Gold Plating Porosity: 4 Causes of Corrosion Under Gold

Gold plating porosity is the network of tiny openings that runs through a gold deposit, and it is the reason a finish that looks perfect can corrode in service. Gold itself does not corrode, but the metal underneath it does, and every pore is a path for flux, moisture and contamination to reach that metal.

Porosity is not a defect that appears on the surface. It is measured by tests that make the underlying metal reveal itself, which is why it is so often found after a field failure rather than at final inspection.

What Pores in a Gold Deposit Are

A plated gold layer grows by nucleation and coalescence, and it never becomes perfectly continuous at the microscopic scale. The grain boundaries, the places where growth fronts met and the sites of co-deposited impurities all leave small channels through the layer.

The number and size of those channels depend on the thickness of the deposit, the condition of the bath and the topology of the surface it was plated onto. A deposit over a rough or contaminated surface has far more pores than one over a smooth, clean surface. A deposit that looks bright and uniform under a lamp can still be porous, because the openings are far smaller than the eye can resolve.

Why Thin Gold Is Usually Porous

Porosity falls steeply as plating thickness rises, because each additional layer of gold covers the openings in the layer below. The relationship is not linear, and it is the reason a minimum thickness is specified rather than a nominal one. The first few tenths of a micron do most of the covering, and the last few add relatively little.

That does not mean that thick gold is automatically safe. A thick deposit with a columnar structure can still be porous, and a deposit that has been plated at an excessive current density can be rough and porous even at a respectable thickness. The structure of the deposit therefore matters as much as the thickness the specification quotes.

The Nickel Layer Under the Gold

In an ENIG finish the metal beneath the gold is a nickel-phosphorus alloy, and its condition sets how much damage a pore can do. A nickel layer with the right phosphorus content resists corrosion well; one that has drifted in composition corrodes readily and the failure spreads under the gold.

A nickel layer that was passivated before gold deposition is the worst case, because the gold then sits on an oxide rather than on metal. Black pad is the extreme version of this mechanism, and the composition of the nickel bath is described in electroless nickel bath control. Phosphorus content, thickness and the absence of passivation together decide whether a pore becomes a pit or stays inert.

Bath Condition and Co-Deposition

Impurities in the gold bath co-deposit with the metal and disrupt the crystal structure. Copper, nickel, iron and organic breakdown products all raise porosity, and most of them arrive as carry-over from earlier steps rather than from the gold chemistry itself.

Current density and agitation have a similar effect through the structure of the deposit. Plating at a density above the qualified window produces a rough, columnar layer whose porosity is high however carefully the thickness is controlled. Agitation that is uneven across the panel has the same effect in the areas where solution exchange is poor.

Corrosion Mechanisms: Galvanic and Creep

Two mechanisms matter. In the first, moisture and an electrolyte reach the underlying metal through a pore and set up a small galvanic cell, because gold and nickel have different potentials. The nickel dissolves and the pore becomes a pit. How fast that happens depends on the electrolyte, which is why a pore that is harmless indoors can fail in a polluted atmosphere.

In the second, corrosion products migrate along the interface between the gold and the nickel, spreading sideways well beyond the original pore. That creep is what turns a microscopic opening into a visible stain or a lifted area of gold.

Testing: Nitric Acid, Electrolytic and Salt Spray

Porosity is usually measured by exposing the surface to a reagent that attacks the underlying metal and then counting the sites that appear. Nitric acid vapour and electrolytic methods are both used, and both need a defined exposure time and a defined magnification for counting. The reagent has to be applied for the same time and at the same temperature for the count to be comparable between lots.

Salt spray and mixed flowing gas tests describe field behaviour rather than pore count, and they take far longer. The choice of method should follow the environment the product will see, and the thickness that supports it is described in plating thickness distribution.

Process Choices That Reduce Porosity

The most reliable lever is thickness, applied within a bath that is clean and inside its window. Beyond that, a smooth substrate, a properly activated nickel surface and controlled current density all reduce the number of pores that remain open. Thickness is the most controllable of those levers, and it is also the one most often reduced to save cost.

A post-treatment can also help, because a thin protective layer on the gold seals some of the openings. The treatment has to be compatible with the soldering and bonding processes that follow, so it is a process decision rather than a finishing preference.

Symptoms Seen in the Field

The visible symptom is a stain, a discolouration or a dark spot on a contact or a solder pad, often after a period in a humid or polluted atmosphere. On fine-pitch features the corrosion can bridge to a neighbour and produce a leakage path. The site often sits where the deposit was thinnest, which is why thickness mapping across the panel is worth keeping.

Another symptom is poor solderability on pads that pass a thickness check. Where the gold is porous and the nickel beneath it has oxidised, the solder cannot wet the pad even though the measured gold thickness is inside specification.

Verification and Records

Verification combines thickness measurement with a porosity test on a sample. Because both are destructive or time consuming, they are usually run as qualification tests and as periodic checks rather than on every lot.

The record should carry the bath analysis, the thickness result, the porosity result and the test method used. Where a customer specifies a maximum pore count, the method and the magnification belong in the specification as well, and the gold bath itself is described in immersion gold bath control, with reference methods published by IPC. Results that are good should be kept as the baseline for comparison after any change of bath or supplier.

FAQ

Does gold plating porosity depend only on thickness? Thickness is the strongest single factor, but bath cleanliness, substrate finish and current density all change the pore count at the same thickness.

Why does corrosion appear away from a pore? Because corrosion products migrate along the interface between gold and the metal beneath, so the visible damage can sit well away from the opening that started it.

Can porosity be tested on a finished board? Some methods are non-destructive enough to use on samples, but most are destructive. The practical approach is to qualify the process and then control it through thickness and bath chemistry.