Like all printed circuit boards (PCBs), an IoT rigid-flex or flex circuit undergoes the reflowing process to make necessary circuit connections on those small boards. A correct and accurate thermal profile is the linchpin in this key reflow process.
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It’s easy to go into all the technical details associated with a thermal profile. But for the purpose of this discussion, let’s simply define it this way: An experienced process engineer on the assembly floor calculates a distinctively tailored temperature formula and enters it into the reflow oven system. That’s basically the thermal profile.
That profile or formula combines a complex set of time and temperature data with the measurement of thermal temperatures in the reflow oven at various zones in the oven. The thermal profile is often measured along a variety of dimensions such as slope, which is the increase in temperature to get to the melting point of the solder paste; soak, the time when the paste has melted; time above liquidus; and peak, the highest temperature or time above liquidus before the temperature is ramped down to the cooling off range.
The important thing to get across here is that thermal profiles aren’t created equal. Each IoT PCB project demands its own unique profile. In this day and age, some electronics manufacturing services (EMS) providers still maintain the old practice of relying on three basic thermal profile models, depending on PCB size and largely based on guesstimates. Those guesses usually result in costly rework and field failures later on.
So, here there is a clear case of caveat emptor. The rationale for this caution is you’re engaging in a dangerous gamble that combines a highly unreliable thermal profile with a highly challenging IoT rigid-flex or flex circuit board.
There are a number of consequences when using a wrong thermal profile for either conventional PCBs or IoT rigid-flex or flex circuit boards. They include a high probability of cold solder joints and/or damaged components.
Suffice it to say that creating the perfect thermal profile for an IoT rigid-flex or flex circuit board is a challenge.
Why? Let’s keep in mind that we’re dealing with very small board real estate that is heavily populated with an array of micro device packages, like micro ball-grid arrays or micro quad flat no leads. With such a small area and small packages, there may or may not be sufficient thermal mass to correctly carry the profile by placing thermocouples on different parts of the board. Thermocouples are small cables attached to a recording device. This combination is called a profiler. This device calculates the temperature on different parts of a PCB regardless of its size. Thermal mass refers to the amount of copper distributed across the surface of the board. Copper is used at various PCB layers, like power and ground planes. When IoT products power up, this copper generates heat, which can be dissipated using the PCB surface with extra non-functional SMT pads, called copper or CU pouring. Copper pouring also creates extra thermal mass.
A great thermal profile creates the ability of an IoT circuit board to properly direct the heat generated in the reflow oven to properly solder the components. A lot of heat energy is required to change the temperature of high-density areas, hence they are said to have a high thermal mass, especially in the rigid board compared to the flex board. But that’s not the case with IoT circuit boards characterized by low thermal mass.
Conventional rigid PCBs have a sufficient amount of copper, allowing them to adequately go through the reflow oven at slower speeds. This is especially true if PCBs are populated with lead-free components, allowing for a 235-250 degree reflow cycle. But for an IoT circuit board, those higher temperature limits of a reflow oven are certain to burn and damage the circuit board and its components. A more prudent approach is to stay with or lower than a 210-215 degree reflow cycle, especially if the IoT circuit board is a leaded product.
It’s also important to keep in mind that there is a thickness difference between rigid and flex circuits. In many IoT applications, the two are used in a combination since the flex circuit is used for the bending and twisting actions of an IoT product. To help offset the thickness difference, the EMS provider should have special fixtures and tools that can be used in this case to assure printing is 100% on the solder pads on both flex and rigid boards. Those same tools are used to support these IoT circuit boards during component pick and place, for automatic optical inspection and when undergoing the reflow process.
Lastly, there are four important steps in achieving a perfect thermal profile for an IoT rigid-flex or flex circuit:
- Conduction oven temperature settings must be lowered to avoid damaging the circuit board and devices simply because there is a combination of rigid and flex boards.
- Special care must be taken to profile leaded and lead-free assemblies. Keep in mind that IoT technology is quickly moving into military and aerospace applications. Leaded devices are used in military and aerospace settings and are more challenging during IoT circuit board assembly and are simply difficult to procure these days. Here, especially, a very fine-line balance between leaded and lead-free components must be achieved, especially if lead-free solder paste is used.
- The best way to obtain temperature and time data is from a profiler and adjust the temperature accordingly.
- Extra and special attention must be devoted to making any small corrections so that the thermal profile is correctly designed for a given IoT circuit board.
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