How to Evaluate Bonding Process (Ball Bonding / Wedge Bonding) Quality: Common Issues and Inspection

Wire Bonding is a key process for interconnecting chips, devices, and substrates. Common processes are divided into gold wire ball bonding and wedge bonding (including deep-cavity wedge bonding). To evaluate bonding quality, one must look at both appearance and parameter stability, as well as the mechanical strength of the solder joints.

Common Issues and Causes

• Unstable tail wire length: Mostly related to the wire clamp system and the wire breaking method. A stepped wire clamp (the first wire clamp located above the wedge bond point, the second wire clamp located at its bottom) can significantly improve the reliability and consistency of the tail wire;
• Wire loop collapse or inconsistent loop height: Related to wire loop programming, bonding speed, bonding force, and wire material condition. Microwave devices are especially sensitive to low-loop bonding;
• Excessive deformation of the first bond point or cratering: Caused by excessively high ultrasonic power, bonding force, and time parameters, and needs to be adjusted according to the device structure;
• Second bond point peeling: Substrate surface condition (oxidation, contamination) and parameter matching issues;
• Limited deep-cavity bonding: Requires specialized wedge bonding capability and capillary length, and has requirements for the equipment's Z-axis travel and vision alignment.

Only with Controllable Parameters Can There Be Reproducibility

Whether parameters such as bonding force, ultrasonic power and frequency, bonding time, and temperature can be fully programmable and stored directly determines whether the process can be reproduced and whether consistency can be maintained across different shifts. The ultrasonic system can switch between 60 kHz and 100 kHz according to substrate type, and the transducer can be replaced on site, which is also a practical plus. For multi-variety, small-batch laboratory and pilot production scenarios, the equipment should preferably provide both manual stepping and automatic production modes, and maintain operational consistency with models in the same series to reduce training and maintenance costs.

How to Inspect Bonding Quality

• Wire Pull: The most direct strength indicator, which can evaluate the first ball bond, second bond point, and the wire loop as a whole;
• Ball Shear: Evaluates the interface bonding of the first bond point;
• Ball Push / Bump Shear: For micro features such as bumps and copper pillars;
• Metallographic cross-section: Examine the solder joint cross-section, intermetallic compound layer, and whether there are voids, cracks, or abnormal growth at the interface.

In terms of international standards, wire pull and shear are commonly based on IPC-9708 / JEITA EIAJ ET-7407, JEDEC JESD22-B115/B116/B117A, and related methods in MIL STD 883. It is recommended to archive the strength data of each batch together with the equipment parameters. When customers ask about process stability, historical curves can be directly retrieved, which is more convincing than supplementary testing on the spot.