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Eight ways to lower defect rates below 100 ppm.

With so many variables, troubleshooting wire bond programs can be tricky. These tips will make the process a bit easier.

Tip 1. Confirm the wire bonder's setup is correct.

1. Install a new wire bonding tool and set screw. If not available, make sure the old bonding tool is the correct part number for the wire size being bonded. Under a microscope, at 10X minimum, inspect the tool and set screw to see if it is worn or dirty. Reinstall the tool and seat the set screw to the manufacturer recommended setting with a torque wrench. Test the ultrasonics; if the ultrasonics are tuned and working properly, proceed with the setup.

2. Install a new spool of wire. If new wire is not available, confirm the wire in the old spool is the correct size and alloy composition. Make sure the wire is not dirty, damaged or spooled incorrectly. Verify the wire feeds smoothly through to the bond tool.

3. Load the secured copy of the wire bond program into the wire bonder. Always keep a copy of the wire bond program in a secure area away from the production floor. You can easily reinstall the program if the production copy becomes corrupt.

4. Using a calibrated thermometer, verify the heated stage is the correct temperature. A thermosonic process, gold wire bonding typically requires the product be heated to 150°C.

5. Using a setup sample, evaluate the program for repeatability and resulting bond strengths.

Tip 2. Bond surfaces must be clean, smooth and contaminant-free. Use cleaning solvents to flush contaminates from bond surfaces. Plasma cleaning will remove micro-contaminates but in most cases will not remove debris or other macro-contaminates. Oxygen plasma will remove organic contaminates. However, removing inorganic oxides requires argon gas or a mix of argon and oxygen gas. The plasma from these gases will remove both organic and inorganic contaminates. Measure the effectiveness of plasma cleaning by placing a drop of DI water on the bond surface and visually examining the contact angle of the water drop. A high contact angle indicates surface contamination. A low contact angle, where the water drop spreads over the bond surface, indicates a clean surface.

Tip 3. The product must be held rigidly. If the product moves during wire bonding, ultrasonic energy is lost and the bond will not form completely. Check the work holder, or confirm the tool is clamping the part firmly before the wire bonding operation starts. The clamping should be repeatable. You may have to put a time delay in the program to permit the clamped part to settle before starting the wire bonding process.

Tip 4. Check for consistent and symmetrical free-air balls. The diameter should be approximately 1.5 times the wire diameter (Figure 1).

Tip 5. Adjust the ultrasonics, force and time. Change parameters individually, as changing all the parameters at once will make it impossible to determine which change has the most impact on improving the wire bond.

Tip 6. Replace the wire bond tool with a different lot number of the same tool, or use an equivalent tool from a different supplier.

Tip 7. Replace the wire with a different lot number from the same supplier, or use an equivalent wire from a different supplier.

Tip 8. Finally, check the equipment calibration, the bond head mechanisms and the operation of the machine.

With good controls on incoming materials, dependable wire bond process control checks, and regular equipment maintenance and calibration, the wire bonding operation will run defects of less than 100 ppm.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.

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