Three ways for die attach on IoT PCBs
In semiconductor lingo, the term die attach simply means a die, or bare chip, is placed and attached with some form of adhesive in a package, like a ball-grid array. However, given the assembly and manufacturing demands of small IoT devices, die attach technology has emerged onto the printed circuit board (PCB) manufacturing floor. This means dies or bare chips are now being directly placed on substrates or small rigid-flex circuits used in small IoT devices. The reason for this is that the circuitry area of those miniature PCBs doesn’t have the luxury to allow bulky or traditional space-consuming device packaging using plastics, ceramics or glass.
What’s also important to know is that all die attachments for IoT devices aren’t the same. There are three different die attach methodologies, each one involving various key differences: epoxy, eutectic and solder attach.
The epoxy method could be silver epoxy glass or a polyimide-based material. It is dispensed using a very fine dispenser that precisely distributes the epoxy. The substrate in this case sometimes needs to be heated to a higher level of temperature, from room temperature up to 200 degrees Celsius, depending on the type of epoxy used. This temperature allows the epoxy to be properly cured so that it adheres to the substrate to precisely create the joint between the substrate and the die.
Eutectic die attach uses a metal layer made of aluminum or gold. Gold has a very high temperature ranging from 230 to 400 degrees Celsius. A eutectic system is a mixture of chemical compounds or elements that form a single composition and solidifies at a lower temperature compared to the individual elements, which have considerably higher individual melting points compared to compounds. The fact that eutectic temperature is more manageable and lower compared to the individual pure elements is important in eutectic bonding.
The third die attach method is solder attach. It is similar to surface-mount technology manufacturing or SMT joint creation. Solder attach is a common type of die bonding because of better thermal conductivity of the solder material itself. Depending on the different methodologies discussed above, that could be extreme variation of temperatures on the die during the die attach process and operation.
Solder attach is an important concept to dissipating heat that is generated from the power device, basically the bond that efficiently creates the joint. When the solder is attached, it is referred to as soft solder attach, as well. Depending on the alloy used, the melting temperature is a lower one compared to other methodologies. Alloy used in solder attach can either be tin lead, which melts at 150 degrees Celsius or indium, which melts at 220 degrees Celsius.
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