Half Ounce Copper: Where 0.5 oz Foil Fits in a Stack-Up

Half ounce copper is foil rolled to about 17.5 micrometres nominal thickness, half the weight of the standard one ounce foil. It is chosen when the design needs finer traces, a thinner finished board or a lighter panel, and it is avoided when the traces must carry current or survive repeated thermal cycling. The trade is simple: thinner copper etches finer and weighs less, and it also has more resistance and less mechanical margin.

Foil weight is measured before lamination and it is not the finished thickness. Plating, oxide treatment and pressing all change what ends up on the board, so a drawing that specifies only finished copper can be satisfied by more than one starting point, and the choice made by the shop may not be the one the design assumed.

What Half Ounce Copper Is and How It Is Made

Foil is produced by electrodeposition onto a rotating drum or by rolling, and its weight is quoted in ounces per square foot. Half ounce copper weighs half of what one ounce weighs, which works out to roughly 17.5 micrometres, and the figure is nominal rather than a tolerance. The tolerance is stated separately, usually as a percentage, and it is wide enough to matter on fine lines.

Thin foil is more delicate in the lay-up room. A wrinkle pressed into half ounce copper cannot be corrected afterwards, and a wrinkle under a trace produces a local thickness change that surfaces later as an impedance or etch anomaly. Flat handling and a clean lay-up surface matter more with thin foil than with one ounce, and the difference shows in yield.

Foil Weight Versus Plated Copper Thickness

Inner layers usually keep their starting foil weight, because they are not plated after lamination. Outer layers gain copper during plating, so the finished copper thickness on an outer layer is the foil weight plus the plating, and a half ounce outer layer can finish close to a one ounce layer in the plated areas.

That is why a stack-up should state foil weight per layer and plated thickness separately. If only a finished number is given, the fabricator may choose lighter foil and plate more. The result is electrically acceptable but it changes hole wall quality, etch control and cost, and it can move plating thickness into a range the design never reviewed.

Current Capacity and Trace Resistance

Trace resistance is inversely proportional to copper cross-section, so a half ounce trace has about twice the resistance of a one ounce trace of the same width. On a wide signal run that is irrelevant. On a power path it sets the voltage drop, and the heating that follows has to be checked against the temperature rise the design allows.

Current capacity should come from the design standard rather than from a rule of thumb. A half ounce internal layer, buried where heat has nowhere to go, carries far less than the same width on an outer layer. Adding a plane or a parallel trace is often cheaper than raising copper weight across the whole board, which is the usual heavy copper mistake.

Where Half Ounce Copper Works Best

Half ounce copper suits fine-pitch digital routing, dense inner layers and thin finished boards where overall height matters. It is also common on layers that are almost entirely planes, because a plane with a thin cross-section still has low resistance and the copper saved reduces laminate thickness and drilling depth.

It fits poorly where traces carry current, where a layer must dissipate heat, or where the board will see repeated thermal cycling. Those cases are better served by one ounce or heavier copper on that layer alone. Mixed foil weight across a stack is normal practice, not a compromise, and it is the cheapest way to buy performance where it is needed.

Lamination and Handling of Thin Foil

Thin foil wrinkles more easily and it also shows the laminate weave more readily, because there is less copper to bury the texture. Where the surface must stay smooth for controlled impedance, a heavier foil or a different resin system is often a simpler fix than tighter handling rules.

During the press, resin flow across a thin layer can drag the foil slightly, and the effect is largest where there is little copper. Registration should be checked after lamination on the layers with the least copper, because that is where movement is greatest and where a drill offset will do the most damage.

Etching Control on Thin Copper

Thin copper etches quickly, so the line speed that suits one ounce foil will over-etch half ounce copper. Etch factor changes with thickness, and the compensation applied to the artwork has to match the foil. One set of etch compensation across a mixed-weight stack produces narrow traces on the thin layers.

Under-etch is the opposite risk when the line is slowed to suit thick layers. The practical controls are to group similar foil weights on the same panel and to verify actual trace width on a coupon rather than trusting the compensation table, because the table describes the line as it was last measured.

Half Ounce Copper in Impedance-Controlled Stacks

Impedance depends on trace width, dielectric thickness and copper thickness. Thinner copper raises the impedance of a given geometry, so moving a layer from one ounce to half ounce shifts every number on it unless the width or the dielectric is adjusted at the same time. The shift is small in absolute terms and large against a tight tolerance.

The field solver used at design time should be told the foil weight you intend to buy, not a default. Where the fabricator is allowed to substitute thickness, the impedance tolerance has to absorb the difference, and a stack that looked comfortable on paper becomes marginal in production.

Cost, Yield and Lead Time

Half ounce foil costs more per kilogram than one ounce foil, and thin foil has a higher handling loss in the lay-up room. The offsetting savings sit in plating time, etch chemistry and sometimes in laminate thickness, so the net cost of a mixed stack is rarely obvious until the board is quoted.

Yield follows the same pattern. Thin layers improve fine-line yield and reduce plating defects, while heavier layers improve thermal and mechanical yield. The right answer for most boards turns out to be a mixed stack with each layer specified for the job it actually does, rather than one weight applied everywhere for convenience.

Specifying Half Ounce Copper on the Drawing

The drawing should name the foil weight for each layer, the plated copper thickness for the outer layers, and the tolerance that applies to both. Where impedance is controlled, the layer reference and its tolerance belong in the same note, so that a substitution cannot be made quietly during fabrication.

A short note listing the current-carrying nets and their minimum width also helps, because it tells the fabricator which traces must not be narrowed during etch compensation. The detail is cheap to add at release and expensive to discover after the panels are etched. Foil weight is one of the few stack-up choices that is visible on the finished board to anyone who looks, and copper foil types are worth agreeing early.

FAQ

Is half ounce copper thick enough for a ground plane? Yes. A plane has a very large cross-section regardless of foil weight, so a half ounce plane still has low resistance. Check the current path rather than the plane itself, because the bottleneck is usually the neck where the plane connects to a connector or a via field.

Can a half ounce outer layer carry power? It can, if the width and the allowed temperature rise are checked from a current capacity table rather than assumed. The plating added during fabrication helps, and the extra copper in plated areas is often the reason a thin outer layer performs better than the foil weight suggests.

Does half ounce copper change the finished board thickness? Only slightly, because the foil is a small part of the stack. The saving matters most on thin boards and on stacks with many layers, where the accumulated difference in copper thickness changes the total height by a measurable amount.