Today, technology patterns are increasingly toward flex circuits or a combination of rigid flex PCB for IoT/wearable PCB designs. You could say those patterns place us on a various footing, so to speak. Therefore, it is essential to get a manage on new design terminology and things that should be factored in as you transfer to this following degree of embedded design.
This includes the following:
– Reduced and high modulus boards
– Bend radius, proportion, and stress
– Dielectric density
– Via placement
– Board layers and linked copper amounts
– Regular copper versus hardened copper
– Copper density
Board modulus refers to its structure– a reduced modulus means a softer construct, while high modulus refers to a harder board with stiffener. A stiffener that offers strength to the flex circuit for secure soldering is revealed; components mounted on the opposite side of the stiffener. Stiffeners are an economical way to rigidize specific areas on the rigid flex PCB boards, such as SMT locations, pin locations, or hole pattern places for part mounting.
SMT areas do not always require stiffeners depending on the elements being mounted at that place. Nonetheless, including a stiffener is mosting likely to include hardly any cost to the assembly. Stiffeners are utilized to strengthen solder joints and are occasionally used to compel bend lines in the selected areas. Stiffeners can be made from FR4, polyimide, copper, or aluminum-based materials.
Coming around the bend
Despite the application, a rigid flex PCB must be flexible and flexible, however the concern is: How flexible and bendable can it be? The jury is still out on the preciseness of bendability (or “bendableness”). As of this writing, the IPC is being rather conservative with its call. So, essentially, the exact interpretation or gauge of bendability hasn’t already been selected and possibly will not be because of its ambiguous nature. The best suggestions offered is to rely on a knowledgeable EMS Supplier 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 stated this, it excels to know terms like bend span, bend ratio, and stress, all which are totally linked. Bend span, as the name indicates, is just how much you could flex that flex circuit before something breaks or incurs a concealed crack. It’s also needed to recognize that the dimension of the bend distance is carried out from the bend’s bottom surface. Again, it’s advisable to companion with a savvy EMS Carrier to analyze and fix bend span concerns and concerns.
The second term, bend proportion, takes into consideration the proportion of the bend span to the density of the flex circuit. For example, the bend ratio for a multi-layer flex circuit for a medical electronic devices wearable tool goes to the very least 20:1. By comparison, for the solitary and double-sided flex circuits the bend proportion should be at the very least 10:1. Tighter bends may develop the threat of circuit damage. It’s always preferable to use more progressive angles instead of an appropriate angle bend with a sharp span. Bend span is determined by measuring the distance from the inside surface area of the bend to the facility of the span.
It’s also essential to understand there are two facets connected with flex PCB circuits flexing. One is static or single bending; the other is vibrant flex entailing multiple flexing operations. The bend radius for fixed flexing need to be at least 10 times the thickness of the wiring and the strain on the crucial layers should be 2.2% or less. On the other hand, the bend distance for vibrant rigid flex PCB must be 25 times or less.
The bend radius for vibrant rigid flex PCB, such as the example populated with µBGA bundles, must be less than 0.8% for 50,000 cycles, less than 0.6% for 100,000 cycles, less than 0.4% for approximately one million cycles, and less than 0.2% for a million cycles or more.
As was noted above, the bend span, bend ratio, and strains developed by the bending action of the boards are inextricably linked. When it comes to pressures, these are already integrated in when the rigid flex PCB manufacturer produces the flex circuit. In other words, stress is inherent in the different circuit layers and can be mitigated with stress relief tools such as stiffeners.
Dielectric materials within the flex circuit can trigger even more stress depending upon their thickness. Dielectrics vary in their proportion of stiffeners to density. Picking a dielectric material according to the underlying application offers the completed flex circuit the quality it requires. In terms of impedance designs, the conductor widths and dielectric thicknesses can be gotten used to satisfy the required resistance outcomes.
As previously kept in mind, high modulus PCBs are difficult boards with stiffeners. Right here, bend span is exceptionally vital and needs to be factored in since the computation of bend proportion need to additionally incorporate the density of the stiffener, consequently boosting the overall density for the flex PCB. Maintaining the bend ratio little rises the flex circuits’ integrity.
This indicates that it is necessary to comprehend the strain at various levels within rigid flex PCB layers. Then, this implies knowing which layers use just what quantities of copper. Changing the quantities of copper has the most negative effect on pressure distinction.
For example, take a rigid flex PCB with a Hoz of copper weight. This will bend with a certain stress quantity with a particular bend ratio. However, if the amount of copper were to be increased to one ounce, the flexibility would certainly be substantially minimized, and the bend proportion would certainly be limited since the copper thickness has increased, therefore creating a total more thick flex design. All this suggests that you have to calculate the bend ratio very thoroughly.
In addition, you need to inspect copper thickness at different layers within the flex material. This is since the thickness affects the bend proportion and the strain variable. You can make use of specific types of flex material for certain applications, so it’s not an instance of “one-size-fits all.”