Glass Fabric Effect on PCB Properties

A glass reinforced laminate is a composite of glass fabric and resin, and the fabric is not a passive filler. The style of the weave, the thickness of the yarn and the proportion of resin determine the dielectric constant, the mechanical stiffness, the drilling behaviour and the way the material moves during lamination. Two laminates with the same transition temperature and the same thickness can behave quite differently if their fabric is different.

The Fabric and Its Styles

The fabric is woven from yarns of glass filament, and the weave pattern sets how the yarns cross. A plain weave has the tightest interlacing and the most crimp, while a twill or a satin weave has longer floats and a flatter surface.

The style number describes the yarn and the weave together. A common style has a defined yarn count in both directions and a defined weight per unit area, and the number is the reference that a fabricator uses when a stack is specified.

The weave matters for the surface. A laminate made with a coarse weave has a surface that follows the yarn, and a fine trace printed over it will see a dielectric that varies along its length.

The Weave and the Dielectric Constant

The glass filament has a dielectric constant around six, while the resin is closer to three. The effective constant of the laminate is a weighted combination, and at a scale comparable to the weave the weighting varies from point to point.

A trace that runs along a bundle of yarns sees more glass than one that crosses between them, so the effective constant and therefore the impedance varies with the position of the trace relative to the weave. The effect is small at low frequency and significant for a very high speed differential pair.

The remedies are a finer fabric, a spread or a flattened yarn, or a design that avoids routing a critical pair along the direction of the weave. The last of these is a layout decision that costs nothing.

Resin Content and Its Effects

The resin content is the proportion of the laminate that is not glass. A higher resin content lowers the effective dielectric constant, because the resin has a lower constant than the glass, and it also lowers the mechanical stiffness.

The resin content also sets the flow during lamination. A prepreg with more resin flows further and fills the cavities around the copper, while one with less resin may leave a void beside a heavy copper feature.

The content should be specified as a range, and the range should be tight enough that the electrical and the mechanical properties are repeatable. A wide range allows the supplier to deliver a material that meets the thickness but not the electrical intent.

Thickness and the Dielectric Layer

The dielectric thickness between two copper layers is the sum of the prepreg layers and the resin that has flowed into the copper features. A heavy copper layer reduces the effective dielectric thickness, because the resin fills the space between the traces.

This is why a stack that is calculated from the nominal prepreg thickness can measure differently on the finished board. The calculation should account for the copper weight and for the flow, and the coupon measurement is what confirms it.

The thickness tolerance also affects the impedance directly. A variation of a few percent in the dielectric thickness produces a comparable variation in the impedance, which is why a tight impedance specification drives a tighter lamination tolerance.

Drilling and the Fabric

The drill passes through glass and resin alternately, and the two materials cut differently. A drill that is set for the resin will chip the glass, and a drill that is set for the glass will overheat the resin.

The result is a hole wall with a rough profile and with resin smear where the heat has softened the resin. The smear has to be removed before plating, and the roughness affects the plating adhesion and the reliability of the barrel.

The yarn size matters as well. A coarse fabric produces larger chips and a more uneven wall, and a fine fabric with a high resin content gives a smoother hole at the cost of a lower stiffness.

Registration and Dimensional Behaviour

The fabric restrains the material in the direction of the yarns more than across them. A laminate therefore moves differently in the warp and the weft directions during lamination, which produces a distortion that is not uniform.

The distortion matters for a multilayer board, where the layers have to line up after pressing. The fabricator compensates by scaling the artwork, and the compensation should be based on the measurement of the actual material rather than on a general rule.

The glass style also affects the movement during drilling and during thermal cycling. A material with a coarse weave and a low resin content moves less but is more prone to the drill wandering along a yarn bundle.

Specifying the Material

A laminate specification should state the glass style, the resin content range, the thickness and the electrical properties, and it should state the method for each. A specification that names only the thickness leaves the rest to the supplier.

Where a design is sensitive to the weave, a spread glass or a specific style should be called out. The cost difference is modest and it removes a source of variation that is otherwise invisible until a link fails on one board in a batch.

The specification should also cover the copper foil. The roughness of the foil depends on the adhesion treatment, which depends on the resin system, so a change of laminate can force a change of foil and therefore a change in the conductor loss.

Verification

The electrical properties should be verified on a coupon. A measurement of the dielectric constant on a resonator or on a known transmission line gives the effective value for the stack, which is what the design uses.

The thickness should be verified by a microsection at several positions on the panel, because the flow varies with the copper density and the thickness is not uniform across a board.

The weave effect is more difficult to verify. A design that is sensitive to it should be evaluated by measuring several traces at different positions, and the variation between them is the evidence that the fabric is affecting the result.

Practical Rules

State the glass style, the resin content and the foil, and verify the dielectric constant and the thickness on a coupon. Avoid routing a critical pair along the weave where the design allows it.

Record the material and the measurement data with the build records and the laminate properties, and review the impedance stackup design and the resin content control when a stack is specified.

FAQ

Why does the weave affect impedance? The glass has a higher dielectric constant than the resin, so a trace over a yarn bundle sees a different effective dielectric constant from one that runs between the bundles.

What does a higher resin content do? It lowers the effective dielectric constant and the stiffness, and it increases the flow during lamination, which fills the cavities around heavy copper more completely.

How is the stack verified? By a coupon that measures the dielectric constant and by a microsection at several positions, because the resin flow varies with the local copper density.