Glass Fabric Selection and Its Effect on PCB Laminates

A laminate is a sandwich of glass fabric and cured resin, and the fabric is the part that most designers never consider. The weave style, the yarn count and the amount of resin around the glass determine the thickness, the mechanical properties, the drillability and, at high frequency, the electrical behaviour of the material. Glass fabric selection is therefore a real part of laminate specification rather than a detail that the mill decides.

What the Glass Fabric Does

The fiberglass fabric provides the mechanical strength and the dimensional stability of the laminate, and in most constructions it is the component that carries the load. It carries the load in the plane of the board, restrains the resin from expanding too much and gives the material the stiffness that allows a thin panel to be handled and processed through the assembly line.

It also sets the thermal limit in a practical sense. The glass itself withstands far higher temperatures than the resin, but the bond between the two determines how the composite behaves when it is heated. A poorly coupled interface fails before either component reaches its own limit, which is why the resin system and the fabric finish are specified together rather than separately.

Weave Styles and Yarn Counts

Glass fabric is woven from yarns, and the weave style describes how those yarns cross each other. A plain weave is the simplest and the most common, while a twill weave produces a flatter surface and less crimp in the yarn, which reduces the thickness for a given weight.

The yarn count determines how many threads run per unit length in each direction, and that figure controls the openness of the weave and the size of the gaps between the yarn bundles. A dense weave gives a smoother surface and a more uniform dielectric behaviour, while an open weave allows more resin to collect between the bundles. The openness also affects how the laminate absorbs moisture and how the drill behaves as it passes through alternating glass and resin.

Resin Content and Its Consequences

Resin content is the proportion of the laminate that is not glass, and it is one of the most influential figures in the material specification. A high resin content gives a smoother surface, better flow during lamination and easier drilling, but it also reduces the mechanical strength and increases the moisture absorption of the material.

A low resin content increases the strength and reduces the moisture uptake, but it makes the laminate harder to drill and can leave the glass more exposed at the surface. The correct value is a balance that depends on the application, and it is normally specified with a tolerance because the two extremes behave quite differently.

Thickness and Layer Count

The laminate thickness is set by the fabric and by the resin content of each sheet, so choosing a thinner dielectric means choosing a lighter fabric rather than simply pressing the same material harder. That is why a stack-up that requires a very thin dielectric also requires a specific fabric style, and why a late change of dielectric thickness usually means a change of material rather than a change of the press recipe.

Where several prepreg sheets are used to build a single dielectric layer, the weave of each sheet can align or offset. Aligned sheets produce a more pronounced periodic variation in the dielectric constant, while offset sheets average it out, and the difference is visible in the impedance of a line that runs across the weave. The stacking order should therefore be stated on the drawing rather than left to the shop.

Electrical Effects of the Weave

At high frequency the fabric is not invisible. The glass has a higher dielectric constant than the resin, so a trace that runs along a glass bundle sees a different value from one that runs between bundles. The effect appears as a periodic variation in impedance along the trace, and it is sometimes described as a weave effect because it repeats at the spacing of the yarn bundles.

The severity depends on the weave density relative to the trace width, and on the direction of the trace relative to the yarn. Fine fabrics and wider traces reduce the effect, and rotating the trace relative to the weave is sometimes used as a mitigation. The subject belongs with the other high frequency considerations described in the guide to high frequency laminates.

Mechanical Effects and Drillability

The fabric determines how the laminate behaves when it is drilled. Dense, fine fabrics drill cleanly and produce a smooth hole wall, while heavy fabrics can leave protruding glass fibres that interfere with plating and with the desmear process. The fibres also change the way the hole wall accepts the electroless copper that follows.

Drillability also depends on the resin content around the glass, because the resin lubricates the drill and carries heat away. A material with a low resin content and a heavy fabric is therefore the most difficult combination, and it may require a different drill bit and a different parameter set.

Supply and Availability

Fabric styles are not equally available everywhere, and a stack-up that depends on an unusual weave may have a long lead time. Standard styles are stocked by most laminate suppliers, while a specialty fabric may have to be woven to order, which adds weeks to the lead time of the first build.

Availability should therefore be checked before the stack is fixed, particularly for a product with a long life. A material that is available for the first build but not for the fifth will force a requalification, which is more expensive than choosing a slightly less ideal fabric at the beginning.

Specifying the Fabric

The specification should name the fabric style, the resin content range, the thickness and the glass transition temperature of the system. Where the electrical performance matters, it should also state the dielectric constant and the loss tangent with the frequency at which they apply.

The specification should be attached to the stack-up drawing so that the fabric and the dielectric thickness are considered together rather than independently. The checks that should be completed before the stack is released are listed in the guide to the fabrication notes checklist.

Verification and Records

Verification is normally done on a coupon or on a sample of the delivered laminate, and it covers the thickness, the resin content and, where required, the electrical properties. The results should be recorded with the lot so that a later investigation has the material data it needs.

Where a product is built over several years, the records also show whether the material has changed. Suppliers revise formulations, and a change in the fabric or the resin can alter the properties without any change to the part number, so the material certificate for each lot is worth keeping with the board records. The comparison of materials is described in the guide to laminate material properties.

FAQ

Does the glass fabric matter at low frequency? Its main effects are mechanical, so at low frequency the fabric choice is driven by thickness, strength and drillability. The electrical effect becomes significant at high frequency, where the periodic variation in dielectric constant along a trace starts to matter.

What does resin content change? A higher resin content improves surface smoothness and flow but reduces strength and increases moisture uptake. A lower resin content does the opposite, and makes the material harder to drill and more prone to protruding glass.

Can the fabric be changed without requalifying the board? Usually not. A change of weave or resin content alters the thickness, the dielectric constant and the drilling behaviour, so it should be treated as a material change and requalified accordingly.