PCB Stiffener Selection And Bonding

A stiffener is a piece of rigid material bonded to a thin or flexible board to control its shape, to support a connector or a component, and to keep the assembly flat enough for the process and for the product. It is common on flex and rigid flex assemblies, and on thin rigid boards that carry a heavy part.

This article covers how the material is chosen, how the bond is made, and what the stiffener does to the rest of the design.

Why A Stiffener Is Needed

A thin board is flexible, and flexibility is a problem in three places. It is a problem at a connector, because the mating force bends the board and the contacts do not align. It is a problem at a component that has to be soldered flat, because the board moves under the placement nozzle and during reflow. And it is a problem at any surface that has to be flat during handling, because a board that flexes under an operator’s fingers damages its own joints.

A stiffener solves all three by adding mechanical support in the form of local rigidity. It does not make the whole board stiff; it makes one region stiff, and the region is defined by where the mechanical load is applied. That is why a stiffener is normally local rather than covering the whole assembly, and why its extent is decided by the mechanical requirement rather than by the available space. The general mechanical arrangement of a board is covered under board outline and mounting design.

Material Choice

FR-4 is the most common stiffener material because it is the same as the board, so its expansion matches and it can be processed on the same equipment. It is bonded with a prepreg or an adhesive film, and the assembly then behaves as a thicker board in that region. Aluminium and stainless steel give more rigidity for a given thickness, which matters where space is tight, and they conduct heat, which can be useful under a power device.

Metal stiffeners bring two complications. Their thermal expansion coefficient differs from the laminate, so a large bonded area develops shear stress as the assembly heats, and the bond has to accommodate it. And they are conductive, so they must be isolated from any conductor unless a ground connection is intended. Where the stiffener has to be both rigid and thermally compatible, a filled polymer composite or a ceramic filled laminate is used instead, at a higher cost. The expansion of the underlying laminate is described under PCB dimensional stability and expansion.

Bonding The Stiffener

The bond is made with a prepreg sheet, an adhesive film or a liquid adhesive, and it is cured under pressure and temperature in a press or with a vacuum bag. A prepreg bond is the most common because it uses the same lamination process as the rest of the board, and it gives a bond whose thickness is defined by the resin content of the sheet. An adhesive film is used where the temperature of a full lamination would damage an assembled part.

Two features of the bond decide whether the stiffener does its job. The first is coverage: a bond that is complete over the whole footprint transfers the load evenly, while a bond with voids or a partial coating lets the stiffener flex and eventually separate. The second is the flatness of the parts before bonding, because a press cannot flatten a bowed stiffener, it can only bond the bow in place. Where flatness is critical, the stiffener is specified with its own flatness tolerance and the bonded assembly is checked afterwards.

Stiffener Geometry And Clearances

The outline of the stiffener has to respect the component placement and the routing. A metal stiffener under a connector needs an opening for the leads and for the solder joints, and the opening has to be large enough that the stiffener does not touch a joint or short a lead. The stiffener also has to clear any cut-out in the board, and its edge is normally placed away from a bend region on a flex, because a stiffener that ends at a bend concentrates the bending at its edge.

On a rigid flex assembly the stiffener is normally placed where the flex meets the rigid section or where a connector is mounted, and its edge defines the transition. The transition should be a gradual change in stiffness rather than an abrupt one; a stiffener with a square edge bonded to a thin flex creates a hard line where the flex cracks under repeated bending. A taper, a chamfer or a soft adhesive at the edge spreads the transition. The overall assembly rules that bound these choices are collected under design guidelines for manufacturability.

Effect On Assembly And Test

A stiffener changes the assembly process in ways that are easy to overlook. It adds thickness, so the board may no longer fit the conveyor rails or the tooling that holds it, and a fixture that was designed for the bare board may not close. It changes the thermal mass, so the profile has to be re-measured where the stiffener is present. And it may obstruct a test probe, so the test point access has to be planned around it.

The stiffener also has to survive the processes it passes through. A polymer stiffener bonded with prepreg goes through reflow with the rest of the board and must not delaminate; a metal stiffener with an adhesive film may be bonded after reflow to avoid the thermal exposure, which changes the sequence of operations. Deciding the sequence early, and recording it, prevents the situation in which the stiffener is bonded at a step that damages an already assembled board.

Specifying And Verifying

The drawing should state the stiffener material, its thickness, its outline and any opening with tolerances, the bond material, the flatness requirement for the finished assembly, and whether the stiffener is electrically connected or isolated. Where the stiffener is metal and isolated, the isolation feature has to be specified, because an exposed metal edge next to a conductor will eventually contact it.

Verification is by flatness measurement on a sample, by a peel or shear test on a bonded coupon, and by visual inspection around the edges of the stiffener for adhesive that has flowed onto a pad or a joint. The most common defect is adhesive squeeze-out onto a pad or into a connector opening, and it is invisible from the top of the stiffener, so the inspection has to be done around the perimeter and where the stiffener meets the board.

FAQ

Can a stiffener be added after assembly? It can where the bond can be cured at a temperature the assembly tolerates. A stiffener bonded before assembly goes through reflow with the board, which is simpler but exposes the bond to more thermal cycles.

Does a metal stiffener need to be grounded? It does not have to be, but it must be isolated from every net if it is not grounded. A floating metal plate close to a circuit can couple to it, so the decision should be deliberate.

How thick should a stiffener be? It is set by the rigidity the region needs, which comes from the mechanical load and the span. The stiffness rises with the cube of the thickness, so a small increase in thickness is usually more effective than a change of material.