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Failures in IoT PCB wire bond loops

Earlier we talked about wire bond testing for IoT printed circuit boards and the types of wire used to connect a bare chip to a substrate or the PCB surface itself. This discussion deals with wire bond loops pull testing and verification, which is critical for IoT device reliability.

Your immediate reaction is probably: What loops are you talking about? To the average everyday person, it would appear a wire goes from point A to point B in a straight line, right?

Actually, when dealing with wire bonding, the wire itself creates a loop or a wire arc between the bare chip and the substrate or PCB surface connection. And therein are a series of failure modes in the wire bonding process for IoT devices. You have to remember these are very fine, very delicate wires. Wire bond loops demand precise pull testing and well-trained technical personnel, as well as wire bond pull tester for reliable wire bond joints. This expertise is critical to quickly spot problematic areas during the assembly and manufacture of IoT PCBs, including rigid and combination rigid-flex circuits.

There are eight modes of failure for wire bond loops. Savvy electronics manufacturing services provider houses assembling and manufacturing IoT PCBs are well supplied with highly reliable wire bond testing for these loops to assure that an IoT PCB doesn’t incur any of these failure modes.

Wire bond loops can break from different segments of the wire length. One of the weakest points of a wire bond is at the tip of the loop where it’s been formed; that is part of the reason it’s the weakest point. This is known as the neckdown breakpoint and is the most common.

Another is a mid-span break somewhere in the vicinity of the middle part of the wire. Two other failures are known as failure in bond. One occurs at the interface between the wire and metallization at the bare die, and the other is at the interface between the wire and metallization at the substrate, package post or other than the bare die or chip.

Two other failures are due to lifted metallization from the die and from the substrate or package post. The last two failures can be due to a die or substrate fracture.

There are a number of reasons these failures can occur in IoT PCB assembly and manufacturing. A major one is the PCB’s surface or the substrate isn’t sufficiently clean. Plasma etch cleaning using argon gas must be used here so that all the residues of any oxidation are completely removed and the surface is 100% clean. This way bonding is reliable and sturdy.

There can be other reasons for wire bond loop failure. The quality of the wire may be bad, or it may be contaminated. Programming the wire bond pull tester may not be properly done so that it’s not optimizing the loops. As a result, loops are created that aren’t sufficiently study for maintaining the bond under a certain test force.

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