Ultra-Thick Copper Multilayer PCB Manufacturing

Boards built for high current are a distinct product family, and they have grown from a specialist curiosity into a sizeable market. They are used wherever heavy current must be transmitted and where power and signal connections have to coexist on the same assembly, and demand has risen with automotive electronics, IGBT assembly, wind power converters and ignition coils. The reason is a change in what a board is expected to deliver: it still provides electrical connection and mechanical support, but it is now also asked to carry integrated power, handle large currents and do so with high reliability — a combination that makes thick copper work commercially attractive.

Where Conventional Processing Stops

Producing very heavy copper with standard processes runs into a wall fairly quickly.

The established methods are to build the copper up by electroplating in successive passes with repeated solder mask printing between them, or to start with ultra-thick copper foil. Both approaches currently reach about 0.41 millimetres, which is roughly twelve ounces per square foot. Beyond that thickness, manufacturing a multilayer board becomes very difficult indeed, and no established breakthrough has taken the process further.

That limit is the reason an alternative approach was developed: instead of depositing copper, embed a copper plate in the laminate. The technique borrows from the manufacture of laminated busbars, and it has produced multilayer boards with copper of 0.5 millimetres, or fourteen ounces per square foot, and above.

A Worked Example

The construction of one such board illustrates how the method works.

The board is a three-layer design. The inner copper is 1.0 millimetre thick and the outer copper 0.3 millimetres, with a minimum outer line width and spacing of 0.5 millimetres. The surface layer is conventional FR-4 copper-clad laminate, 0.3 millimetres thick, etched on one side. The bonding layer is a non-flowing prepreg, 0.1 millimetres thick. The thick copper plate sits embedded in a corresponding cavity in the FR-4 material rather than being formed on top of it.

Process Flow

The sequence follows ordinary PCB practice with one additional mechanical stage inserted into it.

Surface and middle layers are milled, and the thick copper plate is machined by CNC milling. After surface treatment, the assembly is stacked in an integral mould, heated and pressed, then demoulded. From that point the board continues through the standard PCB process to completion.

The interesting part is the detail of each step, because the process succeeds or fails on how the copper and the dielectric materials are made to fit together.

Building the Inner Layer

A copper foil cannot provide the required thickness, so the inner conductor is a 1 millimetre electrolytic copper plate — a readily available material — machined directly to shape on a milling machine.

Around it, the outer profile is filled with FR-4 machined to the same thickness, so that the copper plate sits in a pocket whose walls are dielectric. The gap between the two outlines is controlled to within 0 to 0.2 millimetres, which keeps the fit tight enough for lamination while leaving room for manufacturing tolerance.

The FR-4 filler is doing three jobs at once. It makes up the thickness so the stack is level, it guarantees a tight press fit, and it provides the internal insulation between the embedded conductor and the rest of the structure. Because the copper is inserted rather than grown, the inner copper thickness is no longer limited by plating behaviour, and designs above 0.5 millimetres become practical.

Black Oxide Before Lamination

Thick copper cannot simply be laminated with resin and expected to bond. Its surface treatment has to promote adhesion, which is why the copper is black oxidised before pressing.

The oxide treatment does several things. It increases the surface area in contact with the resin, it improves the wetting of the copper by resin flowing at high temperature, and it allows the resin to penetrate into the structure of the oxide layer so that after curing the bond is anchored rather than merely touching.

The practical benefit is measurable. Black oxidation reduces the white spot and delamination defects that would otherwise occur, and it prevents the surface whitening and blistering that appear after a bake test at 287 degrees Celsius plus or minus six.

<img src="https://www.gopcba.com/wp-content/uploads/2026/05/smart-healthcare.jpg" alt="embedded copper plate laminated into an FR-4 cavity” />

Lamination

The embedded construction makes lamination harder than it looks. The thick copper plate and the FR-4 filler that surrounds it have manufacturing tolerances, so their thicknesses can never be exactly equal. Pressing such a stack conventionally tends to produce white spots and delamination, and the difficulty rises with the copper thickness.

The solution has three elements.

First, an integral mould is used, with steel plates above and below. Rigid tooling defines the flatness of the finished panel and resists the pressure unevenly distributed by an insert of varying thickness.

Second, a silicone pad is placed as an intermediate buffer layer. This is the critical detail: a non-flowing prepreg has very low flow, so a conventional conforming material such as kraft paper cannot distribute pressure evenly across the assembly, and the result is exactly the white spot and delamination defect the process is trying to avoid. A silicone pad does distribute that pressure.

Third, the process parameters are set to suit the material rather than to a default recipe. Pressing pressure is raised from 2.1 megapascals, about 22 kilograms per square centimetre, to 2.94 megapascals, about 30 kilograms per square centimetre, and the temperature is set to 170 degrees Celsius, which is the optimum fusion temperature for the prepreg being used. Temperature, pressure and dwell time are chosen together, and the combination is what produces a sound laminate.

Lamination is the step where most heavy-copper work fails, and the principles involved are the same ones that govern deep stacks, as described in this look at lamination, drilling and plating for deep stacks.

Verifying the Result

Verification is not limited to a visual check.

Pressed boards are inspected against the applicable military specification for acceptable blistering and delamination below the surface, and for freedom from the defects permitted only within the limits defined for surface appearance. Appearance and dimensional requirements are checked in the same pass.

The decisive check is a microsection. A cross-section of the finished board shows whether the resin has filled the spaces around the embedded copper completely, and whether any micro-voids or bubbles remain. A sound result shows fully filled conductor regions with no micro-voids — which is the evidence that the pressure distribution and the fusion temperature were actually correct.

Why the Approach Is Worth the Effort

Heavy copper is not merely a conductor with lower resistance. It changes what the board can do: it carries current that would otherwise require busbars or cables, it spreads heat away from power devices, and it allows power and signal circuitry to be integrated on one assembly instead of being separated.

Those capabilities come with process constraints that affect the rest of the design. Thick copper and fine features pull against each other, so the achievable line width and spacing are larger than on an ordinary board — the example above uses a 0.5 millimetre minimum on the outer layers. Via formation, dielectric thickness and the ability to combine heavy copper with high density interconnect all have to be considered together, and the interaction is examined in this discussion of heavy copper and HDI compatibility.

For assemblies that must also survive a long life in a demanding environment, the heavy copper becomes part of the reliability picture rather than only the electrical one, as set out in this list of traits of a high reliability PCB.

FAQ

What limits conventional thick copper production? Deposition. Building copper up by electroplating or starting from ultra-thick foil reaches about 0.41 millimetres, and beyond that the process becomes very difficult with no established route further.

Why is black oxide necessary on the copper plate? Because it increases the contact area available to the resin, improves wetting by flowing resin and allows the resin to penetrate the oxide layer, so that the bond is mechanically anchored after curing rather than relying on surface contact.

Why use a silicone pad in the press? Because a non-flowing prepreg cannot be pressed evenly with a conventional conforming material. The silicone pad distributes pressure uniformly across a stack whose thickness varies, which is what prevents white spots and delamination.