Understand the basics of multi-tier wire bonding
Electronic systems and equipment OEMs have historically pushed for greater functional integration. This means putting more electronic power on increasingly smaller chips, as well as making sub-assemblies even smaller. All this goes under the banner of increasing performance and decreasing cost. This is truer today for IoT devices than ever before.
Wire bonding plays a big part in these evolving technology trends
Wire bonding, which is the process of connecting a chip to its associated sub-assembly or printed circuit board (PCB), represents a key portion of some of IoT devices’ overall electronics operation. Traditionally, a single wire bonding operation has been used to connect a chip or die to the PCB or substrate.
However, we’re now seeing customers earnestly investigating considerably greater functionality for their next generation IoT products. That means deploying multi-tier wire bonding applications that utilize the same substrate and die real estate.
As the name implies, multi-tier wire bonding ranges from two to four or more sets of wire bonds connecting a highly complex bare die or chip to the PCB or substrate. Single tier wire bonding has reached a high level of efficiency and reliability on the PCB assembly and manufacturing floor.
But there are challenges when it comes to multi-tier wire bonding. For the IoT device OEM taking the multi-tier wire bonding route, it’s best to rely on EMS providers that have solid footing in this emerging technology. That means the OEM must know that multi-tier wire bonding offers them a solution when the number of I/O’s are far beyond the traditional I/O’s that are used in the single wire bonding application.
In multi-tier wire bonding, the different rows of wires are isolated by maintaining a different loop height for each row. This creates a vertical gap between the rows of wire from first row to the second, third and fourth row
Multi-tier wire bonding increases the capacity and the capability of a bare die. If wire bonding is double stacked, I/O capacity is doubled by adding the second set of wire bond pads. If you go on to a third-tier wire bonding, you’re increasing the capacity of I/O’s by three times. Meanwhile, the bare die remains the same. Essentially, you are extracting more functionality out of the same die.
Multi-tier wire bonding challenges
However, there are a few challenges with multi-tier wire bonding. With multi-tier wire bonding, wire bonder precision is critical. You must ensure that the first row of wire bonding is the lowest in height; the second higher than the first; the third higher than the second; and the fourth must be the highest.
A third and quad tier wire bonding machine demands a well-trained operator, top notch precision and process control, exact calculations, and computational knowledge of wire size and bonder restrictions — including x, y and z directional restrictions — among other key requirements. Also, wire looping must be correctly calculated. If it’s not, wiring is prone to sag. The result is the creation of shorts with other rows of wires.
For example, if a third tier of wire bonding is incorrectly performed and its sagging, it creates the possibility of a short with the second tier of wire bonding. Re-working this problem is difficult because wire bonding can only be re-done two or three times. That’s because the pad is worn down after those two or three re-working attempts.
Another major challenge involved with multi-tier wire bonding is wire pull testing (WPT). This PCB microelectronics assembly step focuses on wire bond strength and quality. It involves applying upward force under the wire to be tested, and WPT is applied on every tier. If two to three tiers are not tested properly at the right time, it becomes a challenge to test without damaging certain wire tiers. Therefore, expertise and savvy technicians should know top requirements to assure effective WPT. Each tier of wires must be pull tested before the next tier is bonded.
The future of multi-tier wire bonding
Multi-tier wire bonding is one PCB microelectronics technology that will gain greater interest for newer, smaller OEM products across multiple industries. For example, greater functionality at lower cost is much sought after in medical electronics, IoT devices, wearables and other portable gear.
The savvy IoT device OEM and its product designers will take into account several considerations when seeking guidance for multi-tier wire bonding, which includes optimal design and size of the pad. Pad pitch becomes important, as does the loop height and length of the wires. In addition, the staggering pitch is important, which is basically the pitch from one row to the second to the third, and so on.
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