Machine Configuration and Tools
BY KEVIN SEUFERT
With the recent growth of optoelectronics assembly, the use of ribbon wire for interconnects has become more widely used. This article explores the basics of the ribbon bonding machine configuration and some of the finer points of ribbon wedge tools and wire.
Wedge vs. Ball Bonding
Wire bonding has been around since 1947. Today, it has evolved into a complex, mature manufacturing process with three bonding types: thermocompression (TC), ultrasonic (U/S) and thermosonic (TS). TC wire bonding involves deforming the wire under a bonding tool with force and high heat (>250°C). U/S wire bonding also involves deforming the wire under a bonding tool with force; however, without heat (normally room temperature) and with an ultrasonic movement of the wedge tool. TS wire bonding processing combines both TC and U/S, and involves deforming the wire under a bonding tool with force, heat (lower than in TC, typically ~ 150°C) and an ultrasonic movement of the wedge tool.
Wire bonding falls into two main classifications: ball and wedge bonding. Ball bonding is an omni-directional process. (i.e., the second bond can be at any 360° position from the ball), while ball bonding is used almost exclusively with gold wire and accounts for ~ 98 percent of all wire bonding done today. Ball bonding can be done with a TC or TS process and is faster than wedge bonding. Wedge bonding is an unidirectional process (i.e., the second bond must be aligned with the first bond) and can be done with aluminum, gold or copper wire. Wedge bonding is capable of finer pitch than ball bonding and can bond ribbon wire. Wedge bonding can be performed by TC or TS, and unlike ball bonding by the U/S process it can be performed at ambient room temperature. Some advantages of wedge bonding include finer bond pad pitch, lower loop profiles, controlled wire length and lower temperature when using U/S process.
Ribbon vs. Round Wire
Ribbon wire has a rectangular cross section as opposed to round, and is advantageous for devices using RF. Standard ribbon wire sizes range from 0.00025 x 0.0005″ to 0.002 x 0.020″, and contain different amounts of alloying elements from ribbon manufacturers with different processing methods, which could affect bonding quality.
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Ribbon Wedge Bonding. In the ribbon wedge bonding process, the ribbon wire is positioned under the bonding tool. The bond head of the machine lowers to the first bond, and after the bond is made, the head rises to form a loop and is positioned at the second bond. After the second bond is made, the ribbon is cut and fed back under the tool for the next cycle.
Machine Configuration. Ribbon and round wire wedge bonding machine configurations can have two different types of termination: clamp or table tear. Both manual and automatic bonders can use either type, and some bonders can switch between the two.
Clamp tear — On machines configured for clamp tear, the clamps are located behind the wedge tool and pivot away from the wedge to perform ribbon termination after the second bond is completed. This is the most common process and is reliable because the bonding tool rests on top of the second bond during the termination, keeping it from peeling up. The ribbon will break at the “heel” of the bond because this is the smallest cross-sectional area of the ribbon (Figure 1).
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The standard clamp's “jewel” face typically is made from sapphire or a highly polished metal so as not to damage the surface. For ribbon wire, this face material is changed to ceramic or another slightly rougher surface material to better grip the ribbon during the termination. This is especially critical when using the larger ribbon sizes.
The position of the clamps in relation to the wedge tool is ideal when the wire path through the clamps and the tool slot is at 30°. At this angle, the bottom of the clamps are close to the wedge tool tip and do not allow for clearance to adjacent parts or for getting into deep packages. The wedge tool and clamp position also can be configured at 38°, 45° or 60° feed angles for clearance of adjacent parts or the sidewalls of deep packages.
A problem with these steeper angles is that the ribbon is fed back under the wedge tool at that same steep angle. When the tool approaches the first bond on the next cycle, the ribbon may get “stuffed” back up the feed slot instead of bending under the bottom of the wedge tool. Generally, the tail of the first bond is only ~0.0005″ long so any amount of “stuffing” will cause an improper tail length (Figure 2).
Some machines will allow the clamps to be moved up and away from the wedge tool, but still keep the feed angle at 30°. This allows clamp clearance of adjacent parts and keeps the feed angle at the preferred degree. The problem with having a large gap between the wedge tool and clamps is the ribbon can flex between the two, causing problems with tail length, looping and terminating.
On machines where the clamps open on the horizontal axis, the ribbon has to twist 90° so it is vertical when passing through the clamps and horizontal when passing through the wedge tool. If the clamp is too close to the wedge tool, a “kink” may form during the 90° twist and become jammed in the tool slot. If the machine's clamps open on a vertical axis, the ribbon does not have twist to align with the tool slot, and the clamp may be positioned closer to the wedge tool.
Golden Rule #1: On any wedge bonding machine (ribbon or round wire), the closer the clamps are to the feed slot/hole, the better the tail control, looping control and termination.
- Table tear — On some packages, the bonding is done next to adjacent devices or into deep packages, in which the clamp movement/pivot action during ribbon termination would cause interference. The major difference between table and clamp tear is with table tear, the ribbon clamps only open and close and do not pivot away from the tool for the termination step.
In the table tear process, after the second bond is finished, the wedge tool rises slightly off the second bond, and while the clamp is still open, the machine's X-Y table moves back from the second bond in the Y-axis only. This first table motion feeds out enough ribbon for the tail of the first bond on the next wire. The machine clamps then close and the table continues moving further back, and the ribbon is torn at the weakest point, which is the heel of the second bond.
