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Wire-bond metallization must reach the bonding operation in a defined surface state. Additional firing, cleaning, storage and handling between metallization and bonding can change that state even when the pad geometry is unchanged. The useful process record follows the bond region from its formation to the moment the wire is attached. This helps distinguish a material or surface-history problem from a change in the bonding setup.
Key design decisions
- Identify the final exposed metal and every operation that contacts or heats it before bonding.
- Protect the landing region from uncontrolled handling and residue without assuming that appearance proves bondability.
- Compare bond results with failure location and surface history, not only with a numerical pull or shear value.
Describe the actual metallization route
A fired thick film gold conductor, a plated finish and a thin-film metallization stack present different bonding interfaces. Keep the route and material grade explicit. The same visible color does not establish the same surface chemistry, roughness or mechanical response.
Include any underlying conductor, barrier or bonding constituent relevant to the selected system. Material-specific data is the starting point for compatibility, while the final surface after processing is the state that must be evaluated. Do not describe an unverified finish as interchangeable merely because the intended wire material is the same.
Record the condition in which the pad is formed
Retain the printing and firing history for a thick film pad, including the relevant substrate and load condition. The finished thickness and surface shape may differ from the wet deposit. Keep the actual processed pad state in the bonding record.
Inspect the landing region for topography that could affect tool contact, such as a local ridge, depression or nearby overglaze edge. A large-area thickness measurement away from the pad may not describe the bonding surface. Keep the location of surface measurements tied to the actual landing area.
Separate an average roughness result from a local defect at the intended landing point. A small pit or an isolated contaminant may have little effect on the average while directly affecting one bond. Conversely, a different optical texture can be harmless if the approved bonding process consistently forms the required interface. Select measurements and images that address the actual landing region instead of treating a single surface statistic as a complete bondability test.
Track every thermal exposure after metallization
The pad may see resistor firing, dielectric processing, overglaze firing or package attachment before wire bonding. Each added operation changes the cumulative history. Record which materials are present and whether they can contact or contaminate the exposed bond region during the operation.
Compare bonding results after the intended complete sequence, not only immediately after an isolated conductor firing. If a later step appears to cause a change, retain a matched condition without that step or with heat-only exposure where appropriate. The comparison should separate thermal history from added-material interaction.
Choose cleaning for the bond surface and neighboring materials
Cleaning should address a defined contaminant and remain compatible with the metallization, ceramic and any attached polymer or component. A visually brighter surface is not proof of improved bonding. Residue or surface damage from an aggressive treatment can create a new problem.
Document chemistry, exposure, agitation and drying, then evaluate the actual bond interface. If an unqualified cleaning change is made at the bonding station, the resulting trial no longer represents the controlled surface route. Preserve the original condition and compare a deliberately prepared group rather than repeatedly cleaning until one bond happens to pass.
Control the transfer from clean pad to bonding tool
The period between final preparation and bonding deserves a defined handling method. A pad can be recontaminated by contact with gloves, tools, packaging or an uncontrolled work surface. Identify the permitted contact regions and storage arrangement.
| Checkpoint | Condition to retain | Question when bonding changes |
|---|---|---|
| After metallization | Grade, fired state and landing topography | Was the surface formed consistently? |
| After later thermal processing | Complete exposure and nearby materials | Did heat or material interaction alter the interface? |
| After cleaning | Method, residue and drying state | Did preparation change the bond surface? |
| During storage | Container, duration and environment | Was the exposed pad protected? |
| During transfer | Handling tools and contact regions | Was contamination introduced locally? |
| At bonding | Time since preparation and support state | Does the trial represent the intended process route? |
Hold the bonding setup stable while evaluating surface history
When comparing surface conditions, keep wire, tool, support and relevant bonding settings controlled. Otherwise, a change in bond strength cannot be assigned to the surface treatment. Record the landing position and check whether the tool approaches a different part of the pad on different specimens.
Inspect the bond footprint and the neighboring surface before destructive testing. A placement error can move a bond toward a pad edge or overglaze boundary and produce a different failure mechanism. The surface-history comparison should not silently include a geometry change.
Use a planned order when testing several surface histories. If all specimens from one condition are bonded first and another condition is bonded much later, tool wear, temperature or setup drift can become confounded with surface history. Interleave justified comparison specimens or repeat the starting condition to check for drift. Keep each bond linked to the specimen, landing location and surface sequence so the result can be reconstructed without relying on an operator's memory.
Interpret the interface that actually failed
Record whether failure occurs in the wire, at the wire-to-metal interface, within the metallization or in the substrate. A change in failure location can be more informative than a small numerical change in force. Keep the separated surfaces and their orientation where practical.
Measure relevant electrical behavior before a destructive test changes the joint. If a bond conducts initially but becomes unstable under movement, compare the mechanical and electrical evidence rather than assuming the initial continuity result proves a sound interface. A stronger bonding setting is not the automatic correction for every failure location.
Release a controlled surface route for the drawing
The process definition should connect conductor formation, later thermal operations, cleaning, storage and bonding into one traceable route. State the inspection points and the maximum permitted uncontrolled interval only when it has been established for the material and environment. Do not invent a universal shelf time for exposed pads.
When a material, cleaning method, package attachment or storage container changes, review its effect on the bond region. The goal is a reproducible interface that reaches the bonding operation in the intended state. Clear surface history reduces repeated trial-and-error adjustments at the bonder and makes failures easier to investigate.
Provide the wire-bond surface history
Send the complete route that produces and preserves the landing region.
- Metallization route and grade, pad geometry, substrate and finished surface measurements.
- All post-metallization firing, coating, cleaning, drying and package-attachment operations.
- Storage duration and environment, handling tools and the interval before bonding.
- Wire and tool information, bonding setup, footprint images and mechanical failure locations.
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