PCB Lamination Process Step by Step

Lamination is the step that turns a stack of separate layers into a single board. It is the longest single operation in multilayer fabrication, and it is the point at which the electrical and the mechanical properties of the finished board are decided. Getting it right is a matter of controlling resin flow, pressure and temperature together.

What Goes Into the Stack

The layup is built from cores and prepreg. A core is a cured laminate with copper on both faces, already imaged and etched into an inner layer circuit. Prepreg is the same glass and resin material in a partially cured state, used as the adhesive between cores.

The number and thickness of the prepreg sheets between two cores determines the dielectric thickness at that interface, which is what sets the impedance of the traces routed on the adjacent layers. The thickness is therefore a design figure, not a manufacturing convenience. Our design tolerances article describes how it is specified.

Prepreg and Resin Flow

Prepreg contains resin in a partially cured state, and it must flow during the press cycle to fill the gaps around the inner layer copper. If it does not flow enough, voids remain; if it flows too much, the dielectric becomes thinner than intended and the resin starves at the edges.

The flow behaviour is characterised by the resin content and the gel time, both of which the fabricator controls and monitors. Because the requirement changes with the copper pattern being covered, the prepreg selection depends on the design as much as on the stackup drawing.

The Press Cycle

The stack is placed between heated platens and subjected to a controlled cycle of pressure and temperature. The cycle has three phases: a heating phase where the resin softens and flows, a dwell phase where it fills the gaps and cures, and a cooling phase under pressure where the stack sets.

Pressure must be applied at the right moment. Too early and the resin is squeezed out before it has filled the pattern; too late and the flow is insufficient. Temperature uniformity across the platen matters as well, because a stack that cures unevenly on one side will bow. Our eight layer board fabrication article describes where lamination sits in the sequence.

Registration Through the Press

The layers must remain aligned with each other as the resin flows and the material expands and contracts. That alignment is what allows the drilled holes to hit their target pads on every inner layer.

The material movement during pressing is predictable in direction and roughly consistent in magnitude, so the artwork can be scaled to compensate. The fabricator measures the movement on test panels and adjusts the scale factors for subsequent builds. This is one of the reasons a new stackup takes longer to build well than a familiar one, and why characterisation is worth repeating when the construction changes. Our via placement notes describe the design side of the same tolerance.

Curing and Its Effects

The resin must reach a sufficient degree of cure for the board to be mechanically stable and thermally reliable. Under cured resin remains soft, absorbs moisture more readily and behaves differently during drilling.

The degree of cure is a function of time and temperature, and it is verified by the fabricator through process control rather than by testing every board. Where the material is later exposed to soldering temperatures, an incompletely cured stack is more likely to delaminate or to form measuarable voids.

Defects That Come From Lamination

Delamination is the most serious outcome, appearing as a separation between layers after thermal exposure. It usually begins where contamination or moisture prevented the resin from bonding, or where the flow was insufficient to fill a gap around heavy copper.

Other defects include resin starvation at the panel edges, registration errors that reduce the annular ring on inner layers, and thickness variation across the panel that shifts the impedance of the traces above it. Each of these is traceable to a process variable, which is why the press record is part of the quality documentation. Our PCB quality article describes what that record contains.

Sequential Lamination

Where a board is too thick to press in one operation, or where blind vias are required, the stack is built in stages. A sub-assembly is laminated, drilled and plated, then further layers are added in a second press cycle.

Each additional cycle adds time, cost and risk, because the sub-assembly has already been processed and any damage during the second press destroys work that was previously completed. Sequential lamination is the main reason high layer count boards are quoted in weeks rather than days, and the reason their yield is lower than a simpler board. Our high layer count cost article describes where that money goes.

What the Designer Can Control

Most of the lamination process is invisible to the designer, but three things are not. The stackup sets the prepreg thickness at each interface. The copper distribution sets how much resin has to flow, because a layer that is dense on one side and sparse on the other causes uneven flow.

And the material choice sets the temperature window, the flow behaviour and the moisture sensitivity of the whole build. Keeping the copper balanced, choosing a construction the fabricator has characterised and confirming the material’s availability before the order is placed are the three contributions that most affect the result. Our high Tg material article covers the material decision.

Tooling and the Press Schedule

Lamination is scheduled rather than performed on demand, because the press is the most expensive and least flexible machine in the shop. Panels are collected into a press load that fills the platens, and a job waits for a load to be ready.

That batching is the main reason a small order sometimes takes longer in proportion than a large one, and the reason a fabricator with a light load can quote a shorter schedule. It is also why an unusual stackup has to be scheduled separately, since it cannot share a press cycle with the standard construction next to it. Our PCB lead time article describes how that affects the quoted schedule.

Where a stackup is new to the fabricator, the first build is partly a characterisation exercise. The measured registration and thickness results from that build become the basis for the compensation applied to the next one, which is why repeating the same construction costs less in time and in risk than changing it.

FAQ

How long does a press cycle take? Several hours, and it is the longest single step in multilayer fabrication. It cannot be shortened by running the press hotter without changing the resin behaviour.

What is prepreg in simple terms? Partly cured glass and resin sheet that acts as the adhesive between inner layers and becomes the dielectric between them.

Why does a board delaminate? Usually because contamination or moisture prevented bonding, or because the resin did not flow enough around a heavy copper area.

Can lamination be done in one cycle for a thick board? Up to a point. Beyond the thickness the press can handle uniformly, sequential lamination is used.

How is registration controlled during pressing? By measuring how much the material moves and scaling the artwork to compensate for it on subsequent builds.