Electrochemical Migration and Corrosion on PCB Assemblies

A board that works in the lab and fails in the field after a few months of humidity is usually the victim of an electrochemical process rather than a defective component. The damage is slow, invisible at first, and driven by the combination of moisture, voltage and contamination.

The Mechanism

An electrolyte forms on the surface when moisture condenses into a film that contains ionic contamination. The film does not have to be visible; a thin layer is enough to carry current between two conductors at different potentials.

Metal dissolves at the anode, migrates through the film, and deposits at the cathode. The deposit grows into a dendrite that eventually bridges the gap and shorts the circuit.

Contamination Sources

The contamination is usually left by the process. Flux residue, plating salts, etching chemicals and even the salts transferred from bare hands all provide the ions that make the surface conductive.

Cleaning removes most of it, but only if the cleaning reaches the surface. A no-clean flux that is acceptable in a dry environment is not the same as a clean surface, and the difference appears only at high humidity. Our conformal coating notes describe how the surface must be prepared.

Where the Risk Is Highest

The risk rises where the voltage is high, the spacing is small and the surface is not protected. A fine pitch area with a bias between adjacent conductors is the classic case.

Condensation is more likely where the assembly sees thermal cycling, because a surface that cools faster than its surroundings collects moisture from the air. Enclosures that are not sealed are the usual locations. Our mixed signal notes describe the layout measures that reduce exposure in this area.

Design Measures

Increase the spacing between conductors that carry a sustained voltage difference. The gain is not linear, because the field that drives the migration falls as the distance rises.

Remove the bias where it is not needed. A guard structure or a ground between two biased conductors interrupts the path, and a slot in the board stops surface migration across a boundary completely. Our design notes cover the guard structures that suit a mixed signal board.

Coating and Encapsulation

A coating blocks the formation of the electrolyte film, provided it adheres to the surface and covers the conductors completely. A coating over a contaminated surface traps the contamination underneath.

The edges, the connector interfaces and the areas around the mounting hardware are where the coating is most often incomplete, and they are also where moisture collects.

Process Controls

The cleaning process should be specified by its measured result, which is the ionic contamination per unit area or the surface insulation resistance, not by the chemistry used.

Handling after cleaning should be controlled, because gloves that have been worn elsewhere transfer salts. The cleanliness requirement should be stated on the assembly drawing so that it is not treated as optional. Our fabrication notes notes list the attributes that should be stated.

Testing the Susceptibility

The test is a temperature and humidity bias test with an applied voltage while the insulation resistance is monitored. A drop in resistance over time is the signature of the mechanism starting.

The test should use the real layout rather than a standard coupon where the risk is understood, because the failure depends on the geometry of the conductors that are actually biased. Our test coupon notes describe how the standard structures relate to the board.

Process Control and Verification

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Verification and Records

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

FAQ

Does a no-clean process prevent migration? It reduces the residue but it does not guarantee a clean surface. Where humidity and bias coexist, the surface should be measured rather than assumed.

Is a thicker coating better? Only if it adheres and covers. A thick coating with a pinhole at a biased edge is worse than a thin one that is continuous, because it hides the defect.

What does gopcb provide for migration control? We provide cleaning processes specified by ionic contamination measurement, spacing and guard structures for biased conductors, coating and encapsulation coverage at the edges and interfaces, controlled handling after cleaning, and humidity bias testing with resistance monitoring.