By flex PCBs
In flex PCBs, the pads and traces need special care to accommodate for torsional stresses. For example, the copper needs to be thicker and more rugged than in rigid-circuit boards. This allows for the copper to stretch further before fatigue cracking occurs, which means a longer life for flex circuits. Additionally, pads should be designed with an offset between the top and bottom of the pad to prevent a solder joint from detaching from the underlying copper in a dynamic bend area.
The design process also has a large impact on how well a flex circuit will hold up in the field. It’s important to determine up front if the flex circuit sections are expected to be static or to be used in a dynamic bending environment, as this will affect layer count and stack-up options. For instance, if the flex section is going to be in a fixed position – like in a handheld ultrasound device – you can use lower layers and more conductive material (both copper and dielectric). If the flex sections will be continuously bending or moving then you’ll want to consider adding stiffeners that are made of rigid materials such as FR4 or metal (PCBWay has a detailed calculator on how this will affect the manufacturing cost).
Stiffeners can be placed in selected areas on the flex pcb to provide mechanical stability. They should be positioned in the areas that are expected to experience high amounts of stress or deflection. The stiffener material should be a different dielectric than the rest of the flex section and should be laminated to a flex substrate. The most common choice is polyimide which provides superior flexibility and strength compared to rigid-board materials.
How do flex PCBs accommodate for torsional stresses?
Another key factor is to avoid sharp angles when routing signals. This is because they can cause a compression strain that leads to i-beam effect and cracking. To counter this, it’s best to stagger traces and pads when possible. Also, opt for rounded corners instead of sharp ones since this will reduce stress concentration and improve material strength.
Copper is infamous for work-hardening and fatigue. If a flex circuit will be subjected to repeated creasing or movement then the choice of copper should be higher-grade Rolled Annealed (RA) foil. This allows the copper to stretch more before fatigue cracking and is softer in the z-deflection direction – exactly what you’ll need for a flex circuit that will be constantly bending.
Another tip is to keep the number of plated through holes to a minimum. This will reduce the amount of copper that is exposed and will improve overall reliability. In addition, the flex section should have plated copper anchoring stubs and reduced coverlay access openings. This can be accomplished using the CAD software by placing an opening in the solder mask layer to define the coverlay at the stubs (similar to how it’s done for vias). This will anchor the stubs and help them stay put during reflow, and also reduce the amount of solder that may get funnelled into the flex zone when a component is inserted.