Choosing A Flex Substrate Or A Rigid Board

The material at the bottom of a board is usually decided by the layout engineer on the basis of what the previous design used. That habit is reasonable for a product that has not changed, and it is a source of avoidable cost and avoidable failure when the mechanical or thermal requirement has moved. The three families in common use behave differently enough that the choice deserves an explicit decision.

This article sets out how to make that decision from the requirement rather than from the precedent, and what each choice implies for the assembly process.

The Three Material Families

The rigid laminate is a woven glass fabric bonded with a resin, and it provides mechanical support, electrical insulation and a surface that can be drilled, plated and soldered with well established processes. The flexible substrate is a thin polymer film with a rolled copper conductor, and it provides the same electrical function while allowing the board to bend and to fold.

The metal core board replaces the insulating base with a metal plate beneath a thin dielectric, and it is chosen when heat has to leave the components through the back of the board. Each family occupies a different position, and the requirements that drive the choice are mechanical, thermal and dimensional rather than electrical in most cases.

Mechanical Requirement Comes First

If the product has a moving part, a hinge, or a shape that has to wrap around another component, the substrate has to bend, and that requirement removes the rigid laminate from consideration regardless of the electrical merits. The flexible substrate allows the board to be folded into a three dimensional form, which removes connectors and the cable assemblies that would otherwise be needed.

The bending requirement also brings its own constraints: a minimum bend radius, a limit on the number of cycles, and a ban on plated through holes and stiffeners inside the bend region. Where only part of the board needs to move, the rigid flex construction keeps the rigid sections for connectors and dense circuitry and uses the flexible sections only as interconnections. That hybrid is usually more expensive than either pure form and is justified when the connector count and the assembly steps it removes outweigh the panel cost.

Thermal Requirement And Material Selection

If a component dissipates enough power that its temperature cannot be held by the copper on a laminate, the material has to conduct heat away, and the metal core board becomes the candidate. The decision is quantitative: a thermal calculation or a measurement on a prototype shows whether the laminate can hold the junction temperature within its limit, and the answer decides whether the metal core is needed.

The metal core brings electrical consequences with it. The base is a conductive plane that changes the effective dielectric constant of the traces above it, so an impedance critical design has to be recalculated rather than copied. The dielectric between the circuit and the metal sets the voltage rating and the isolation, which becomes a safety parameter in equipment that connects to a chassis.

Dimensional Stability And Assembly

A flexible substrate moves during assembly. It is thin enough to sag in a stencil printer, it can be distorted by the vacuum of a placement table, and it needs a carrier to be processed on an automated line. A rigid laminate holds its shape, which is why the tooling for a flex assembly is more elaborate and why the first production run usually costs more than the board itself suggests.

Dimensional stability also matters for the finished assembly. A laminate expands with temperature in a way that is well characterised, while a metal core board moves with the metal and constrains the dielectric bonded to it. Where a component with a different expansion is soldered to the board, the mismatch is carried by the joint, and the temperature cycle test is what reveals whether the joint survives it.

Rigid Flex As A Compromise

The rigid flex construction uses a flexible core with rigid sections laminated on both sides, so the interconnections are made in the flexible layers and the components are mounted on the rigid sections. The electrical performance at high frequency is better than a cable and connector assembly, because the transition is a controlled transmission line rather than a connector interface, and the mechanical reliability is better because there is no mating interface to wear.

The cost is in the fabrication. The stack up has to be designed so that the flexible region survives the lamination cycles, the coverlay has to be opened where the bend occurs, and the panel has to be handled on carriers. Rigid flex is therefore chosen when the space or the reliability requirement cannot be met any other way, rather than as a general improvement. The stack up rules for the rigid sections follow the usual practice described for layer stack up, and the mechanical constraints follow the same reasoning as board outline and mounting design.

Cost And Volume

For a simple two layer digital board the laminate is the cheapest option by a wide margin, and the tooling is minimal. The flexible substrate costs more per unit area and requires carriers and fixtures, and its advantage appears only when the mechanical function it provides would otherwise have to be built from cables and connectors. The metal core board costs more than a laminate and less than a ceramic, and its advantage appears when the thermal requirement cannot be met otherwise.

The right question is therefore not which material is better but which requirement is binding. A design that answers that question explicitly, and that confirms the answer with a measurement or a calculation, avoids both the cost of an unnecessary upgrade and the failure of an inadequate one. The fabrication consequences of each choice are covered in the general discussion of PCB design and fabrication.

Process Control and Verification

On a design of this kind, material selection is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

FAQ

Can a flex substrate be processed on a standard surface mount line? Usually it needs a carrier or a stiffener to be handled, because the material is too thin and too compliant to be printed and placed on its own. The tooling is part of the cost of the choice.

Is a metal core board always better for a power design? Only when the thermal calculation shows that a laminate cannot hold the temperature. It brings electrical and process consequences that are not worth accepting otherwise.

When is rigid flex worth the extra cost? When the assembly would otherwise need several connectors and cable assemblies, or when the reliability of a mating interface is the limiting factor in the product.