High Tg Material Selection Guide

A high tg laminate is one whose glass transition temperature is well above the ordinary range. Above that temperature the resin stops behaving as a glass and becomes more compliant, so its expansion rises sharply and its mechanical stiffness falls. A material with a higher transition temperature keeps its rigid behaviour over a wider temperature range, and that is what makes it useful for a board that sees repeated thermal cycles or a lead free assembly process.

What the Glass Transition Temperature Means

The glass transition temperature is the point at which the resin changes from a glassy to a rubbery state. Below it the material is stiff and its expansion is low, and above it the expansion rises by a factor of two or three.

The transition is not a sharp melting point. The property changes gradually over a range, and the figure quoted on a datasheet is obtained by a specific method, so two materials measured by different methods cannot be compared directly.

The importance of the figure is what happens at assembly and in service. A board that is heated above its transition temperature during reflow will expand more, and the plated holes and the copper will be stressed by that expansion.

Why Lead Free Changed the Requirement

A lead free reflow profile reaches a higher peak and holds it for longer than a tin lead profile. The board therefore spends more time at a temperature that is close to or above the transition point of a standard material.

The consequence is a higher expansion in the z direction during the process. The copper in a plated hole does not expand at the same rate, so the barrel is strained, and the strain is cumulative with each reflow pass.

A high tg material reduces the strain by keeping the resin rigid through more of the profile. It also reduces the movement of the layers relative to each other, which improves the registration of the inner layers after pressing.

The Measurements That Matter

The glass transition temperature is the first figure, and it is usually quoted in the range from 130 to 180 degrees for standard materials and above 170 for the high tg grades.

The decomposition temperature is the second and it is arguably more important. It describes the temperature at which the resin begins to break down chemically, and a material that decomposes at a temperature close to the process peak will be damaged even if its transition temperature is high.

The thermal expansion below and above the transition point is the third. A material with a low expansion above the transition is more tolerant of a lead free profile than one whose expansion rises steeply, even if the two have the same transition temperature.

Other Properties That Travel With It

A high tg material usually has a higher filler content, which lowers its expansion in all directions and improves its dimensional stability. The same filler makes it more brittle and harder to drill.

The moisture absorption is usually lower, which is an advantage for a reflow process because there is less water to be released. The dielectric properties may be slightly different from a standard material, which matters only for a high frequency design.

The drilling behaviour deserves attention. A harder, more highly filled material produces more tool wear and a greater risk of a rough hole wall, and the drilling parameters may need to be changed rather than reused.

When a High Tg Material Is Justified

The requirement appears when the assembly is lead free, when the board is thick, when the layer count is high, when the product has to pass a thermal cycling qualification, or when the board is expected to survive a rework step.

The requirement is weakest for a thin, simple board assembled with a low temperature alloy and used in a benign environment. In that case a standard material will meet the reliability target and the high tg premium buys nothing.

The decision should be tied to a measurement or a qualification rather than to a habit. A board that fails a thermal cycling test with a standard material is evidence, while a board that has never been tested is not.

Stackup and Design Consequences

A high tg material is often used together with a thicker dielectric between the plated hole and the planes, because the two measures address the same strain from different directions.

The via design should account for the expansion of the material in the z direction. A thick board with a high expansion and a small via produces a high strain on the barrel, and the aspect ratio should be kept within the process capability rather than at its limit.

The copper balance is as important as with any material. A layer with a large area of copper and one with very little will laminate differently, and the difference appears as a thickness variation that affects the impedance and the registration.

Qualification and Testing

The material should be qualified with the process that will be used. A thermal cycling test on a coupon that contains the smallest via and the thickest stack is the standard evidence, and a test that uses a thin coupon does not demonstrate the same result.

The tests that reveal a marginal material are the thermal shock and the reflow simulation. A reflow simulation that repeats the profile several times represents the multiple passes that a board with two sided assembly will see.

The results should be recorded with the material batch, because a change of supplier or of resin content can move the properties without any change to the specification.

Suppliers and Documentation

The datasheet should give the transition temperature, the decomposition temperature, the expansion in the z direction, the moisture absorption and the dielectric properties, and it should state the test method for each.

A material that is described as high tg without a figure is not a specification. The figure and the method are what allow two materials to be compared and what allow a batch to be checked against the requirement.

Where a supplier changes the formulation, the datasheet should be reissued, and the change should be treated as a change of material for the purpose of the qualification. A resin content change that moves the decomposition temperature is a different material in every way that matters.

Practical Rules

Base the choice on the process peak and on the expansion above the transition, not on the transition figure alone. Confirm the decomposition temperature and the z axis expansion, and qualify the material with the real stack.

Record the material data with the build records and the laminate properties, and review the dimensional stability data and the hole copper specification when a stack is qualified.

FAQ

What does the glass transition temperature describe? The point at which the resin changes from a glassy to a rubbery state. Above it the expansion rises sharply and the material loses stiffness.

Is a higher transition temperature always better? Not on its own. The decomposition temperature and the expansion above the transition matter as much, and a harder material is more difficult to drill.

How should a high Tg material be qualified? With a thermal cycling or reflow simulation test on a coupon that contains the real stack, the smallest via and the thickest construction.