Teflon PCB Design Guide

A teflon board, more precisely a board built on a polytetrafluoroethylene laminate, is chosen for its electrical properties. The dielectric constant is low and stable with frequency and the loss is very low, which is what a microwave circuit, an antenna or a very high speed link needs. The same material is soft, it moves when it is heated and its surface does not bond easily to anything, so the fabrication process has to be adapted rather than used as it is.

The Electrical Case

The dielectric constant of a teflon laminate is around two, compared with four or more for a glass reinforced epoxy. A lower dielectric constant means a wider trace for the same impedance, a faster propagation and less capacitance per unit length.

The loss tangent is the property that matters at high frequency. A teflon based material has a loss tangent an order of magnitude below a standard laminate, which is why it is used for a long feed line, a filter or an antenna where a small loss is repeated many times.

The stability of both figures with frequency and with temperature is part of the value. A material whose dielectric constant changes with frequency makes a broadband design difficult, and one whose properties change with temperature shifts the tuning of a filter in service.

The Material and Its Forms

Pure polytetrafluoroethylene is soft and it creeps under load. To make it usable as a laminate it is usually reinforced with a glass fabric or with a ceramic filler, and the reinforcement changes both the mechanical behaviour and the electrical properties.

A glass reinforced material is stronger and easier to handle, with a slightly higher dielectric constant. A ceramic filled material has more uniform properties in all directions and a lower expansion, at a higher cost.

The choice should come from the design requirement. A thin antenna substrate that will not be mechanically loaded can use a soft material, while a multilayer board that will be drilled and pressed needs the reinforcement to keep the layers in registration.

Drilling

The material is soft and abrasive at the same time, because the filler is hard. A standard drill wears quickly and produces a rough hole with a smear of resin on the wall, and the smear has to be removed before plating.

The drilling parameters are gentler than for an epoxy board. The feed rate is lower, the speed is set for the material rather than for the copper and the drill is changed more often, because a worn drill tears the wall rather than cutting it.

A plasma treatment is the usual method for removing the smear and for preparing the wall. The treatment also raises the surface energy, which is what makes the plating adhere to a material that is otherwise almost impossible to bond.

Plating and Bonding

Polytetrafluoroethylene is chemically inert, which is the property that makes it useful and the property that makes plating difficult. A plated through hole requires a surface treatment before the plating, and the treatment has to be applied consistently.

A sodium etch or a plasma process is used to modify the surface. The etch produces a micro rough surface that the plating can key into, and the process window is narrow because an over treated surface becomes weak and an under treated one does not plate.

The same problem appears in lamination. The bond between a teflon layer and the prepreg or the adhesive has to be made with a material that is compatible, and the surface has to be prepared. A bond that looks complete at the edge can be separated in the middle, so the process should be qualified by a peel test rather than by inspection.

Dimensional Stability

The expansion coefficient of an unreinforced teflon material is very high, and it changes more with temperature than an epoxy laminate. A board that is drilled at one temperature and assembled at another will have moved in between.

The movement matters for registration on a multilayer board, where the layers have to line up after pressing, and for a design with a tight tolerance on a feature position. The stackup should be arranged so that the critical features are placed with the movement in mind.

The material also creeps under a sustained load. A connector that applies a compression force to the board and a screw that is tightened into the laminate both produce a slow movement that appears as a loss of contact or as a cracked joint.

Multilayer Stackups

A multilayer board in teflon material is built with a bonding layer between the core layers, because the core alone will not fuse. The bonding layer is usually a lower melting point material, and its dielectric properties and its thickness both have to be included in the electrical design.

The pressing cycle is different from an epoxy board. The temperature, the pressure and the dwell all have to suit the bonding material, and a cycle developed for epoxy will produce a board with voids or with a distorted registration.

The layer count should be kept to what the circuit needs. Each additional lamination step adds a chance for the registration to drift on a material that moves more than the usual laminate.

Design Rules for the Layout

The trace width for a given impedance is wider on a teflon substrate than on an epoxy one because the dielectric constant is lower. A design transferred from an epoxy stack will have the wrong impedance until the geometry is recalculated.

The thermal relief and the pad sizes should account for the lower stiffness of the material. A pad that is pulled by a heavy component or by a connector lead stresses the laminate more than on a rigid board.

The ground structure is as important as on any high frequency board. Since the material is chosen for the electrical performance, the design should also provide a continuous reference plane and a controlled via transition, because a good material with a poor layout produces a poor result.

Assembly Considerations

The board is more sensitive to the assembly process than an epoxy board. The reflow profile has to be within the material rating, the bake before assembly has to be controlled because the material absorbs very little moisture but tolerates less heat, and the handling has to avoid a permanent deformation.

A soldering iron applied directly to a teflon board can deform it. The repair should be done with a controlled temperature and a short contact, or with a method that heats the joint rather than the board.

The enclosure and the mounting should not clamp the board tightly. A mounting scheme that applies a steady compression will produce the creep that the design was meant to avoid.

Practical Rules

Recalculate the geometry for the dielectric constant rather than reusing a stack, treat the drilling and the plating as material specific processes and control the bake and the reflow. Avoid a sustained compression on the board.

Record the material and the process data with the build records and the PTFE property data, and review the laminate properties and the high frequency laminate guide when the stack is selected.

FAQ

Why is a teflon board difficult to plate? The material is chemically inert, so plating does not adhere without a surface treatment such as a plasma or a sodium etch, and the process window for the treatment is narrow.

What changes in the layout? The trace geometry. A lower dielectric constant gives a different width for the same impedance, so the stack has to be recalculated rather than reused.

Why is dimensional stability an issue? The material expands more with temperature and creeps under load, which affects layer registration and any joint that is held under compression.