Laminate Selection: Glass Transition Temperature And Loss

The laminate under the copper determines how the board behaves when it is heated, how fast a signal travels along a trace, how much of that signal is lost on the way, and how well a plated hole survives a lead free reflow. Selecting it is therefore an engineering decision that has to be made with the process and the product in view, not a default choice of the material the fabricator happens to stock.

This article explains the thermal, electrical, and mechanical properties that matter, how they are traded against one another, and how the choice is documented and verified.

The Thermal Properties

The glass transition temperature is the point at which the resin changes from a glassy solid to a softer material. Below it the laminate is rigid and its expansion is modest, and above it the expansion rises sharply and the mechanical properties fall. A standard laminate has a transition around one hundred and thirty to one hundred and forty degrees, a mid grade material between one hundred and fifty and one hundred and seventy, and a high grade above that, and the lead free reflow temperature is well above the transition of the lower grades.

The transition temperature is not the only thermal limit. The decomposition temperature, at which the material loses a defined fraction of its weight, matters for the same reason, and the CTE above the transition, which is the z axis expansion, is what stresses a plated hole during reflow. A thick board with a low transition temperature can crack the copper in its holes during a single lead free excursion, which is why the layer count and the material grade are chosen together.

The expansion is quoted in three directions for a reason. The x and y expansion is close to that of copper, which is what allows a heavy copper layer to stay attached through thermal cycling, while the z expansion is several times larger and acts directly on the barrel of every plated hole. A board that is thick in the z direction accumulates more expansion than a thin one, so a twenty layer backplane is a much harder case than a four layer control board of the same material.

The Electrical Properties

The dielectric constant sets the impedance of a controlled line and the velocity at which a signal travels along it. A higher constant means a slower signal, a shorter wavelength for a given frequency, and a lower impedance for the same geometry, and it also makes the line more sensitive to the uncertainty in the constant. A standard laminate sits around four, and it varies with the resin content, with the frequency, and with the amount of moisture the material has absorbed.

The loss tangent sets the attenuation at high frequency, and it is the property that separates the material grades for high speed work. A standard laminate has a loss tangent around two hundredths, a mid loss material around one hundredth, and the low loss and ultra low loss grades below that. The loss rises with frequency, so a material that is adequate at one gigabit per second may be marginal at ten, and the total loss depends on the length of the channel as well as on the material.

Mechanical And Process Properties

Copper peel strength determines how well a trace or a pad resists being lifted, during assembly, during rework, and in service, and it depends on the resin chemistry and on the treatment of the copper foil. The dimensional stability of the material during lamination affects the registration of the layers, which matters most on a high layer count board with small features, and the resin content and flow determine whether a thick stack can be laminated without resin starvation.

Two further properties are specific to particular applications. Resistance to conductive anodic filament growth, which is the migration of copper along a fibre path between two plated holes under bias and humidity, is a requirement for a dense board in a humid environment and is tested on a coupon that is biased in a humidity chamber. The comparative tracking index, which describes how well the surface resists tracking under a high voltage, is a requirement for a power board. Both are properties of the material rather than of the layout, although the layout can make them worse.

How The Choice Is Made

The choice is a matrix rather than a single number. A consumer product with a lead free process needs a material with a transition temperature high enough to survive the reflow and the subsequent thermal cycles, which in practice means the mid grade rather than the standard one. A high speed channel needs a low loss material, and the number of layers, the trace length, and the bit rate decide whether a mid loss grade is sufficient or whether the low loss grade is required.

An automotive or industrial product adds requirements that are not visible in the datasheet headline: resistance to conductive anodic filament growth, a wider operating temperature range, and a stable dielectric constant over temperature. A radio frequency design may need a specialised material with a very low constant and a very low loss, such as a filled polytetrafluoroethylene or a ceramic filled laminate, which behaves differently in fabrication and costs considerably more.

Cost, Availability And Consistency

The price of the material rises steeply with the electrical grade, and the higher grades are available in fewer thicknesses and with longer lead times. Some of them also require different lamination conditions, which changes the press cycle and the panel handling, and a fabricator who is not familiar with the material may need to qualify it before production. A design that specifies a low loss material for a short, low speed connection therefore pays for a property it does not use.

Consistency matters as much as the initial choice. The dielectric constant of a laminate varies from lot to lot within a tolerance that the supplier quotes, and a controlled impedance design has to allow for that variation rather than assume a nominal value. Where the material is critical, the supplier and the grade should be named on the drawing, and a change of supplier should be treated as a change of design. The layer arrangement that the material sits in is described under layer stack up from one to eight layers, the high speed consequences under multilayer advantages at high speed, and the fabrication sequence under PCB design and fabrication.

FAQ

Is a higher glass transition temperature always better? It is better for a lead free process and for a thick board, because it reduces the z axis expansion and the stress on the plated holes. For a simple board built with a low temperature alloy it adds cost without a benefit.

How do I know whether a low loss material is needed? From the channel length, the data rate, and the loss budget. A short connection at a moderate rate is served by a standard material, while a long backplane channel needs a low loss grade.

Does moisture affect the electrical properties? It does. Absorbed water raises the dielectric constant and the loss, which shifts the impedance and increases the attenuation, and it also lowers the transition temperature of the resin.