The thermal problem
A heavy copper plane is an excellent heat spreader. That is why it is there: it carries current and pulls heat out of the components. During soldering, the same property works against the process — the plane pulls heat out of the joint.
On a 10 oz inner layer or a 20 oz outer layer, the copper mass is large enough that a hand iron or a short reflow profile cannot bring the pad to temperature. The failure looks like poor wetting, but it is a heat balance problem.
Pre-heat, from below
The standard answer is to raise the whole assembly closer to process temperature before the local heat is applied:
- Reflow: a longer soak and a slower ramp, verified with a profile measured on the board rather than read from the oven display.
- Selective and hand soldering: a hot plate or bottom-side pre-heater, so the iron only has to add the last few tens of degrees.
- Wave soldering: adequate preheat and a pallet that does not insulate the joint from the wave.
The failure mode to avoid is overheating the laminate to compensate: too much top-side heat lifts the copper or blisters the board long before the joint is right.
Thermal relief and neck-downs
Design and assembly meet here. A pad connected to a plane through four narrow spokes heats faster than one connected by a full pour, and it is easier to solder without damaging the laminate. Where a pad cannot have relief — a high-current terminal, for example — the assembly process has to supply the heat instead.
The trade-off is deliberate: relief makes soldering easier and current capacity slightly lower; a solid connection carries more current and demands more heat.
Stencil, paste and profile
- Aperture area increases on pads tied to heavy copper, so the joint has enough paste to form a fillet.
- Aperture shape avoids long narrow prints across wide pads, which bridge rather than release.
- Paste type stays inside the qualified window; a paste chosen for fine pitch may not release well from a large aperture.
- Profile is set for the thermal mass, with a soak that lets the board catch up and a peak that stays inside component ratings.
Through-hole and connector joints
Press-fit and heavy through-hole terminals are where the heat problem is worst, because the barrel is surrounded by copper. Selective soldering with a defined preheat, or a wave with a long contact, is more reliable than a hand iron. For press-fit, the hole and pin tolerances matter more than the solder, since there is none.
Inspection
X-ray for the joints the copper hides, AOI for placement and visible fillets, and cross-sections on the coupon for hole-wall copper. A sample pull test on power terminals converts "looks fine" into a number, which is what a customer asking about current capacity actually needs.
Workmanship is judged to IPC-A-610. The board-level rules are on the heavy copper design guide, the process limits on the heavy copper PCB page, and the assembly gates in the inspection note.
