Design Guidance

2022-10-27 BEST

Stiffeners/Reinforcements & Tear Guards

Stiffener/Reinforcements

Single sided, Double sided, and Multi-layer Flex circuits  can be stiffened in specific area’s by adding rigid material. This material can add support for mounting components, increasing strength, thickness and rigidity. The thickness of flex legs can be adjusted for component needs as well, such as the “Ziff” end of a flexible circuit. Kapton (Polyimide), FR4 and stainless steel materials are commonly used for stiffener(reinforcements), these materials can be attached with thermally cured acrylic adhesive or pressure sensitive adhesive (PSA). Stiffeners (Reinforcements) should overlap bared coverlay by .030” to relief stress.

l  Standard FR-4 material thicknesses range from 0.006" (0.15mm) to 0.125" (3.18mm). Typical thickness for Polyimide stiffeners is 0.005" (125μm), but 0.001" (25μm), 0.002" (50μm), and 0.003" (75μm) are also available, Polyimide stiffeners are less expensive than FR-4 stiffeners because they are punched on a die instead of routed with a drill machine. The Polyimide stiffener lay-up procedure is performed with alignment pins, therefore the registration is better. The Polyimide stiffeners are trimmed with the cover on the final blanking procedure, which guarantees perfect outside alignment.

 

Stress Point 30 Mils Overlap


l Stiffeners can be utilized to force a bend line in selected areas. BEST TECH can provide epoxy fillets for the edges of the FR-4 stiffener, where flexing occurs. 

l Stiffeners reinforce solder joints and increase abrasion resistance.

l Circuits may be attached to a stiffener pallet (multiple parts) to provide easier handling for automated pick-and-place and component soldering. Circuits can be held together for processing on the pallet, then DE-panelled (clipped free) after wave soldering and circuit testing.

l Stiffeners can be silk-screened with component mounting locations for rapid assembly.

l When using multiple stiffeners, maintaining the same stiffener thickness consistent throughout the entire construction will help keep costs under control.

Stiffeners (Reinforcements) Considerations

• Maintain the same stiffeners (reinforcements) thickness when using multiple reinforcements to lower cost.

• Stiffeners (Reinforcements) should come to at least two edges of the board

• Stiffeners (Reinforcements) reinforce solder joints and increase abrasion resistance.

• Stiffeners (Reinforcements) can be used for strain relief and heat dissipation.

For the stiffeners boards, there are three ways in the end of the coverlay and the bend line at P used:

 

Stiffeners Considerations


Stiffeners/Reinforcements are added to flexible circuitry to provide strain relief when attaching connectors and/or other terminating devices.

There are two(2) types of stiffening methods used:

1)Use of Rigid material with “pre-casted” adhesive (usually acrylic adhesive).

2)Use of Adhesive Coated Kapton material.

Type “1” is the most common method. It provides a highly rigidized surface for component attachment. Since the flexible circuit can no longer bend in the solder joint area, the stress on the solder joints is thus eliminated. Typically the holes in the stiffener should be larger than the circuit pad to prevent damage to the pads during lamination and/or adhesive squeeze into the holes.

This material is available in various thicknesses from .005” to .125” with .031” being the most common thickness used. Type GF material is typical used; with GI used in high temperature applications.

Hard Tooling such as Lamination Fixtures, Rout Fixtures, NC Drill and Rout Programs are required to bond Stiffeners. Therefore, common tooling holes should be incorporated into the Flexible Circuit and Stiffener for this purpose only.

Example of flexible circuit with “rigidized” stiffener

Rigidized Stiffener

Type “2” is used when only a slight additional stiffness is needed. This material is typically 5 mil Kapton coated with Acrylic Adhesive. They can be applied to both sides of the flexible circuit. This method although not nearly as effective as rigidized stiffeners are generally less costly to apply than using rigidized stiffeners since they are attached when the coverlay is applied and do not require the tooling cost.

Example of flexible circuit with “Adhesive Coated Kapton” reinforcement

Adhesive Coated Kapton Reinforcement

 

Tear Guards

We recommend the use of Tear Guards which will help to reinforce the flex material along inside bend radius’s. This will help prevent tearing of the flex material. Avoid any discontinuation of materials close to bend area’s and try to use a liberal bend radius avoiding sharp corners.

 

Avoiding Sharp Corners

                                                                                                                Tear Stops

 

possible-tear-point tear-stops.jpg

 

All inside corners should have a radius to prevent tearing. A “tear stop” which is an extra piece of copper will further will help stop a tear from entering the conductive path area.ca