Grain Structure:
Grain structure can affect the properties of the wire bond being made, especially in a gold ball bond.
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During the ball formation (EFO), the properties of the wire directly above the FAB (free air ball) are altered. The HAZ (heat affected zone) is mechanically weaker than the original wire, so this directly affects the loop formation and stability. A fine grain structure introduces a shorter HAZ providing lower loop heights. A long HAZ assists in loop shapes that are naturally higher without relying on the capillary trajectory.
Spool Size:
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There are two main considerations to take into account when specifying what spooling requirements you need for your bonding wire. The first is the spool type itself, which needs to be compatible with the wire bonding system you are working with. As the wires are subject to contamination which can affect bond quality and strength, as well as the fine nature of the wire, reworking or re-spooling the wire is not recommended, making the spool specification more critical.
The second criterion to take into account is the length of the wire you require on the spool as well as how many layers are wound on the spool. Multi-wound spools that have more than one layer of wire wound onto the spool will hold an increased length of wire, but this increases the risk of wire becoming snagged from the spool as it is fed into the wire bonding machine. Fine wire SiAl is more prone to becoming snagged if oxide layers start to form from incorrect storage. This is especially true for small bobbin-like spools that have multiple wires, as this wire tends to have “memory” so it will try to return to the wound shape if left unused on the machine for too long.
For manual wire bonding machines it is recommended to use a single wound layer to prevent this. If using a fully automatic wire bonder, then changing wire spools means additional down time which will affect production and throughput each time a machine is stopped for a spool change, so a multi-wound spool may be more desirable.
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