Prepreg and Core Selection Guide

Every multilayer board is built from two materials: a core, which is a sheet of laminate with copper on both faces, and prepreg, which is partially cured resin and glass that bonds the cores together. The choice between them sets the dielectric thickness of each layer, the impedance of every controlled line and the flatness of the finished panel. It is a materials decision that the layout cannot correct afterwards.

What Prepreg and Core Are

A core is a fully cured sheet of laminate, with copper foil bonded to both sides. It is rigid, it defines a layer pair, and its thickness is fixed by the supplier.

Prepreg is glass fabric that has been impregnated with resin and dried but not cured. It is soft and slightly tacky, and it flows during the press cycle to fill the space around the copper.

A multilayer stackup alternates core and prepreg, and the prepreg layers are the ones that bond the stack together. The number of prepreg sheets in a layer determines its thickness, since a single sheet is thin.

The properties of both materials are listed in the laminate properties guide, and the dielectric constant and the loss that appear there are the values used for impedance calculation.

Resin Content and Flow

Resin content is the fraction of the prepreg that is resin rather than glass, and it is quoted as a percentage by weight. A high resin content gives more flow and a better fill around heavy copper, while a low resin content gives a more dimensionally stable laminate.

Flow is the movement of resin during the press cycle. Enough flow is needed to fill the gaps between traces and to exclude air, and too much flow leaves the layer thin and the glass starved.

The copper distribution on the layer being bonded affects the flow. A layer with heavy copper needs more resin to fill around it, and a layer with no copper needs less, so the same prepreg behaves differently on two different boards.

The resin also determines the dielectric constant of the layer, so a change of resin content changes the impedance. That is why a stackup should be approved as a whole rather than sheet by sheet.

Thickness and the Stackup

The finished thickness of a prepreg layer is not the sum of the dry sheet thicknesses. The resin flows and the stack compresses, so the supplier gives a cured thickness for a given combination of sheets and a given press cycle.

The laminate thickness of the finished panel is therefore a press result rather than a drawing dimension, and the shop holds it by choosing the sheets and by controlling the press. A tolerance of about ten per cent on the total thickness is normal.

The controlled impedance layers are the ones where the thickness matters most, because the impedance depends on the dielectric height. A layer that is built thinner than the calculation assumes gives a line that is faster and lower in impedance than intended.

The stackup guide shows how the layers are assembled, and the layer count decision has to be made before the materials are chosen.

Choosing a Glass Style

The glass fabric comes in styles that are identified by a number, and the style sets the thickness of the weave and the weight of the fabric. A thinner style allows a thinner dielectric, which is useful for a fine impedance line.

The weave also has a periodic structure, and on a very fine differential pair the bundle of glass and the gap between bundles give two different dielectric constants. That variation is the glass weave effect, and it appears as a skew between the two lines of a pair.

A spread glass or a flattened weave reduces the effect by making the dielectric more uniform across the surface. It is more expensive and it is used where the skew budget is tight.

The glass style also affects the mechanical properties, since a heavier fabric gives a stiffer board with a better dimensional stability. The choice is a compromise between the electrical and the mechanical requirements.

Storage, Shelf Life and Handling

Prepreg absorbs moisture from the air, and a wet prepreg produces voids and delamination during the press. It is stored in a sealed bag with a desiccant and it has a shelf life measured from the date of manufacture.

A prepreg that is past its shelf life can still be pressed, and the result is a bond that looks sound and that fails in a thermal cycle. The failure is a delamination at the interface between a core and a prepreg layer, and it appears only after the board has been through soldering and a temperature cycling test.

The material is also sensitive to temperature, because the resin advances slowly at room temperature. A roll that has been left out for weeks will not flow in the same way as a fresh one, and the press cycle may have to be adjusted.

The shop records the batch number and the date for every layer of every panel, which is what makes a failure traceable. A delamination that appears in one batch and not another is a material or a storage issue rather than a design issue.

Where the stackup is unusual, the shop should be asked to confirm the build before the order is placed, and the confirmation should be a stackup drawing with the sheets named. A verbal agreement is not enough, because the shop may substitute a sheet that is electrically similar and mechanically different.

The design should avoid a stackup that depends on a special sheet which only one supplier makes, because the lead time and the cost both rise. A standard combination is easier to source and easier to press.

Design Review

The review starts with the target dielectric thickness for every layer, which comes from the impedance requirement. The stackup is then built from available sheets that meet those thicknesses.

The second item is the symmetry of the stackup around the centre line, since an asymmetric build warps during the press and after the soldering. The dimensional stability rules apply to the whole build.

The third item is the copper balance on each layer, which affects the flow and the thickness of the adjacent prepreg. A layer with a large area of copper and a layer with almost none will not bond to the same result.

The last item is the material specification, which should name the resin system, the glass style and the thickness rather than a generic description. The shop builds what the drawing says, and a vague drawing is built to a default.

FAQ

What is the difference between prepreg and core? A core is fully cured laminate with copper on both sides. Prepreg is partially cured resin and glass that flows and cures in the press to bond the cores together.

Why does prepreg thickness vary? Because the resin flows and the stack compresses during the press. The supplier quotes a cured thickness for a combination of sheets and a press cycle, not for a single sheet.

Does the glass weave affect impedance? It does on a very fine line, because the dielectric constant is different over a glass bundle and over the gap between bundles. A spread glass reduces the variation.