Wearable/IoT PCB technology is ushering in new design terminology and steps that need to be factored in. Two particular areas that need to be understood deals with (1) low and high modulus boards and (2) flex circuit bending.
Board modulus refers to its structure; low modulus means a softer structure, while high modulus refers to a harder board with stiffener. Fig. 1. shows a stiffener that provides rigidity to the flex circuit for stable soldering, and components are mounted on the opposite side of the stiffener, Fig. 2. Stiffeners are an inexpensive way to rigidize certain areas on the flex boards, such as SMT areas, pin areas or hole pattern locations for component mounting.
SMT areas don’t always need stiffeners depending on the component being installed on the location. However, adding a stiffener is going to add very little cost to the assembly. Stiffeners are used to reinforce solder joints and are sometimes used to enforce bend lines in the selected areas. Stiffeners can be made from FR4, polyimide, copper or aluminum- based materials.
As far as flex circuit bendability, the jury is still out on it. Long story short, the exact definition of bendability hasn’t been pinned down and probably won’t due to its nebulous nature. The best advice given is to rely on an experienced EMS Provider that has several wearable/IoT PCB designs under its belt and has a storehouse of critical nuances associated with flex circuit bendability.
Check out our EE Times article to get more detail on these design aspects as well as others.
In the meantime, here are some terms that’ll get you well on the way to gauging your new wearable/IoT PCB designs.
- Bend radius is how far you can bend that flex circuit before something breaks or incurs a latent fracture.
- The measurement of the bend radius is performed from the bend’s underside surface.
- Bend ratio (not to be confused with bend radius) takes into account the ratio of the bend radius to the thickness of the flex circuit.
- Bend radius is calculated by measuring the distance from the inside surface of the bend to the center of the radius.
- There are two aspects associated with flex circuit bending. One is static or one-time bending; the other is dynamic flex involving multiple times of bending.
- Bend radius for dynamic flex circuitry populated with µBGA packages should be less than 0.8% for 50,000 cycles, less than 0.6% for 100,000 cycles, 0.4% for up to one million cycles, and less than 0.2% for a million cycles or more.
Bend radius, bend ratio, and strains created by the flexing action of the boards are inextricably intertwined. In the case of strains, they’re already built in when the manufacturer produces the flex circuit. In other words, strain is inherent in the different circuit layers and could be mitigated with strain relief devices such as stiffeners.