Resin Bleed on PCB Surfaces: Causes and Control

Resin bleed is a thin film of resin that spreads from the laminate onto the copper surface during drilling, lamination or a thermal process. It is rarely visible without magnification, and it is often present over an area much larger than the feature that produced it. Because it sits between the copper and whatever is applied next, resin bleed affects solderability, mask adhesion and plating, and it is one of the reasons a board that looks clean behaves oddly in assembly.

What Resin Bleed Is

The resin in a laminate is a partially cured polymer. When it is heated, or when it is smeared mechanically, it can flow a short distance across the copper and then cure in place as a thin, hard film. The film is electrically insulating, chemically resistant and difficult to detect, which is why it is usually found by the defect it causes rather than by inspection.

It differs from the resin smear found inside a drilled hole, although the two share a cause. Smear coats the barrel and is removed by desmear; bleed sits on the surface, where the processes that follow may or may not remove it depending on where it lands.

Where It Comes From

Drilling is the largest source. The drill heats the laminate by friction, and the softened resin is carried out of the hole onto the surrounding pad by the flutes and by the drill’s exit. A dull drill or a high hit count increases the amount of material transferred, and the effect grows quickly once the cutting edge has rounded.

Lamination is the second source. The resin flows under heat and pressure, and at the edges of a panel or around a cavity it can emerge onto the surface. Thermal operations after lamination, including reflow and any baking step, can also mobilise resin that is close to the surface.

Effect on Solderability

A film of resin on a pad prevents the solder from wetting the copper. The flux cannot remove it, because it is not an oxide, and the resulting joint shows dewetting or a non-wetting surface with the solder sitting as a ball on top of the pad.

The effect depends on thickness. A very thin film may be broken down by an active flux and produce a marginal joint, while a thicker film produces a definite non-wet. Because the film is uneven, the defect often appears as a pattern on one part of the board rather than across the whole panel. The general controls for maintaining bare copper before assembly apply here as well.

Effect on Solder Mask and Coating

Resin bleed under a solder mask reduces adhesion. Surface cleanliness is therefore judged on the laminate as well as on the copper. The mask bonds to the laminate and to the copper, and a resin film that is partially cured provides a weak interface that fails during thermal cycling or during a tape test. The failure appears as a lifted edge at a location that does not correspond to any mechanical damage.

Under a conformal coating the effect is similar, with the added complication that the coating may bond to the resin film rather than to the board. The joint is initially sound and fails later, which makes it harder to attribute.

Drilling and Desmear Contributions

Drill parameters control the amount of resin that reaches the surface. Feed rate, spindle speed, entry material and hit count all affect the temperature at the cutting edge, and the burr that is lifted at the hole entrance carries resin with it. A board with a heavy entry burr usually has more bleed around the holes.

The desmear step removes resin from the hole wall, and the same chemistry will attack bleed on the surface if it is exposed to it. Where desmear is applied through the holes only, the surface film survives. The desmear control regime and the drill parameters therefore have to be looked at together.

Etching and Plating Contributions

Etching removes copper, not resin, so a resin film on the surface protects the copper underneath and leaves a raised area after etching. The result is a line that is wider than the artwork at that point, or a patch where copper remains between two traces and produces a short after assembly.

Plating cannot deposit through a resin film either, so the effect on a plated surface is a void or a thin patch. Where the film is on a resist surface it can also prevent proper development, leaving an area of unremoved resist that appears as a defect at a later stage.

Detection and Measurement

Visual inspection under magnification with a low-angle light shows bleed as a change in surface reflectance, but the method is unreliable on a matt finish. A water break test is more useful: a clean copper surface wets uniformly, while a resin film causes the water to draw back into droplets.

Chemical detection is possible with a dye that stains resin but not copper, and it is used to establish whether a problem exists rather than to inspect production. Where the question is whether a specific pad is contaminated, a solderability test on that pad is the most direct answer.

Corrective Actions

The action depends on where the resin came from. Bleed around the holes is answered by reducing the drill temperature through feed, speed and entry material, and by tightening the hit count. Bleed from lamination is answered by reviewing the press cycle and the handling of the panel after pressing.

Where the film has already formed, removal requires a mechanical or chemical step. A light mechanical scrub or a pumice treatment removes surface resin, and an alkaline or plasma treatment can be effective depending on the chemistry. Whichever is used, the treatment has to be verified on the surface rather than assumed, because an incomplete removal leaves a thinner film that produces marginal joints instead of obvious ones.

Records and Process Control

The record should carry the drill parameters and hit counts, the entry material, the desmear condition and any surface treatment applied. Since bleed is a surface condition, the records for the processes that touch the surface are the ones that matter.

Where a soldering problem is traced to the board rather than to the assembly process, resin bleed is one of the candidates to eliminate early, because the evidence is easy to obtain and the remedy is usually in the process records rather than in the assembly line. Checking the surface before assembly costs far less than investigating a batch of non-wetting joints afterwards.

FAQ

How does resin bleed affect soldering? It forms an insulating film between the solder and the copper, so the solder cannot wet the pad. Flux cannot remove it because it is not an oxide, and the joint appears non-wetting.

Is resin bleed the same as drill smear? They share a cause and a chemistry, but smear is inside the barrel and is removed by desmear, while bleed is on the surface where the desmear chemistry may not reach it.

How is resin bleed detected? A water break test on the copper is the quickest method, with a dye stain used to confirm the diagnosis. A solderability test on the affected pad gives the most direct answer.