Resin Content Control in Laminates

Resin content is the fraction of a laminate that is not glass fabric, expressed as a percentage by weight or by volume. It sounds like a manufacturing detail, but it is the variable that sets the effective dielectric constant, the flow during lamination, the finished thickness and the drilling behaviour all at once. A stack that is specified by thickness alone leaves the resin content to chance, and the electrical result follows.

What the Figure Describes

A prepreg is a glass fabric that has been impregnated with resin and dried to a partly cured state. The resin content is the proportion of the weight that is resin, and the remainder is the fabric.

The figure is quoted for the prepreg as supplied and it changes during lamination, because resin flows out of the prepreg and into the spaces around the copper features. The content of the finished laminate is therefore lower than the content of the prepreg where the flow has carried resin away.

The distinction matters when a stack is being calculated. A design that uses the prepreg figure for the final dielectric thickness will overestimate the thickness in an area with heavy copper, because some of the resin has filled the space between the traces.

Effect on the Dielectric Constant

Glass has a dielectric constant around six and a typical resin around three, so the effective constant of the laminate is a weighted average of the two. A higher resin content gives a lower constant and therefore a faster propagation and a wider trace for a given impedance.

The relationship is not linear with weight, because the two materials have different densities. The mixing rule that is used should be the one the supplier quotes, and the comparison between two materials should be made on measured data rather than on the theoretical curve.

The sensitivity is modest but not negligible. A change of a few percent in the resin content moves the dielectric constant by a fraction of a percent, which is enough to matter for a tight impedance tolerance or for a narrow band filter.

Effect on Flow and Filling

Resin flows during the press cycle and fills the gaps between copper traces and around heavy features. A prepreg with more resin has more material available to flow, which reduces the risk of a void beside a heavy copper plane.

Too much flow is also a problem. Resin that travels far leaves a starved area behind it and produces a thin dielectric at the edge of a large copper area, and the thickness variation changes the impedance across the board.

The flow is controlled by the resin content, the press cycle and the copper distribution. The copper balance is therefore part of the thickness control, which is why thieving is used around a sparse area.

Effect on the Finished Thickness

The finished dielectric thickness between two layers is the sum of the prepreg layers after flow. A prepreg that is 0.1 mm thick with a low resin content will end up thinner after pressing than one with a high resin content, because the resin has moved into the copper features.

Because the thickness sets the impedance, a change in the resin content of a stack that is otherwise identical will move the impedance. The effect is usually small, but it is the reason two fabricators can deliver different results from the same drawn stack.

The thickness should be verified on a section rather than assumed, and the measurement should be taken at a position that represents the average copper density rather than over a large plane or in an empty area.

Effect on the Drilling

A laminate with a higher resin content is softer and easier to drill, and it produces less tool wear. The trade is a lower stiffness and a greater tendency for the resin to smear during drilling.

A laminate with a lower resin content is harder and more abrasive. The drilling generates more heat, the tool wears faster and the hole wall is more likely to be rough where the drill meets a yarn bundle.

The desmear process has to suit the material. A chemistry that removes the smear from a high resin material may attack the glass in a low resin one, and the process should be established on the actual laminate rather than on a general recipe.

Effect on Mechanical Properties

Resin is more compliant than glass, so a higher resin content gives a softer laminate with a higher expansion and a lower modulus. That is an advantage in a thin board that has to bend and a disadvantage in one that has to stay flat.

The creep behaviour also depends on the resin content. A laminate with more resin creeps more under a sustained load, which matters for a connector that applies pressure or a screw that is tightened into the board.

The glass transition temperature is a property of the resin system rather than of the content, but the content changes how the transition affects the whole laminate, because more resin means more material whose expansion changes at that temperature.

Specification and Control

A prepreg specification should state the resin content as a range with the test method, the resin flow, the gel time and the volatile content. Those figures together describe the material’s behaviour during lamination.

The thickness of the prepreg should also be specified, and the two figures interact: the same resin content with a different fabric weight gives a different thickness. A specification that names only the thickness permits a range of resin contents.

The purchase specification should require notification of a change of resin content or of fabric, because either can move the electrical properties without any change to the drawing. The change should be treated as a material change and requalified.

Verification

The finished laminate can be checked by a microsection, which shows the resin rich areas, the glass distribution and the thickness. The section should be taken from a position that represents the copper density of the product.

The dielectric constant is measured on a coupon as an effective value for the stack. A change from the expected value points at either the thickness or the resin content, and the section distinguishes the two.

The drill quality is verified on a sample by a section and by a thermal stress test. A barrel that passes the plating thickness check can still fail the stress test if the hole wall is rough or the resin has smeared.

Practical Rules

Specify the resin content and the flow together with the thickness, verify the dielectric constant on a coupon and the thickness on a section. Treat a change of resin content as a material change.

Record the specification and the measurements with the build records and the laminate properties, and review the glass fabric effect and the impedance stackup design when a stack is calculated.

FAQ

Why does resin content change the dielectric constant? The laminate is a mixture of glass and resin, which have different dielectric constants. A higher resin content lowers the effective value for the composite.

How does it affect thickness? Resin flows into the spaces around copper features during pressing. A prepreg with more resin ends up thinner after lamination for the same starting thickness.

Why treat a resin content change as a material change? It moves the dielectric constant, the flow and the drilling behaviour at once, so a stack that was qualified with one content cannot be assumed to perform the same with another.