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Wire pull is done using a hook. Hooks come in many shapes and sizes. Standard hooks are made from bent round wire and defined by their foot diameter, foot length and shaft diameter. A nice long hook length like 30mm is useful especially when deep access is a requirement, but also for vision lines and because length has very few downsides.
Hook diameter is an important consideration. As a rule of thumb, one should choose a hook with a diameter at least 3 times the diameter of the wire to be tested. This reduces the stress in the wire bent over the hook, so that its strength is similar to the maximum tensile load of the wire ensuring the highest load on the bond possible. The hook should not cut into the wire.
The foot length is normally about 3-3.5 times the hook diameter. However, for fine pitch applications, a shorter length is required to avoid pulling multiple wires at once.
Wires or ribbons with extremely low loop heights may require special tooling to get underneath. A flat wire hook may fit, where regular hooks are too high. Alternatively, one can opt for a tweezer pull, with a set closing distance (or grip force). USB Tweezers are very suitable for this.
A suitable hook material must be,
Tungsten often meets these requirements and is widely used. The material pulled by a hook is typically softer than the hook and so hook wear is rarely a problem. Poor hook quality is normally due to damage.
Most hooks are delicate and easily damaged if you touch them. The most common causes of damage are being hit by he sample or work holder when changing the sample to be tested.
A separate hook from the hook body is preferable because there is more chance of successful repair after a collision. Also, the concentricity of the hook can be readjusted (see below). A damaged hook should not be used, as it will affect your measurement results.
A concentric hook is one where the main shaft of the hook, in the vicinity of the foot, does not move when the hook is rotated. It is important to have a concentric hook, especially when using some form of automation, like auto hook or full automation. If the hook is not concentric and you rotate it when you are below the wire, you may bend the wire and test two wires at once, or miss the wire altogether. Good hook concentricity also makes manual alignment much easier.
Given the very small gap between fine pitch wire bonds hook eccentricity may need to be no greater than 5µm. Because it is not possible to manufacture a sensor spindle and hook to this accuracy, XYZTEC developed the Concentric Tool Holder. Using this tool holder, an experienced user can obtain concentricity easily. Of course, all hooks are shipped concentric.