Solder Joint Corrosion in Humid and Polluted Environments

A solder joint is a piece of metal in a chemical environment, and that environment contains water, oxygen, flux residue and whatever the atmosphere brings with it. Corrosion is the result, and it appears as a change in resistance, a growing leakage path or a joint that fractures at the interface months after assembly. It is a slow failure, which is what makes it dangerous: the product passes every test at the factory and fails in the field.

How a Solder Joint Corrodes

Corrosion needs three things: a metal, an electrolyte and a potential difference. On a printed board the electrolyte is a film of moisture with dissolved ions, the metals are the solder, the copper and the surface finish, and the potential difference comes either from the different metals themselves or from an applied voltage.

The joint is usually the cathode rather than the anode, because solder is more noble than copper in most environments. The copper then becomes the anode and dissolves, so the result is a joint that still looks intact from the outside while the connection beneath it has been eaten away. The corrosion product also occupies more volume than the metal it replaced, so a joint can crack as the deposit grows inside a confined space such as a plated through hole.

The Role of Flux Residue

Flux residue is hygroscopic, which means it absorbs moisture from the air and holds it against the surface. That is exactly what an electrolyte needs, and it explains why a no clean process depends so heavily on the residue being benign rather than merely present.

A residue that is thin, hard and fully reacted is a very different proposition from a sticky, partly reacted layer. The first can be left in place for many products, while the second collects dust and moisture and becomes a permanent source of ionic contamination. The amount that can be tolerated depends on the circuit, since a digital board with low impedance nodes is far more forgiving than an analog front end measured in megohms.

Halides and Activation

Halides are the most effective activators in a flux and the most aggressive once they are left on the board, because the halide ion forms soluble complexes with the metal and keeps the reaction going. The classification of a flux states whether halides are present and at what level.

A halide activated flux that is cleaned properly carries little risk, while the same flux left unwashed on a high impedance circuit is a reliability problem waiting to appear. The choice of flux, the cleanliness requirement and the cleaning process have to be treated as one decision rather than three. Where encapsulation follows, the residue is sealed in place and the opportunity to clean it has gone, which is why the sequence of steps matters as much as the individual ones.

Humidity, Condensation and Temperature Cycling

Humidity supplies the water and temperature cycling supplies the condensation. A board that cools below the dew point of its environment will form a film of liquid water on its surfaces, and every surface, including the inside of a connector, will be wet.

Where the environment also contains sulphur, chlorine or salt, the film becomes a much better electrolyte, and the rate of attack rises accordingly. Industrial and coastal installations are therefore the hardest environments for an unprotected assembly, and they are the ones where coating is normally specified from the start, as the wider catalogue of solder defects and board failures shows.

Electrochemical Migration and Dendrites

When a voltage is present across a wet, ionically contaminated surface, metal can dissolve at the anode and deposit at the cathode. The deposit grows as a branched structure known as a dendrite, and it eventually bridges the gap between two conductors and produces a short circuit.

The process is fastest where the gap is small, the field is high and the contamination is concentrated, which is why fine pitch assemblies and high impedance analog circuits are the most vulnerable, and the visible sign is often no more than a faint discoloration recorded by automated optical inspection. The failure is often intermittent at first, which makes diagnosis at the bench difficult and slow. Before a short appears, the same process raises the leakage current between the affected conductors, and that rise is often the first measurable sign that something is wrong.

Contamination from Handling and Processes

Not all contamination comes from flux. Fingerprints deposit salts and oils, some cleaning agents leave residues when the rinse is inadequate, and plating chemistry trapped in a via or under a component can bleed out later and attack the surface.

Packaging and storage materials contribute as well, since some plastics release compounds that condense onto the board. The practical rule is that anything touching the assembly can leave something behind, and a cleanliness specification is what keeps the total within limits. Storage belongs to the same picture, because a board left in a humid room for a week before coating will have absorbed moisture that the coating then traps.

Protective Measures: Cleaning, Coating and Design

The first measure is cleanliness, achieved with a cleaning process matched to the flux and verified by measurement. The second is a conformal coating, which forms a barrier between the surface and the environment and is the standard answer for a humid or polluted installation.

Design contributes as well. Adequate spacing, the removal of exposed conductors where they are not needed, and the avoidance of traps where moisture can collect all reduce the risk without adding a process step to the line. Tented or plugged vias, and slots that drain rather than collect, are small design choices that make a large difference in a wet environment.

Testing for Corrosion Risk

The classic test is surface insulation resistance measured under a bias in a humid chamber, using a coupon with a defined pattern. The change in resistance over the test period shows whether the combination of flux, cleaning and coating is acceptable for the environment.

Ionic contamination testing measures the residue that can be extracted from the surface and reports it as an equivalent figure per unit area. Both tests are useful and they answer different questions: one measures behaviour, the other measures the amount of material present. Both should be run on the actual combination of flux, cleaning and coating that production will use, because a result obtained with a different combination does not transfer.

Prevention in the Process Specification

The specification should state the flux family, the cleaning requirement, the cleanliness limit, the coating type and the storage conditions. It should also state the handling rules, because gloves and controlled packaging are cheap compared with the cost of a field failure.

The specification is only useful if it is verified. A cleanliness test at a defined frequency and a coating inspection to confirm coverage are what turn a requirement into a controlled characteristic rather than an intention. The test methods themselves are described in the guide to solderability testing.

FAQ

Does no clean mean no residue? No, it means the residue has been qualified to remain on the board for the intended environment. A no clean flux used in a humid, polluted location may still need cleaning or coating.

Can corrosion be seen on a board? It often appears as a green or white deposit on copper or solder, but the most damaging forms begin underneath a joint or a component where nothing is visible. Optical inspection finds the obvious cases only.

Is coating a substitute for cleaning? No. A coating applied over ionic contamination traps the contamination against the surface, and the corrosion continues underneath with less oxygen available. Cleaning comes first.