SMT soldering: when to use hotplate, convection reflow, or vapour phase

Fast choice by board and BOM

Use a hotplate when the PCB is ≤60×60 mm, copper is ≤1 oz, it’s single‑sided, and packages are up to 0.5 mm‑pitch QFN with no BGAs; choose profiled convection reflow for double‑sided work, BGAs/QFNs on mixed‑mass boards, or areas >60×60 mm; choose vapour phase when you need board‑level ΔT ≤5°C on high‑thermal‑mass builds or 2–4 oz copper while staying within J‑STD‑020 peak limits.

What really changes between hotplate, reflow, and vapour phase?

Heat transfer mode. A hotplate is predominantly conduction from the bottom; dense layouts often see top‑to‑bottom gradients of 20–40°C unless you add targeted top‑side hot air. Convection reflow uses forced hot air; with a tuned soak and good airflow distribution, ΔT across a populated board is typically held to ~10–15°C, which is important for BGAs. Vapour phase condenses an inert fluid on all exposed surfaces; the surface temperature is clamped near the fluid’s boiling point, and well‑designed bench systems often hold ΔT to ±2–5°C with very low oxidation.

When is a hotplate the right tool?

Small, single‑sided prototypes with modest copper and no bottom‑side shadows. Place a thermocouple on a large ground pad near the heaviest copper, preheat the board to 150–170°C, and use a narrow hot‑air nozzle to drive the local area past alloy liquidus. Expect more variability near big connectors or RF cans and higher voiding on QFNs unless you windowpane the paste. Double‑sided on a hotplate is only reliable if both sides use low‑mass parts and identical alloys; otherwise the first side will re‑melt or skew.

Convection reflow profiles that hold ΔT for BGAs

Start from IPC/JEDEC guidance (IPC‑7530, J‑STD‑020) and verify with 2–3 thermocouples on light and heavy zones. In small batch ovens, a soak profile usually beats straight ramp‑to‑peak on mixed‑mass boards. Keep board‑level ΔT within ~10°C at peak if any BGA is present.

Profile targets that actually work

• SAC305 (Sn96.5/Ag3.0/Cu0.5; liquidus ~217°C): ramp 0.5–2.0°C/s to 150–180°C soak for 60–120 s; time above liquidus 30–90 s; peak 235–245°C (do not exceed the most sensitive component’s J‑STD‑020 rating)

• Sn42Bi58 (liquidus ~138°C): ramp ≤1.5°C/s; peak 160–170°C; time above liquidus 30–60 s; useful for plastics and sensors that dislike ≥230°C but joints are more brittle, so avoid high‑strain locations

Is low‑cost vapour phase practical in the lab?

Yes—if you control condensation and ramp. Match the fluid to your alloy (≈230°C class for SAC305, ≈170°C for SnBi). Preheat the load to ~140–170°C to reduce condensation shock, limit effective ramp to ≤1.5–2.0°C/s via lift speed, and hold time above liquidus for ~30–60 s. A lid and condenser are non‑negotiable to minimize drag‑out. Verify with thermocouples on both a high‑mass ground pad and a light area. Vapour phase is not an oil bath; silicone oil introduces contamination and different wetting behavior.

Copper balance and stencil data that cut defects

Quantify copper balance in layout: keep local copper density within ±10–15% across any 10×10 mm window so reflow time constants are similar. Use thermal reliefs to pads on planes: 4 spokes, each 0.25–0.30 mm wide for 1 oz copper (increase to ~0.30–0.40 mm for 2 oz). For QFNs and other BTCs, windowpane paste to 50–65% pad coverage to reduce voids (ref. IPC‑7093); a 3×3 or 4×4 grid with 0.3–0.6 mm apertures works on 0.10–0.12 mm stencils. For 0402–QFN mixes, stencil thickness of 100–120 µm is a good starting point; consider step‑downs to 80–90 µm near 0.5 mm‑pitch parts to limit bridging. Keep solder mask dams ≥0.10 mm between 0402 pads to preserve paste separation. For double‑sided SAC305, do the heavier side first; second‑pass peak ≤245°C and TAL ≤90 s help the first side survive without glue.

How to instrument and verify?

Use at least two thermocouples: one taped or soldered to a large copper pad tied to ground (worst‑case thermal sink) and one on a light component region. Validate that your measured ramp, soak, time above liquidus, and peak meet the alloy and component J‑STD‑020 limits; adjust conveyor speed or dwell to keep ΔT within your BGA margin.

Quality and compliance we can prove

HyperPCB operates an ISO 9001:2015 QMS certified by SGS (certificate no. CN‑2023‑Q1156, valid until June 2027). We return quotes within 24 hours and DFM feedback within 1 business day. Boards can be built and inspected to IPC Class 2 or Class 3 acceptance criteria, with RoHS and REACH compliance for hazardous substances supported.