Pinpoint IoT PCB wire bonding reliability

IoT devices are spreading into a variety of markets well beyond the consumer area. These small to medium-size devices are finding their niche in industrial, military and aerospace, medical and other associated areas.

In such mission-critical applications, the top requirements are for reliability and ruggedness. IoT devices must be well-built and able to withstand considerable wear and tear, while still being able to operate efficiently over long periods of time.

Wire bonding reliability is at the heart of these demands. As noted in my last blog, wire bonding is increasingly being used today in IoT printed circuit boards (PCBs). It’s used to connect bare chips or dies to a substrate or the small rigid or rigid-flex board. Device packaging is taking too much valuable space on these tiny boards, hence chips must be placed and connected on the board.

There are 10 key principles that must be adhered to with pinpoint accuracy in assembly and manufacturing so that IoT PCB wire bonding maintains high reliability. Some are interrelated. However, as standalones, each one is critical.

1. First and foremost, metallization is extremely important. This means creating a bond between the substrate on the PCB and the die. If the surfaces aren’t clean and oxidized, the strength of the wire bond is highly questionable.
2. Different nuances are associated with gold, aluminum, copper and silver wiring. There must be assurances from wire bonding and PCB assembly experts that using a certain type of wire is compatible with a given application.
3. Flip chips or chip scale packaging must be mounted properly on the board for wire bonding to work, and must be securely attached before wire bonding starts. If not, the created joint will not be reliable. When pull strength testing is applied to the wire bonded joint, the wire will come off.
4. Virtually all wire bonding requires a fixture to lay the substrate flat. Knowledge and expertise are critical here for precisely creating those fixtures for ultra-fine devices to be wire bonded. Tolerances must be extremely tight to properly hold the substrate before it gets wire bonded.
5. An experienced programmer is required to deal with an array of wire bonding variables. That programmer must also be savvy enough to perform wire bonding accurately and calculate and verify bonding strengths, bond integrity and bond placement before doing the actual bonds.
6. Get assurances the electronics manufacturing services provider knows to perform plasma cleaning using argon gas to clean substrates as needed. The surface of those substrates must be 100% defect- and oxidation-free for perfect wire bonding.
7. The chip and the wire itself must be 100% clean and free of oxidation. Otherwise, a wire bond joint will be unreliable.
8. A fixture must be used to hold the substrate rigid, especially for flex circuits.
9. An experienced wire bonding technician that knows the fixture assures that the substrate will remain rigid and co-planar to avoid bends, twists and/or uneven surfaces.
10. Most often, gold wiring is used. A caveat when using gold wire is to ensure stabilized gold is used. Stabilized gold has a different chemistry with beryllium mixed into gold to increase the life of the gold wire.

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