White residue on solder joints: flux, tin corrosion or moisture

White fuzz that sits on bare solder and nowhere else, on a board that spent years in a box but never powered up, is corrosion of the tin or hardened halide flux residue — not battery leakage. If no battery shares the box, that cause drops out first. The fix is mechanical: reflow the crusted joints, clean the whole board in 99% isopropyl alcohol, and put it away sealed with desiccant held at or below 40% RH. Storage decides the outcome, and the dew-point arithmetic below shows why. At 50% RH a joint only has to cool about 11 C (20 F) under the room before moisture films form on it. A cardboard box on a garage shelf sees that swing on any clear night. Below you get the deposit signatures that separate the three causes, the cleaning sequence, and the worked number that sets the storage target.

What the deposit tells you

Read the deposit against where it sits before the iron comes out of the drawer. Two minutes with a magnifier or a USB microscope puts most boards into one of these rows:

• Crust on exposed joints only, tracks under the green resist clean. Atmospheric corrosion of the tin. The resist blocks the air; the bare joints take the whole attack.
• Dull white film, sticky or waxy under magnification, on hand-worked joints only. Activated flux residue. It follows the iron, not the air.
• White crust with green or blue-green growth spreading from one point. Electrolyte or battery leakage. Find the source before you clean anything.
• White powder over gray-black, pitted joints. Corrosion that has eaten into the solder. Plan on reflow and part replacement, not just washing.
• Film under conformal coating or inside a component. Contamination from before the coating went on. Surface cleaning will never reach it.

Colour adds a second clue, and it costs nothing to use. Red or orange powder points at rust or clay film from the box itself; the red residue guide covers that case. White on bare metal is tin oxide or flux nearly every time, and where it sits beats how it looks: crusted joints beside clean resist-coated tracks is the airborne signature, not the mark of a spill.

The causes, most likely first

Atmospheric corrosion of the exposed tin

Tin oxidizes in ordinary air, and humid air carrying nitrates, sulfates or road-salt chlorides speeds the attack up sharply. The products sit on the joint as a white, faintly fuzzy crust. The lead in tin-lead solder dulls to dark gray alongside. That white-on-tin, dark-on-lead pairing is what decades of open storage in a cardboard box produces. The resist-coated tracks stay clean because the coating denies the air access, exactly the pattern your inspection should show.

Hardened halide flux residue

Activators in rosin-activated and some no-clean fluxes are halides, meaning chloride and bromide ions. Left on a tin-lead joint through humid storage, those ions react with the lead and build white lead salts. That reaction is the documented chemistry behind white patches on stored, hand-soldered work. Under magnification the deposit reads viscous and waxy, and it sits only where the iron touched. If the fuzz follows a flux-spatter pattern instead of covering every bare surface, score this cause level with corrosion.

Water that reached the board

Condensation during storage or a single wetting event leaves the same white crust plus tide lines and pitted traces. On a never-powered board the attack runs slow and chemical; on a powered one it turns electrolytic and eats traces fast. The moisture guide separates failed capacitors from water ingress, and it is the right next read when tide lines show up. A leaky cell adds white-green crust around its own contacts. Boards that share a case with a battery keep the two compartments apart, as the circuit board and battery layout guide sets out.

How to tell the causes apart

Three checks on your bench separate the causes without any lab:

• Swab test. Touch a crusted joint with a cotton swab wetted in 99% isopropyl alcohol. Flux dissolves and wipes away. Corrosion softens but scrubs off as gritty powder, leaving dull or pitted metal behind.
• Location test. Deposit on hand-worked joints alone points at flux. Deposit across every bare surface, joints and legs alike, points at the air.
• Legs and pads. Inspect component legs and pad rims under magnification. Corrosion pits metal; flux never does.

After cleaning, run the follow-up pass: pits in pads, dull grainy joints, and any open trace need rework rather than a second wash. The PCB corrosion diagnosis walkthrough covers the repair side once your board is clean.

The fix: rework, then clean

Wear nitrile gloves for this job: a corroded tin-lead board carries soluble lead salts. Isopropyl alcohol is flammable too, so keep the tray away from heaters and sparks.

1. Rework every crusted joint: pull the old solder off with a pump or braid, add fresh flux, and flow new solder. A joint that stays dull or grainy after reflow gets its leg re-tinned or the part replaced.
2. Scrub the whole board in 99% isopropyl alcohol with a soft brush. A 93% mix works; weaker ones leave more water behind and dry slower.
3. Rinse in fresh alcohol, or in deionized water when the salt loading was heavy. Keep tap water off the board, because it lays down a mineral film of its own.
4. Stand the board on edge for a full hour at room temperature before applying power, so trapped solvent under the components can escape.
5. Inspect again under magnification for pits, corroded-through traces, and joints that refuse to take shine.
6. Meter every suspect trace end to end. An open trace gets a jumper wire, not a second wash.

Worked example: dew point in a storage box at 20 C and 50% RH

Inputs. Storage air at 20 C (68 F) and 50% RH: ordinary indoor conditions, and an example assumption, so substitute readings from your own shelf and the arithmetic holds. The case is the first row of the symptoms list, a board in a cardboard box with free air exchange, so the trapped air tracks the room. The formula is the Magnus approximation with the standard psychrometric constants a = 17.62 and b = 243.12.

Rule. Moisture films form on any surface once that surface cools below the air's dew point. The approximation computes it in two steps: γ = ln(RH/100) + a·T/(b + T), then dew point = b·γ/(a - γ).

Arithmetic. ln(0.50) = -0.693. 17.62 × 20 / (243.12 + 20) = 352.4 / 263.1 = 1.339, so γ = -0.693 + 1.339 = 0.646. The dew point is 243.12 × 0.646 / (17.62 - 0.646) = 157.1 / 16.97 = 9.3 C (49 F).

Result. At 20 C and 50% RH, any surface at or below 9.3 C grows a moisture film, and bare solder is the first surface to show it.

Decision. The joints wet when the box cools 20 - 9.3 = 10.7 C, call it 11 C (20 F), under the daytime peak. At 40% RH the same formula gives γ = -0.916 + 1.339 = 0.423 and a dew point of 243.12 × 0.423 / 17.20 = 6.0 C (43 F), so the swing needed grows to 14 C (25 F). Hold the case at or below 40% RH and a clear night must drag the shelf down 14 C to wet a joint instead of 11 C. That 3 C of extra margin is the whole argument for the 40% target over 50%.

Keeping it from coming back

A sealed case stops rain and spray on its own merits. Condensation is a different mechanism, set by the dew point rather than the gasket, and a high ingress rating alone cannot stop it; the condensation guide walks through that mechanism. Storage then comes down to three moves:

• Seal it with desiccant, held at or below 40% RH. Corrosion handbooks put the critical humidity for most metals near 60% RH, the point where surface films go continuous and the attack accelerates; 40% leaves margin for the swings in the worked example. A 10 g sachet of silica gel adsorbs about 25% of its own weight in water at room conditions, roughly 2.5 g, enough to dry a storage case's trapped air several times over. Reactivate spent gel at 120 C (248 F) for 2 h, the reactivation point desiccant datasheets publish, and it goes back to work.
• Watch the indicator card monthly. It tells you the desiccant is spent before the crust returns, not after.
• Add a vapor-phase corrosion inhibitor for hostile rooms. In a shed, a garage or coastal air, a VCI emitter lays a molecular film over the bare metal and slows the attack; the vapor phase inhibitor guide covers placement and dose.

Swap the cardboard and styrofoam out for the sealed case when you store long-term. Cardboard is hygroscopic, and it re-wets the trapped air with every swing of the room.

When the storage needs re-specifying

Boards kept in sheds, garages, or any space without climate control end the cycle with a sealed stainless case. Our wall-mount stainless line runs from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D), each with a hinged gasketed door and a removable panel. Every case is rated NEMA 4X / IP66 by the manufacturer; not UL listed. It is delivered in 10 business days, counted from your order; for custom work, from your approval of the drawing. Size the case for the board, its tallest parts, and a desiccant sachet with room to swap. Browse the line at stainless wall-mount and HMI enclosures and set width, height, depth and cutouts to the job.

The decision, case by case:

• Climate-controlled room, box opened weekly: rework and clean, then a sealed case with a 10 g sachet and a monthly card check.
• Garage or shed: the same kit plus a VCI emitter, and conformal-coat the board if it powers up in place.
• Coastal or industrial air: 316L rather than 304, the VCI emitter, and the coating; chlorides in that air attack bare solder hardest.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions