The demand for and technology of IoT devices are rapidly passing the consumer market and landing squarely in medical electronics, IT/enterprise, industrial and military markets. And those IoT markets are experiencing dramatic growth.
However, those markets are highly demanding when it comes to high reliability. IoT devices in these instances must be reliable to the point where there can be no flaws or failures, and definitely no latent failures — meaning the IoT device shouldn’t fail once it’s being used in the field.
How do you maintain IoT device high reliability especially at the printed circuit board (PCB) level?
Remember, as I said in an earlier blog, most IoT PCBs are not conventional printed circuits. Rather, in most cases, they are small combinations of rigid and flexible circuits or flexible circuits alone. It can be argued that some IoT products may be larger and, in those cases, more of a conventional rigid PCB is used. But in a majority of cases, IoT products are small and call for the rigid-flex or flex circuitry as the basic PCB.
Vias take on special significance for IoT rigid-flex and flex circuit reliability. Vias are tiny drilled openings that create electronic or power connections between a circuit’s multiple layers. Via placement is of paramount importance in maintaining high reliability. Location of via placement is critical because flex circuitry has bending curvature and radius, which can weaken these vias after long periods of time.
The general rule of thumb for IoT circuits is for these drill holes to be as small as possible due to limited space. Sticking to five to six mil finished drill hole size is a good compromise. Going below four mils, like drilling at three mils, calls for laser drilling, which is more time-consuming and costly. Going higher, for example at six to seven mil via hole drilling, means too much valuable real estate is consumed, and that’s not a good move for a small IoT PCB design.
Keeping smallness in mind, those IoT rigid-flex and flex circuits have limited space. Components placed on those small circuits are miniscule as well.
For instance, capacitors are part of an IoT circuit’s electronics. The trend has been to capsulize capacitors into increasingly smaller packaging like the 01005 package, which is a challenge in terms of placement and inspection. This means the IoT rigid-flex and flex circuit assembly house must have sufficient experience with its various processes so it can effectively produce highly reliable products. In particular, savvy assembly houses must have extensive knowhow about solder joints for IoT circuits since devices and surface mount pads are so miniscule.
A misstep during the pick-and-place and reflow process can create latent failures as mentioned earlier. So, solder printing, placement of components and reflow temperature have to be accurately dialed-in and maintained.
Printing, in particular, takes on special meaning. If overprinting is inadvertently performed, shorts may result between the leads or balls of a micro BGA, QFN or flip chip packaged device. A short or solder bridge is a solder extension from one lead or ball to the next and forms a short circuit.
However, if enough paste isn’t dispensed on the extremely small surface mount pads, there will be a shortage of solder paste, thereby creating an open (a broken interconnect of a component’s package) in the worst case scenario or big voids in the best case scenario.
Also, one has to be wary about using through-hole components in an IoT design. One reason is they lack the important functionality. Also, they have a high probability of adversely affecting reliability. Through-hole technology is older and was routinely used for earlier conventional PCBs before surface mount technology came into prominence. Still, there can be some cases where through-hole components are used for IoT products.
Through-hole components are larger compared to similar ones in surface mount packaging such as micro BGAs, CSPs and QFNs. Thus, through-hole components placed on small rigid-flex and flex circuits during the pick-and-place assembly operation will pose difficulties. In effect, they create the probability of reliability issues, again due to the bending and curve radius that are associated with flex circuits.
In summary, distancing an IoT design from using through-hole components is a good idea. Also, printing consistency is highly critical, as well as proper solder joint consistency. These key steps are the foundation of high reliability, which is absolutely required in specific IoT medical, industrial and military/aerospace applications.
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