Hole Wall Resin Smear And Desmear

Drilling a hole in a printed circuit board generates heat, and the heat softens the resin around the hole. As the drill turns and withdraws, the softened resin is smeared across the hole wall like a thin film, and it can cover the copper of an inner layer entirely. The plating that follows is deposited onto that film rather than onto the copper, and the electrical connection becomes a mechanical contact that fails later.

This article explains how smear forms, which drilling parameters cause it, how it is removed, and how it is detected before it becomes a field failure.

How Smear Forms

The drill removes material by cutting, and the cutting action converts mechanical energy into heat. The temperature at the cutting edge can exceed the glass transition temperature of the resin, and above that point the resin becomes soft and plastic rather than brittle. A soft resin does not cut cleanly; it deforms and is dragged along the wall by the rotating flutes. The same mechanism produces the resin film that is left behind when the drill withdraws.

The film is not always uniform. It is thickest where the drill has been hottest, which is usually near the entry side and where the hole passes through a thick resin rich region. It is thinnest where the laminate is heavily glass reinforced, because the glass fibres resist deformation and the resin around them is constrained. The result is a hole wall whose condition varies along its length and around its circumference, which is why a single measurement of smear is difficult to interpret. The consequences of such variation for the plating are described under copper plating defects prevention.

Drill Parameters That Cause It

Four parameters decide how much heat reaches the resin. The first is the surface speed of the drill, set by the spindle speed and the diameter; too high a speed raises the temperature at the cutting edge. The second is the feed rate per revolution, because a low feed means the drill rubs rather than cuts and generates heat without removing material. The third is the condition of the bit, since a worn cutting edge rubs and a dull bit is the most common cause of a sudden increase in smear. The fourth is the number of hits per bit, which is the parameter that controls wear in production.

Backing material and entry material change the picture as well. A proper entry foil lubricates and cools the cutting edge at the top of the hole, and a backing board supports the laminate and reduces exit burrs. Where these are absent, the drill runs hotter and the smear increases at both ends. Panel stacking matters for the same reason: more panels per stack raises the temperature at the bottom of the stack and produces more smear on the lower panels than on the upper ones.

What Smear Does To Plating

The plating process cannot plate onto resin. The sequence that makes the wall conductive, and then the electroplating that builds the barrel, both depend on an activated surface, and a resin film blocks the activation. Where the film covers an inner layer connection, the copper of the inner layer never joins the plating, and the only thing holding the barrel to the inner layer is whatever mechanical bond the film allows.

The result is a joint that passes continuity when it is made and becomes intermittent later. Thermal cycling widens the gap, because the differential expansion loads a connection that has no metallic bond, and the failure appears as an open circuit after a period in service. Because the defect is inside the hole, it survives electrical test at the end of the line and appears only in the field, which is what makes it one of the more expensive failure mechanisms in a board shop. The metallurgy of the deposit that has to bridge the connection is covered under electroplating additives for PCB.

Desmear And Its Control

Desmear removes the film before plating. The common process is a permanganate treatment, in which an alkaline solution swells the resin surface and the permanganate oxidises it, converting the smooth film into a roughened surface that the plating can grip. A neutraliser follows to remove the manganese residues, and a light etch of the copper removes the oxide and the damaged metal at the inner layer connection.

The process has to be controlled to a narrow result. Too little treatment leaves a film that is only partly removed, and the plating bonds to a weak surface. Too much treatment attacks the resin around the hole and can undercut the inner layer pads, widening the entry and thinning the dielectric. Both are process control problems, and both are verified by the same coupon used to check the plating thickness. Ensuring the process is capable at all is a matter for the prototype programme described under multilayer prototype requirements.

Detecting Smear

The direct method is a microsection of a drilled and plated hole, examined at magnification. Smear appears as a dark line or a band of resin between the plating and the inner layer copper, and it is most visible at the inner layer interface. A section that shows a clean interface with a continuous metallic connection has no significant smear at that location, which is a statement about one hole rather than about the panel.

Two indirect methods support the section. The first is a resistance chain on a coupon that runs through many holes; a connection with an interposed film raises the resistance, and the effect is visible on the distribution even when individual holes are satisfactory. The second is the drill bit count, which is a record keeping control rather than a measurement. Where the bit life is exceeded, the smear increases on every panel until the bits are changed, and the chain test is what detects the resulting drift.

Process Control Around Drilling

Control rests on recording and enforcing four things: the spindle speed and feed for each hole size, the bit type and its life, the stack height, and the entry and backing materials. These are set once per product and then checked per run, because the drill programme is one of the few parts of the process that can be changed by an operator without changing the artwork or the chemistry.

The other control is the drill room environment. Temperature and humidity affect the laminate and the bits, and a shop that runs its drills hot in summer will see a seasonality in its smear results. Keeping the room controlled, and checking the first panel of each lot by section rather than the last, turns a problem that appears intermittently into one that is detected at the point where it starts.

Additional Considerations for This Build

Practical attention to resin smear pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating resin smear explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Deliberate attention to plating adhesion pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating plating adhesion explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

Does a new drill bit eliminate smear? It reduces it, because a sharp edge cuts rather than rubs. The heat generated by the cutting action itself still softens the resin, so desmear is required even with a fresh bit.

Is smear worse on thick panels? It is worse on thick panels and on tall stacks, because the drill stays in the material longer and the heat has less chance to escape. Both increase the temperature of the cutting edge and the amount of resin that is deformed.

Can smear be seen without sectioning? Not directly. A resistance chain shows its electrical effect and a drill bit count shows the risk of it, but the position and extent of the film require a section through the hole.