Stacked-die wire bonding for IoT devices can get tricky

Stacked die is a newer technology gaining traction in a variety of electronics applications, due to the fact that OEMs continue to demand greater capabilities and performance in smaller printed circuit boards (PCBs). As the name implies, stacked die means one bare chip is placed on top of another one or it could be a spacer rather than a bare chip, then another chip is placed on top of those and perhaps even a third one, and so on. Multiple rows of wire bonding loops are arranged in sets, each going to a separate die or spacer. This way, the same real estate can be utilized for packing considerable functionality in the same small die placement area. Circuitry placement is performed this way to save valuable space on the PCB.

IoT devices are at the top of this stacked die agenda because PCB assembly and manufacturing companies have little space to work with on these extremely small rigid and combination rigid-flex circuits.

With stacked dies being ushered to the IoT PCB manufacturing floor, OEMs have to understand that basic surface-mount technology manufacturing must have the companion microelectronics manufacturing expertise to effectively manufacture IoT PCBs populated with stacked dies.

Here, the electronics manufacturing services (EMS) provider must have both manufacturing capabilities in-house to effectively coordinate surface-mount technology and microelectronics assembly. On top of that, the OEM should expect the EMS provider to have the expertise not only for basic wire bonding for bare chip on-board connections, but also of the wire bonding challenges stacked die applications pose. Some of those manufacturing challenges deal with low wire loops between the bare chip and the substrate or PCB, as well as multiple-level and higher overhang wire loops.

With a multistacked die, one set of wire bonding is being created, which is the lowest one first and has the smallest loop angle. Then, a second set of wire bonding is created and, in some cases, a third and a fourth. All the while, the same PCB real estate, the same wire bonding techniques and the same substrate are being used.

Smaller bond pads pose yet another challenge associated with multistacked dies. These pads are much smaller than usual and they are packed closer and more tightly. In this case, the IoT device OEM needs to know that creating more bonds for multistacked die demands more bond pads.

All told, IoT device OEMs should become more familiar with the advanced stages of today’s wire bonding. That includes gaining an understanding of state-of-the-art testing and inspection tools associated with advanced wire bonding. Moreover, it’s a good idea to learn about how an IoT device EMS provider maintains process-related and engineering training to keep updated on newer developments in this field.

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