How do I solder a PCB prototype board reliably?

Use flat ENIG pads (Ni 3–6 µm, Au 0.05–0.10 µm), keep ≥0.10 mm solder‑mask dams, size stencil and apertures to meet area‑ratio ≥0.66, and run a verified SAC305 or Sn63Pb37 profile (TAL 45–90 s; peaks 235–245°C or 205–215°C) while controlling vias and thermal reliefs.

Which surface finish matches my assembly?

ENIG provides planar pads suited to ≤0.5 mm pitch BGAs/QFNs; a typical stack is 3–6 µm electroless Ni with 0.05–0.10 µm immersion Au, which wets reliably with SAC305 or Sn63Pb37. Lead‑free HASL leaves a thicker, non‑uniform Sn layer; reserve it for through‑hole and ≥0.8 mm pitch where planarity matters less. If you expect multiple rework cycles on fine pitch, ENEPIG’s Au–Pd–Ni stack resists Ni corrosion and maintains bondability. Keep copper‑to‑mask clearances tight and avoid exposed Cu at pad edges to prevent dewetting.

Stencil, aperture, and solder mask geometry

For mask, use 50–75 µm expansion around pads and maintain ≥100 µm solder‑mask dam between adjacent pads; for 0402, 80–100 µm dams are common. Control paste volume with stencil thickness and aperture reductions: 0.10–0.12 mm stencils suit 0.5 mm pitch and 0402; 0.08–0.10 mm helps at 0.4 mm pitch and 0201. Use the area‑ratio criterion for clean release: for a rectangular aperture, area ratio = (L×W) / [2T(L+W)] and keep it ≥0.66. Example: a 1.0×0.25 mm aperture in a 0.10 mm stencil gives (0.25)/(0.2×1.25)=1.0, good; at 0.4 mm pitch a 0.8×0.20 mm aperture in 0.10 mm stencil yields 0.8, also acceptable. Reduce aperture area 5–10% for gull‑wing leads to limit heel beading. On thermal pads (QFN/DFN), use windowpane patterns targeting 50–70% paste coverage to vent flux and control voiding.

What solder paste and particle size should I pick?

Match paste flux to your cleaning plan: no‑clean for prototypes that will not be washed; water‑soluble (OA) only if you can clean thoroughly, as ionic residues under QFNs/BGAs cause leakage. Choose particle size by pitch: Type 3 for ≥0.5 mm and 0603; Type 4 for 0.4–0.5 mm and 0402; Type 5 for 0201/0.3 mm pitch. For hand soldering, SAC305 tips typically run 350–380°C; Sn63Pb37 320–350°C. Preheat boards with large copper to ~100–130°C before hand rework to reduce dwell and pad lift risk.

How do I set and verify a reflow profile?

Use board‑mounted thermocouples (small passive, fine‑pitch IC site, and a large copper anchor) to capture worst‑case thermal loads. Target ramp 1–3°C/s to soak at 150–180°C for 60–120 s, time‑above‑liquidus ~45–90 s, and cool ≤3°C/s to limit intermetallic growth and component stress. Aim for ≤10–15°C peak delta across the PCB.

• SAC305: liquidus ~217°C; set 235–245°C peak for mixed‑mass builds to ensure complete wetting without overstressing packages

• Sn63Pb37: liquidus ~183°C; set 205–215°C peak to avoid voiding and spattering while achieving full reflow

Match any rework profile to the original alloy and keep total heat cycles per pad low to preserve adhesion.

Vias near pads: tenting or via‑in‑pad?

If a via is near an SMT pad, tent or mask‑plug it to prevent solder wicking. For fine‑pitch BGAs, via‑in‑pad should be filled and capped; unfilled via‑in‑pad risks voids and opens. On early prototypes without via filling, keep vias at least one pad diameter from the pad edge, or tent from the solder side. For QFN center pads, either fill microvias or tent and use windowpane paste to avoid drain‑out.

Large copper and through‑hole heat management

Solid planes sink heat and cause insufficient wetting. Add thermal reliefs on through‑hole and pad connections: four spokes, each ~0.30–0.40 mm wide, balance heat without compromising current. For through‑holes, target finished hole 0.20–0.30 mm over pin diameter to promote capillary action and barrel fill without excessive dwell. When hand soldering, heat from the high‑mass side and feed solder from the opposite side to draw alloy through the barrel; limit each dwell to ~3–5 s and allow cooling between attempts.

How should I inspect prototype joints?

Inspect under 5–10× magnification for full wetting, toe/heel fillet geometry, and bridges. X‑ray BGAs/QFNs when available; otherwise, use boundary scan or functional tests for coverage. Reject dull, grainy fillets, icicles, void clusters on thermal pads, and any pad cratering or lifted mask.

Build and compliance options at HyperPCB

HyperPCB can build and inspect boards to IPC Class 2 or Class 3 acceptance criteria, and supports RoHS and REACH for hazardous substance control. Our ISO 9001:2015 certified quality management system (certificate no. CN-2023-Q1156, issued by SGS, valid until June 2027) underpins documentation and traceability. Quotes are returned within 24 hours and DFM feedback within 1 business day, so finish, mask, and stencil decisions can be locked before assembly.