Just like the big boys (meaning conventional printed circuit boards), IoT PCBs fall under the categories of Class I, II and III.
Earlier, I noted that Class I IoT PCBs are pretty routine; those end applications cover virtually every consumer wearable you can think of. Class I IoT PCBs don’t demand a great measure of quality and reliability, for the simple reason that these products aren’t subjected to harsh or demanding environments that can affect proper operation.
So, in essence, Class I IoT PCBs don’t require the extra design, fabrication and assembly considerations that Class II and III IoT PCBs demand. For example, a Class I wearable product could be a sensor-based health-monitoring device that measures your heart beat, number of consumed calories or number of walking steps taken.
Let’s now look at Class II. These are mostly for industrial and commercial applications, meaning that these IoT devices aren’t as rigorously designed or tested as Class III IoT circuit boards. IoT devices are made acceptable under the standards of Class II by assuring good and reliable solder joints are in place and are making connections within the rigid-flex or flex circuitry.
Home automation is one major application that comes to mind. In home automation devices can connect through the internet to your iPhone or handheld device. The best thing is that home automation doesn’t incur vibration shocks or mechanical or thermal shocks.
All home appliances, from refrigerators to televisions to microwaves, are connected to home automation software through the internet. You can switch these devices on and off, and you can change the different programs or modules. You can also increase or decrease the flow rate of these devices. You can keep an eye on what’s happening inside the house through the home automation devices (for example, cameras or locks), and you can remotely turn on or off locks or cameras using your handheld device. That handheld device is another IoT product that doesn’t require extremely high reliability because environments they operate in aren’t harsh and don’t pose harm to their operation.
On the other hand, requirements for Class III IoT devices are very specific. IoT applications falling into this category include military/aerospace and medical electronics. In the assembly process for any type of printed circuit board, either surface mount technology or through hole is used to place components onto an IoT’s rigid-flex or flex circuit.
As the name implies, surface mount is just that — components and devices are placed with their connections on the surface of the circuit board. Through hole means tiny holes are drilled into the circuit board to insert component leads through them, thereby making electrical connections throughout the circuit board. If through hole components are used for an IoT device’s circuit board, a major Class III requirement, for example, is to have the barrel of each through hole to be filled with at least 75% or more solder. This is required so that the IoT device can sustain harsh environments and keep joints intact in all kinds of conditions. This Class III requirement is also known as the J Standard applied in military/aerospace and similar demanding applications. In particular, IoT targeted at aerospace applications must be extremely reliable. You not only have to prove through hole barrels are filled, but you also must have 100% verification using x-ray or similar inspection and verification system. Here, there’s no human intervention, guessing or judgment calls.
Also, it’s important to note the assembly process for Class III IoT rigid-flex or flex circuit boards becomes considerably more challenging than that for conventional PCBs. That’s because IoT PCBs are mainly based on rigid-flex or flex circuits that demand a host of critical considerations.
Here are some: You have to know how and where to place vias in the bend areas of the flex circuitry, or better yet, avoid putting them in if at all possible. You also have to assure stiffeners are kept at the right places; stiffeners are used to keep flex circuitry stable at certain locations.
Also, IoT rigid-flex or flex circuits have growing numbers of layers. In some cases, there are 12 or 14 layer flex circuits. These become exceedingly difficult during assembly and manufacture because different materials are involved. All are bending and twisting at different ratios, levels and angles. Each layer has different electrical characteristics associated with it, along with different thermal traits.
Further, inspections are vital for Class III with the J Standard applied to military IoT devices. It’s a good idea for assembled IoT devices to go to a third party for verification dealing with environmental testing, thermal shock and temperature cycling. The latter refers to subjecting the IoT device from -100° to +125° in a very short period of time. This ages the product; cracks and weaknesses start showing up in the weaker areas of the circuitry, especially when the flex circuitry is merging into the rigid circuitry.
The third party performing this important inspection should be without any bias and not interested in trying to meet shipping deadlines. Inspection companies like these are unbiased judges for testing your product; assuring there aren’t any flaws or issues that prevent them from meeting Class III standards is critical.
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