PCB Hole Processing and Rigid-Flex Fabrication

Two processes decide how much of a circuit board’s reliability is real rather than assumed: the treatment of the holes that connect its layers, and the construction of the flexible and rigid sections in a board that has both. The first governs whether a multilayer stack conducts as designed; the second governs whether a board that has to bend survives the bending. This article covers the hole processing sequence in fabrication and the additional steps that rigid-flex construction requires.

Hole Types and What They Demand

Three kinds of hole appear in multilayer fabrication, and each has its own difficulty. A through hole passes through every layer and can be drilled mechanically at a range of diameters. A blind hole connects outer layers to inner ones without passing through the board, and a buried hole connects inner layers only; both are formed by drilling before the layers are bonded, or by laser after.

On high density interconnect boards the smallest holes are laser-drilled and can be well under 100 µm in diameter. Position accuracy at that scale has to be held to a fraction of the hole size, and the drill programme, the panel registration system and the material all have to agree for the hole to land where the design intends.

Drilling

Mechanical drilling uses a bit spinning at high speed and advancing at a controlled rate, with entry and exit materials above and below the stack to protect the copper from burrs and the hole walls from tearing. The parameters that matter are the bit geometry, the spindle speed, the feed rate, the number of hits a bit may take before replacement, and the stability of the machine.

Laser drilling is used for microvias, where mechanical bits are impractical. It removes dielectric without cutting through the copper beneath, which requires the copper to be treated with a surface that stops or absorbs the beam predictably.

Both routes share a consequence: drilling generates heat at the point of contact, and that heat melts the resin binder. The result is a smear of resin across the hole wall, covering the copper it is supposed to bond to.

Desmear and the Three Routes to a Clean Wall

Removing that smear is the step that makes the rest of the process possible. Three approaches are used, and they are chosen on material rather than convenience.

Sulphuric acid dissolves epoxy resin chemically and is the traditional route. Permanganate, usually in a multi-stage alkaline sequence, both removes the smear and roughens the resin surface so that the metal deposited later has something to grip; it is the most widely used. Plasma etching uses ionised gas in a vacuum to attack the resin, and it is the usual choice for high-frequency materials, whose chemistry and fillers do not respond well to the wet routes.

Each process has to be controlled for its own variables. Under-treatment leaves smear behind and the plating that follows will adhere poorly; over-treatment roughens too far and can leave the dielectric itself degraded. The check is a routine one: examine the hole wall under magnification and confirm both that the smear is gone and that the resin texture is what the process is meant to produce.

Electroless Copper and Plating

The hole wall is a dielectric, so it cannot be plated electrically. It has to be made conductive first. The panel passes through a catalyst that deposits palladium on the treated surfaces, and then through an electroless bath where copper precipitates chemically onto that catalyst without any applied current, building a thin continuous layer typically a fraction of a micrometre to about a micrometre thick.

That layer is the seed. Electrolytic plating then builds the copper to the required thickness, on the hole walls and, depending on the process, across the surface as well. Some processes plate the whole panel first and then plate again after imaging; others image first and plate only the conductors and the barrels. The two routes differ in how much surface copper is removed later and in how uniform the barrel thickness turns out.

Thickness in the barrel is the number that matters. A hole that is plated thinly in the middle and heavily at the entry passes a surface measurement and fails in service, so plating distribution is controlled by current profiling, anode geometry and shielding rather than by the setpoint on the rectifier.

Checking the Barrel

Inspection of holes is a combination of methods because no single one sees everything. Optical inspection finds surface defects around the hole but cannot see inside it. X-ray inspection reveals voids, cracks and separation in the barrel, and it becomes essential as hole diameter falls. Thermal stress testing, holding a coupon in molten solder for a few seconds and repeating, then examining the barrels for cracking or barrel-to-land separation, is the classic check on whether the plating will survive assembly and thermal cycling.

On boards intended for long service life, the same tests are run on coupons from each lot rather than once at qualification, since the property being checked is a process condition rather than a design property.

Rigid-Flex: Materials and Stack-Up

A rigid-flex board combines rigid sections that carry and support components with flexible sections that bend to connect them. The materials differ accordingly.

The flexible cores are polyimide laminates, usually thin enough to bend without cracking. The rigid sections use conventional FR-4 cores and prepreg, but the prepreg must be selected for low resin flow, since excessive flow at the boundary is what produces the defects this construction is known for. A coverlay, a polyimide film with openings for the pads, protects the flexible conductors.

The stack-up has to treat the rigid-flex boundary as the critical feature it is. That transition is where stress concentrates when the board is bent, and it is where the design uses a stepped transition and, where needed, tear-drop reinforcement in the conductor pattern so that the foils taper rather than step abruptly into the bend region.

Opening the Windows

Removing the rigid material where the board has to bend is done by machining or by laser. Machining is cheaper and can leave burrs and a rougher edge; laser cutting is more precise and is the route for fine flexible traces. Whichever is used, the flexible region’s edge must be finished so that the layers cannot separate there, and the shape of the opening needs to match the bend radius the design assumes.

Lamination of Mixed Dielectrics

Bonding polyimide and epoxy in one stack is where rigid-flex fabrication departs furthest from ordinary pressing. The two materials do not flow the same way at the same temperature, so the profile is built in stages: a low-temperature, low-pressure stage that evacuates air and lets the stack settle, a higher-temperature medium-pressure stage where the resin crosslinks, and a cooling stage performed under pressure rather than after it.

Pressure control is tight, because the two failure modes pull in opposite directions. Too much pressure squeezes resin out of the flexible region, thinning it and leaving the conductors poorly supported; too little leaves voids in the rigid region that later appear as delamination.

Drilling and Plating Where It Bends

Drilling a stack that contains polyimide behaves differently from drilling epoxy. The flexible material is more prone to producing raised edges and local deformation, so the practice is to support it with a backing material and to change the drilling parameters as the bit passes from rigid into flexible material, reducing the feed so the cut is gentler at the interface.

Desmear is also different. Where wet chemistry removes epoxy smear effectively it may leave polyimide residue behind, so plasma treatment is commonly used for the flexible layers. Plating then has to cope with the larger expansion of the flexible material, which is why an activation step before electroless copper is used to improve adhesion, and why some processes use pulse plating to produce a denser, finer-grained deposit in the barrel.

Coverlay and Stiffeners

The coverlay is laminated over the exposed flexible conductors after the board has been processed, using a press cycle at moderate temperature and pressure. Its openings have to be clean and accurately placed, because they define the areas available for soldering.

Where a flexible section carries a connector or has to be handled during assembly, a stiffener of FR-4 or steel is bonded to the back so the area behaves as a rigid region. Stiffeners are not structural decoration: a connector soldered to an unsupported flexible tail will eventually crack the joints every time the tail is moved.

Testing Rigid-Flex and Flex Boards

The acceptance testing adds items that rigid boards do not need. Bend life is measured by cycling the flexible region through the intended radius until it fails, with the requirement expressed as a number of cycles rather than a qualitative judgement. Ionic contamination is measured on the finished surface, because residue left on flexible circuits can cause corrosion under conditions the board will actually see. And the rigid-flex transition is examined with X-ray for separation between layers, since that is where delamination begins.

Where the Process Is Going

Two directions are visible. Direct metallisation methods aim to make the hole wall conductive without a full electroless sequence, using sputtered or conductive-polymer layers; they are attractive for small, deep blind vias where wet chemistry struggles to reach and rinse. And in rigid-flex construction, any-layer interconnection and embedded devices are pushing the same miniaturisation seen on rigid HDI boards into boards that also have to bend.

Both directions raise the same requirement: the process has to be measured, not assumed. Boards that pass a functional test on the bench can still fail later at a burned through-hole or a cracked bend region, and the controls that prevent it are inspection of the barrel under magnification, test coupons from each lot, and the discipline of a quality management system that treats a process change as something to be verified. Blind and buried via construction is covered further in the note on HDI blind via fabrication.

FAQ

Why does desmear matter so much? Because plating adheres to the resin surface. Smear left on the hole wall prevents reliable adhesion and produces barrels that pass inspection and fail under thermal stress.

When is laser drilling used instead of mechanical? For microvias and blind holes in HDI construction, where the diameter is too small for a mechanical bit and the depth has to stop on a specific copper layer.

What makes rigid-flex harder than a rigid board? The press has to bond two materials with different flow behaviour in one stack, and the boundary between them is a stress concentration that has to be designed and inspected.