Key takeaways
- A busbar defect is usually a local resistance or insulation problem, so the failure appears at a joint, a bend or a hole.
- Cracks start at a bend or a sheared edge, delamination starts at a bond line, plating voids start at a contaminated surface and burrs start at a cut.
- Visual inspection finds burrs and some plating faults, but cracks and delamination need a bend check, a cross-section or a thermal test.
- The cheapest control is the process step before the defect, not the inspection after it.
Why a small defect matters in a busbar
A busbar carries a large current through a small section. A defect that would be minor in a bracket can be serious in a bar. A crack raises the local resistance and creates a hot spot. A delamination reduces the insulation between layers and can lead to a partial discharge or a flashover. A plating void exposes the base metal to corrosion and raises the contact resistance at a joint. A burr reduces the effective contact area and can puncture insulation.
The common thread is localisation. A busbar is designed as a current path with a known resistance, and a defect changes that resistance at one point. The rest of the bar may be perfect, but the hot spot sets the temperature and the life of the joint.
That is why the inspection should look at the places where a defect can start: the cut edge, the bend, the hole, the joint face and the layer bond. A general visual check of the flat faces misses most of them.
Cracks at bends and sheared edges
A crack in a copper busbar usually starts on the outside of a bend, where the material is stretched. The risk rises when the bend radius is small relative to the thickness, when the temper is hard, and when the bend line runs across a sheared edge or a burr. A crack can be visible, or it can be a fine line that only opens after the bar is loaded.
The second common site is a sheared edge. A punch or a shear leaves a cold worked zone and a burr. If the bar is bent near that edge, the worked zone can crack. If the bar is bolted, the burr can reduce the contact area or create a gap that lets moisture in.
Prevention is mostly in the process. Use a bend radius that suits the thickness and the temper, deburr before bending, and keep the bend line away from a sheared edge where the design allows. For a high-stress bend, specify a stress relief or a softer temper and check the first article with a bend test or a dye penetrant check.
- Set the inside bend radius from the thickness and the temper, not from the tool that happens to be fitted.
- Deburr the cut edge before bending.
- Keep the bend line away from a sheared edge.
- Check the first article at the outside of the bend.
- Ask for a stress relief where the bend is highly strained.
Delamination in a laminated busbar
A laminated busbar is a bond between copper and a dielectric. Delamination is a separation at that bond, and it can be a void that is present from the start or a crack that grows in service. The usual causes are a contaminated copper surface, an incomplete cure, trapped air, a mismatch in expansion or a mechanical load at a termination.
A small delamination may not show on a short insulation test. It can, however, reduce the dielectric thickness at one point, concentrate the electric field and start a partial discharge. Under thermal cycling the void can grow, and the layer can separate further.
Detection is not easy from the outside. A partial discharge test is more sensitive than a simple withstand test. A cross-section on a sample shows the bond line and any voids. A thermal cycle followed by a partial discharge check is a stronger qualification than a single room temperature test.
A passed hipot is not a clean bond
A short voltage test can pass a laminate with a small void. Partial discharge and a cross-section show what the test missed.
Plating voids and poor adhesion
A plating void is an area where the deposit is missing, thin or not bonded. It can come from a contaminated surface, an incomplete cleaning step, a poor current distribution in the bath, a rack contact that shields the part or a mask that was not removed. A void exposes the base metal, and on a joint face it becomes a corrosion site and a point of higher resistance.
Adhesion is a related problem. A deposit that is mechanically weak can flake or blister when the bar is bent or when the joint is tightened. A blister on a joint face can hold the faces apart, so the contact is made through a smaller area than the designer intended.
The usual checks are a visual inspection at a stated magnification, a thickness measurement and an adhesion test such as a bend or a tape test on a sample. For a critical joint, a cross-section shows the deposit and the interface. A salt spray or a humidity test on a sample gives an indication of the corrosion resistance, but it is not a substitute for a clean process.
| Defect | Where it starts | How to detect | How to prevent |
|---|---|---|---|
| Crack | Outside of a bend or a sheared edge | Visual, dye penetrant, bend test | Radius, deburr, temper and stress relief |
| Delamination | Bond line in a laminated stack | Partial discharge, cross-section, thermal cycle | Surface clean, cure control, edge margin |
| Plating void | Contaminated or shielded surface | Visual, thickness, adhesion, cross-section | Cleaning, rack contact, bath control |
| Burr | Cut edge or punched hole | Visual, touch, profile check | Tool sharpness, clearance, deburr |
The controls are process controls first. Inspection confirms the process, it does not create the quality.
Burrs at cut edges and holes
A burr is raised metal at the edge of a cut or a punched hole. It reduces the flat contact area at a joint, it can puncture a dielectric or a coating, and it is a sharp point that concentrates an electric field. A burr on a hole wall can also damage a stud or a fastener during assembly.
The burr direction matters. On a punched bar, the burr is usually on one side. If that side faces a joint face or an insulation layer, the effect is worse than if it faces away. The drawing can specify the burr direction and the allowable height, and the process can control it with tool clearance and sharpness.
Deburring adds a step, but it is cheaper than a rejected joint. A tumbled or a brushed edge is common for a small bar. A large bar may need a manual or a machine edge treatment. The inspection should check the edge at the joint face and around every hole, not just the overall profile.
- Specify the burr height and the permitted direction.
- Check the tool clearance and sharpness on a punching line.
- Deburr before plating, because a burr under a deposit is a weak point.
- Inspect the joint face and the hole edges, not only the outline.
- Protect the finished edges in packing.
Building an inspection plan that finds them
The inspection plan should match the defect to the step. After cutting, check the burr and the edge. After bending, check the radius and the outside of the bend for cracks. After lamination or coating, check the bond, the thickness and the edge coverage. After plating, check the thickness, the adhesion and the masked areas. After assembly, check the joint resistance and the torque marks.
For a first article, add the checks that show the process is capable: a cross-section, a partial discharge measurement, a dye penetrant check on a bend and a contact resistance reading on a sample joint. For production, use a shorter set that still covers the critical features, and keep the records so a trend can be seen.
When a defect is found, contain the batch and check the process step that caused it. A burr that appears on one batch may be a worn tool. A plating void that appears on one lot may be a contaminated bath. The corrective action should change the process, not just sort the parts.
What to send with a quality enquiry
Send the drawing with the burr, radius, plating and insulation requirements. Add the sample size, the inspection level and any standard the part must meet. If a defect has been found in service, send the location, the operating current and the joint temperature. A photograph with a scale is more useful than a description.
If the part is laminated or coated, say which test the defect was found by. A delamination found by a partial discharge test and one found by a visual check are different problems, and the corrective action is different.
- Drawing revision and the critical characteristics
- Defect location, size and photograph
- Operating current and joint temperature
- Test that found the defect
- Batch, date code and quantity affected
- Inspection level and standard
Frequently asked questions
What is the most common busbar defect?
Burrs and edge defects are the most common, because every cut and every punched hole can produce one. Cracks at bends and plating voids are next. Delamination is specific to laminated bars but is the most serious for insulation.
How do I detect a crack that is not visible?
Use a dye penetrant check on a sample, a bend test on a first article or a cross-section at the bend. A visual check finds open cracks but misses fine ones. For a high stress part, a stress relief and a process check are more useful than a single inspection.
Can a plating void be repaired?
A small void on a non-critical surface may be acceptable if the specification allows it. A void on a joint face or a high current area is usually a reject, because a repair can leave a different surface and an uncertain bond. The part should be re-plated or replaced.
Why does a laminated busbar delaminate after thermal cycling?
The copper and the dielectric expand at different rates, so the bond line sees stress at every cycle. A void, a contaminated surface or an incomplete cure starts the separation. A thermal cycle followed by a partial discharge check is the practical way to find it.
Sources and further reading
- IEC 60664-1, Insulation co-ordination for equipment within low-voltage supply systems
- IPC-A-600, Acceptability of printed boards, as a reference for plating and edge criteria
- Qumek busbar inspection and defect reviews
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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