IoT devices are entering a distinctly new realm when it comes to printed circuit board (PCB) assembly. The reason for that is IoT devices require both traditional surface mount technology (SMT) assembly, as well as microelectronics assembly. The latter involves chip on board, wire bonding, die attach, flip chip and other associated technologies.
Microelectronics is used for very small form factor flex and rigid-flex circuits populated with bare unpackaged die, which are placed on a substrate or printed circuit via wire bonding. Examples of IoT devices that undergo both SMT and microelectronics assembly include: medical electronics, industrial and network edge systems.
For effective SMT and microelectronics assembly of IoT PCBs, the OEM provider must precisely implement three key steps, which are:
- Protecting sensitive microelectronics areas
- Properly using epoxies
- Addressing thermal profiling and heat dissipation
Protect sensitive microelectronic areas
It’s critical to protect IoT PCB microelectronic areas from residue fluids, solder, fluxes or paste. Plasma cleaning is used at times to ensure a clean surface. Plus, total assurances are needed for appropriate glob top materials and fluids that can efficiently interact with the human body without adverse effects.
After SMT and microelectronics assemblies are complete, specialized cleaning, effective glob topping, final rinse and cleaning are performed. It’s important to note that these and other PCB hybrid manufacturing processes demand extensive experience. Thus, electronics manufacturing service providers with this high level of knowledge are the ones that know how to overcome cleanliness and multi-glob top application and protection challenges.
Use the correct epoxy
Thermally conductive and non-conductive are the two types of epoxies. Several factors are involved when selecting the one to use in an application. For example, you should consider curing temperatures, pot life, thermal conductivity and glass transition temperature (Tg).
Epoxies are comprised of one or more compounds or elements, which have to be considered when undergoing curing. For example, consider the generic epoxy EPO TEK 930 -4. It’s composed of two compounds and is cured at 150° C for 10 minutes, or it could be cured at 80° C for six hours. Operating temperature is an associated factor here. A maximum temperature could be 200°C or 150°C, which also depends on epoxy compounds and their curing conditions.
Pot life ranges from two to six hours or as long as 20–30 days and refers to a certain period of time before an epoxy expiries. Generally, epoxy manufacturers cure their own products and savvy OEM customers provide the PCB house with the exact epoxy to be used for their IoT applications.
Thermally conductive epoxy is used when the base of the die has either a ground or solid plane. When current is flowing from point A to point B, the die’s bottom surface is used to migrate the temperature from those two points. However, if the die’s bottom side has no substrate or ground, thermally non-conductive epoxy is used because no current flows through the epoxy from top of the die to the bottom.
Address thermal profiling and heat dissipation
Tg is a gradual and reversible transition in materials from a hard and brittle state into a viscous or rubbery state. Tg is an important aspect during assembly because substrates and epoxies, which are assembled, need to have compatible Tg.
Thermal conductivity, as it pertains to epoxies, is a measure of how quickly the heat dissipates. Values can range from 0.3 or 0.4 watts (W) per Milli-Kelvin (mK) at the lower scale, meaning heat dissipates very slowly, to 1.7 to 2 W/mK at the high side, meaning it quickly dissipates.
When it comes to thermal profiling, Tg has to be considered. Analysis needs to be performed among the substrate, epoxy and die using the different datasheets for all of them. Then, you can decide on the ones most conducive among themselves.
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