 Ethernet Link
PP24 Composite Video
PP25 L Audio
PP26 R Audio
PP27 USB VCC
PP28
PP35 VCC +5V
PP36 USB D-
PP39 PP40
PP41 USB D+
PP42 USB D-
PP43 USB D+
PP44 USB D-
PP45 USB D+
PP46 USB D-
PP47 USB D+
PP48 GND
PP49 GND
PP50 GND
PP51 GND
PP52 Ethernet TX+ ?
PP53 Ethernet 3v3 ?
PP54 Ethernet RX+ ?
PP55 Ethernet TX- ?
PP56 Ethernet RX- ?
PP57 Ethernet 3v3 ?
PP58
PP59
PP60