Today, innovation patterns are progressively towards flex circuits or a combination of rigid flex PCB for IoT/wearable PCB designs. You can claim those fads place us on a different ground, so to speak. For that reason, it’s important to obtain a manage on brand-new design terms and points that should be factored in as you relocate to this following degree of embedded design.
This includes the following:
– Low and high modulus boards
– Bend distance, proportion, and pressures
– Dielectric thickness
– Via placement
– Board layers and associated copper quantities
– Normal copper versus hardened copper
– Copper density
Board modulus describes its construct– a reduced modulus implies a softer structure, while high modulus refers to a harder board with stiffener. A stiffener that supplies rigidness to the flex circuit for stable soldering is revealed; components mounted on the contrary side of the stiffener. Stiffeners are a cost-effective means to rigidize certain locations on the rigid flex PCB boards, such as SMT areas, pin areas, or hole pattern places for element placing.
SMT areas do not always require stiffeners depending on the parts being set up at that location. Nevertheless, adding a stiffener is mosting likely to include hardly any price to the assembly. Stiffeners are utilized to enhance solder joints and are sometimes used to force bend lines in the chosen locations. Stiffeners can be made from FR4, polyimide, copper, or aluminum-based materials.
Occurring the bend
Regardless of the application, a rigid flex PCB must be flexible and flexible, yet the question is: Just how pliable and flexible can it be? The court is still out on the accuracy of bendability (or “bendableness”). As of this writing, the IPC is being instead conservative with its call. So, basically, the precise interpretation or scale of bendability hasn’t already been pinned down and most likely will not be because of its nebulous nature. The very best recommendations provided is to rely upon a knowledgeable EMS Provider that has a number of wearable/IoT PCB designs under its belt and has a warehouse of crucial nuances associated with flex PCB bendability.
Having said this, it readies to recognize terms like bend span, bend ratio, and stress, all of which are totally intertwined. Bend span, as the name implies, is just how far you could flex that flex circuit before something breaks or incurs an unexposed fracture. It’s likewise required to understand that the measurement of the bend radius is performed from the bend’s underside surface area. Once more, it’s a good idea to companion with an intelligent EMS Supplier to evaluate and settle bend radius concerns and problems.
The 2nd term, bend ratio, thinks about the ratio of the bend radius to the density of the flex circuit. For instance, the bend proportion for a multi-layer flex circuit for a medical electronic devices wearable device is at the very least 20:1. Comparative, for the solitary and double-sided flex circuits the bend proportion must go to the very least 10:1. Tighter bends may create the danger of circuit damages. It’s always more effective to utilize more progressive angles as opposed to an appropriate angle bend with a sharp radius. Bend span is computed by gauging the range from the inside surface area of the bend to the facility of the distance.
It’s also vital to recognize there are two facets connected with flex PCB circuits flexing. One is static or one-time bending; the other is dynamic flex entailing numerous bending procedures. The bend span for fixed bending should be at least 10 times the thickness of the wiring and the strain on the critical layers will be 2.2% or less. On the other hand, the bend span for dynamic rigid flex PCB will be 25 times or less.
The bend span for vibrant rigid flex PCB, such as the example inhabited with µBGA bundles, ought to be less than 0.8% for 50,000 cycles, less than 0.6% for 100,000 cycles, less than 0.4% for as much as one million cycles, and less than 0.2% for a million cycles or more.
As was kept in mind over, the bend radius, bend ratio, and pressures created by the flexing activity of the boards are totally linked. In the case of stress, these are currently integrated in when the rigid flex PCB manufacturer creates the flex circuit. To puts it simply, stress is inherent in the various circuit layers and can be minimized with pressure relief devices such as stiffeners.
Dielectric thickness
Dielectric materials within the flex circuit could cause more stress relying on their density. Dielectrics differ in their proportion of stiffeners to density. Choosing a dielectric material inning accordance with the underlying application offers the completed flex circuit the quality it requires. In regards to impedance designs, the conductor sizes and dielectric densities can be adapted to fulfill the needed impedance results.
As previously noted, high modulus PCBs are difficult boards with stiffeners. Right here, bend radius is very essential and has to be factored in due to the fact that the estimation of bend ratio must also include the thickness of the stiffener, thereby raising the total density for the flex PCB. Maintaining the bend ratio little boosts the flex circuits’ dependability.
This suggests that it is essential to understand the pressure at different levels within rigid flex PCB layers. Consequently, this means understanding which layers utilize exactly what quantities of copper. Altering the quantities of copper has the most unfavorable result on stress difference.
As an example, take a rigid flex PCB with a Hoz of copper weight. This will flex with a certain stress quantity with a certain bend ratio. Nonetheless, if the amount of copper were to be increased to one ounce, the versatility would be considerably decreased, and the bend ratio would be restricted since the copper thickness has doubled, thereby creating a total thicker flex design. All of this suggests that you need to calculate the bend proportion very meticulously.
Moreover, you need to inspect copper density at various layers within the flex material. This is because the density influences the bend proportion and the stress variable. You can utilize certain sorts of flex material for certain applications, so it’s not a situation of “one-size-fits all.”