CAF: Design Rules and Process Limits
A conductive anodic filament is a path of copper that grows through a laminate along the interface between the resin and the glass, and it grows only when three conditions are present at once: a voltage, moisture and an ionic path. CAF resistance is therefore a design and material property rather than a manufacturing defect.
What the Filament Is
The filament grows from the anode along the glass fibres, following the resin to glass interface where the bond is weakest, and it deposits copper as it moves. When it reaches the cathode the resistance falls and the two conductors are shorted, and the current that flows then usually burns the path open again.
The failure is intermittent for that reason, and it appears after months or years in service rather than during manufacture. That combination of a late failure and an intermittent symptom is why the topic is treated with design rules rather than with inspection. A filament is only a few micrometres across when it is first detected and it widens as current passes through it, so the same failure can present as a small leakage current at first and as a burnt open circuit later.
The Conditions It Needs
The three conditions are a direct current bias between conductors, moisture absorbed into the laminate and an ionic species that can dissolve the copper. Remove any one of them and the filament does not grow, which is why the countermeasures are arranged around the three.
A board in a dry, sealed enclosure with no bias is a very different case from a board in a humid environment with a permanent voltage between two holes. The bias does not have to be high, because the filament follows the field rather than a breakdown, and a few volts is enough given time and moisture. The requirement is therefore written per product rather than as a general rule, and it is assessed together with the coating and the enclosure.
Where Growth Starts
The usual starting point is a drilled hole wall, because the drilling damages the resin and leaves a path along the fibres. The direction of the glass weave relative to the two holes matters as well, since a filament moves faster along a fibre bundle than across one.
Hole-to-hole and hole-to-trace gaps are therefore the geometries that matter, and the hole spacing is the figure that appears in the design rules. The weave direction is rarely visible on a production board, so the rules are written to be safe whichever way the glass runs rather than to depend on knowing it. Our hole copper notes describe how the wall is examined, which is also how a filament path is confirmed in a section.
Spacing Rules
A working rule for a general product is a minimum of about 0.5 mm between the walls of two plated holes, and a smaller figure where the material is specified as CAF resistant. The figure is measured between the conductive surfaces rather than between the hole centres, which is where the two are often confused.
The spacing also has to be maintained between a hole and an inner layer plane or trace, not only between holes. An inner layer that passes close to a drilled hole is exposed to the same interface, and the gap there is the one that is hardest to see on a drawing. A common error is to apply the rule between hole centres, which overstates the true conductive gap by one hole diameter and leaves the design short of what was intended.
Material and Stackup Choices
CAF resistant laminates use a resin chemistry and a glass finish that bond more strongly to the fibre, and some use a filler that blocks the interface path. Higher glass transition temperatures help indirectly by keeping the resin rigid and the bond intact at temperature.
The laminate material properties that matter are the moisture absorption and the CAF resistance rating, and both are quoted by the supplier. Where a product needs the resistance, the material is named in the stackup rather than left to the fabricator’s standard.
Manufacturing Factors
Drilling quality is the first factor, because a rough wall exposes more of the fibre interface, and desmear is the second, because a wall that is not properly prepared leaves a layer that traps moisture. Lamination parameters that leave the resin under-cured have the same effect.
Handling and storage also enter, because a board that has absorbed moisture before assembly carries it into the finished product. A panel held in an uncontrolled store releases that moisture during soldering and leaves both voids and a weakened resin to glass interface behind it. Our fabrication notes describe how the drilling and lamination conditions are recorded so that a change in either can be seen against the CAF resistance of the product.
How CAF Resistance Is Tested
The standard test places a coupon with closely spaced biased holes in a chamber at 85 degrees Celsius and 85 percent relative humidity, applies a direct voltage and monitors the insulation resistance for several hundred hours. The appearance of a low resistance path is the failure criterion.
The coupon geometry is the variable that matters most in interpreting the result, because spacing and weave direction are built into it. Two suppliers quoting a CAF rating from different coupons are not offering comparable figures, which is why the test condition is quoted with the rating.
Symptoms and Diagnosis in the Field
In the field the symptom is a resistance that falls, an intermittent short that clears when current flows, or a leakage current that rises with humidity. Confirming it requires a section through the two conductors, followed by an elemental analysis of the filament.
The analysis is what separates a CAF failure from a plating nodule, a solder ball or an ionic residue, and it is the reason the failure mode is not simply called a short. A resistance that recovers while the board dries is a strong indication, since the path itself remains but its conductivity depends on the moisture present. The evidence is taken from the failed board rather than from a coupon, because the coupon proves the material rather than the defect.
Design Rules and Qualification
The rules that prevent the problem are a minimum conductive spacing at holes, a material specification with a CAF rating, a bias and humidity assessment of the product and a coating where the environment demands it. They are written into the design guide and carried into the drawing as a spacing note.
Where a product is qualified by test, the coupon geometry, the spacing, the material and the storage condition are recorded together with the result. A qualification that quotes the outcome without the conditions cannot be applied to the next design, which is the most common way the work is wasted.
FAQ
Is CAF the same as dendrite growth on the surface? No. A dendrite grows over a surface, and a CAF grows through the laminate along the fibre interface. Both are copper migration under bias and moisture, and the countermeasures are different.
Does a conformal coating prevent CAF? It reduces the moisture that reaches the board, and it cannot stop a filament that has already begun inside the laminate. The coating helps the product and does not replace the spacing and material rules.
Can a board with a CAF failure be repaired? The filament path is inside the laminate, so there is nothing to clean or remove. The assembly is dispositioned rather than repaired, and the design is changed if the failure is a design outcome rather than a one-off.
