Haloing Around a Hole: What the Laminate Is Telling You

Haloing is the pale ring that appears in the laminate around a drilled hole, visible on a finished board as a lighter area and in a microsection as a separation between the resin and the glass fabric. The laminate has not been removed; it has been damaged, and the damage takes the form of a network of small cracks and a loss of bond in the region around the hole.

Whether a halo matters depends on its size, its position and the duty of the board. A small halo around a hole in a low stress area is usually acceptable, while a halo that reaches a conductor or that surrounds a plated barrel in a thermally cycled product is a reliability risk. Reading the size and the shape of the halo is therefore the first step in deciding what to do.

What a Halo Is

A halo is a zone of damaged laminate around a hole. The glass fibres and the resin separate, small cracks form along the fibre and resin interface, and the optical appearance changes because the two phases are no longer in intimate contact. In a section it appears as a light band around the barrel, sometimes with individual cracks radiating outwards.

It is different from resin recession, which is a loss of resin at the wall, and from smear, which is resin spread over the copper. A halo is subsurface damage, and it is usually larger than the visible surface effect, which is why the surface appearance alone understates the extent.

Why It Forms

The immediate cause is mechanical and thermal damage during drilling. The drill removes material by cutting, and the cutting edge generates heat and a local stress field that extends beyond the hole. Where the laminate is brittle, or where the resin and the glass are poorly bonded, that stress field opens the interface and the halo forms.

The heat is the second contributor. A drill that is dull, a chip load that is too low or a spindle speed that is too high raises the temperature at the cutting edge, and the resin softens and expands at a different rate from the glass. The differential movement damages the interface, and the damage remains after the board has cooled.

Drilling Parameters and Heat

The parameters that matter most are the cutting speed, the feed per revolution and the condition of the tool. A high surface speed with a low chip load rubs rather than cuts, which generates heat without removing material efficiently. The correct combination produces a chip that carries heat away and a hole that is dimensionally accurate.

The tool condition is the variable that changes without any setting being touched. A drill that is worn at the outer corner cuts less efficiently and generates more heat, and the halo grows before the hole diameter goes out of tolerance. The geometry that changes with wear is described in the notes on drill regrind and bit life.

Resin, Glass and Moisture

The laminate itself sets how much damage a given drilling operation produces. A material with a high glass content and a brittle resin system is more prone to haloing than one with a tougher resin, and a material that has absorbed moisture is more prone still, because the water flashes to steam at the cutting temperature.

Storage therefore matters before drilling. A laminate that has been kept in a humid room holds more water than one from a dry store, and the difference is enough to change the halo size on the same drilling line. The drill room environment and the storage of panels are described in the notes on drill room humidity control.

Reading a Halo in Microsection

The section is prepared through the hole axis, and the halo is measured as a distance from the barrel wall to the furthest extent of the damage. The measurement is taken at several points around the circumference, because the damage follows the glass weave and is therefore not uniform, and the maximum is reported.

The section also shows the nature of the damage, which is more useful than its size. Cracks that follow the fabric suggest a mechanical cause, a generalised lightening of the resin suggests thermal damage, and a separation that follows the wall suggests a combination of both. The preparation of a section that preserves those features is described in the notes on hole wall pullaway.

Halo and Reliability

The reliability risk comes from two effects. The first is mechanical: the damaged laminate no longer supports the barrel as well as sound material, so the barrel carries more of the strain during thermal cycling. The second is chemical: a path along a separated interface can absorb process chemistry and release it later, and it can provide a route for moisture and for conductive migration.

Whether a given halo is acceptable is therefore a question about the thermal and the electrical duty of the product. A board that will be cycled between wide temperature extremes needs a smaller halo than one that operates at a stable temperature, and the acceptance limit should reflect that rather than being a single number for all products.

Process Window Adjustments

When haloing is found, the first response is to review the drill parameters against the tool and the material. A higher feed per revolution with a matched spindle speed usually reduces the heat, and a new or correctly reground tool removes the wear component. The entry and backup materials are part of the same adjustment, because they affect the heat generated at the entry and the exit.

Where the parameters are already correct, the material is the next item to examine. A different laminate, a different glass style or a lot with a different resin content can produce a different halo on an unchanged line. That is a supplier conversation supported by sections from both materials rather than by a specification comparison.

Detection and Sampling

Detection in production is done on a section from a drilled coupon, because the surface appearance is unreliable. The coupon should carry the smallest hole and the thickest stack of the panel, since those are the hardest to drill without damage, and it should be sectioned at the same interval as the plating coupons.

A quick check that needs no section is to look at a drilled hole under magnification with a low angle light, where a halo shows as a lighter ring. That check detects the gross cases and not the marginal ones. For a proper trend the section result is required, and it should be recorded with the tool number, the stack configuration and the parameters that produced it.

Records and Change Control

The records are the laminate lot, its storage, the drill parameters, the tool number and its regrind count, the stack configuration and the section result. With those fields, a change in halo size can be attributed to a tool, a parameter or a material within a day, which is the difference between containing the problem and shipping it.

Change control is what keeps the process stable. A new drill supplier, a new entry material or a new laminate lot changes the halo behaviour, and a change that is made without a section check will be discovered by the customer if it matters. One coupon and one section per change is a small cost for that assurance.

FAQ

Is every halo a reject? No. Small halos are common and acceptable for many products. The limit depends on the thermal and electrical duty of the board.

Does haloing affect the hole diameter? It can. Severe haloing removes support from the wall and the hole can enlarge or become out of round after plating.

Can haloing be seen on the finished board? It often shows as a pale ring near the hole, and the visible ring understates the damage beneath it. A section is the only reliable check.