The feed hole on the bonding wedge is likely the most critical aspect of the wedge configuration. The feed hole has the responsibility of guiding the wire accurately to the center of the bond foot. The feed hole's most significant contribution is how it defines the looping performance for our customers.
What do we mean by looping performance? Our customers use our product to attach wires between two points. This wire needs to have an arc shape in order to prevent shorting on other circuitry within the electronic device. If a wire is laid flat, it would sag down and touch other electrical points in the customer's product. If the wire is shaped into an arc similar to the shape of a bridge spanning a stream, then the wire is stronger, just like the bridge is stronger. This wire shape is what we call the loop profile. The loop profile can be very low or it can very high for a variety of different reasons. The loop profile will be dictated by the length of the wire, the diameter of the wire and the height difference between the first and second bond. Our customers have to consider all these factors when deciding which wedge to utilize.
There is a very simple guideline to start with when selecting the diameter of the feed hole. Generally speaking, the feed hole diameter will be twice the diameter of the bonding wire. A .001" diameter wire will use a .002" diameter hole. Customers deviate from this general guideline for a variety of reasons.
The feed hole plays such a critical role that it can also be the cause of many performance problems. These problems can be very difficult for our customers to resolve quickly because the symptoms can be caused from several different sources. These problems include poor wire feeding, poor looping, wire damage, poor wire termination and difficulty threading the wire. This is why accuracy on the feed hole diameter, the feed hole length, and the funnel depth are extremely critical.
The feed hole gains control over the wire by producing "wire lock.” Wire lock is when the wire cannot slide through the feed hole. The wire is thus "locked" inside the feed hole. Wire lock takes place when the angle of the wire moves significantly away from the angle of the feed hole. In other words, the wire angle and the hole angle need to be at the same angle in order for the wire to freely pass through the feed hole. If the angles of the wire and hole are not the same, then there will be friction on the wire. The greater the angle differential the greater the friction. Eventually the friction is high enough to achieve wire lock. Then the tool is in control of the wire and in control of the loop profile. A couple of feed hole aspects can cause significant wire lock problems. The two primary aspects would be the hole diameter in relationship to the wire diameter, as well as the feed hole length.
One of the other factors that contributes to wire lock performance is the length of the feed hole. The length of the feed hole is altered when the depth of the funnel is altered or if the "T" dimension of the tool is modified.