Introduction
Copper strip used in SMT lead frames and co
ector stamping must be flat, dimensionally stable, and free of residual stresses that can cause warpage or springback after forming. When a coil of copper strip exhibits coil set—a tendency to retain the curved shape of the coil—or contains locked-in residual stresses from rolling and slitting, downstream stamping and assembly operations suffer. Parts bend unpredictably, die alignment drifts, and automated feeding systems jam. Understanding and controlling residual stress and coil set is therefore essential for high-yield SMT production.
What Is Coil Set and Why It Matters
Coil set is the curvature that a strip retains after being uncoiled from a wound roll. When copper strip is wound tightly on a mandrel during rolling and recoiling, the i
er layers are compressed and the outer layers are stretched. After unwinding, the strip does not fully return to a flat state because some of this deformation has become permanent. The result is a strip that bows upward or downward instead of lying flat on a die or feeder table.
In progressive die stamping, even a few millimeters of coil set can cause the strip to lift off the die surface during feeding. This misalignment changes the effective punch-to-die clearance, produces uneven cuts, and can shift the part geometry by tens of micrometers. For fine-pitch lead frames where pad positions must align within 50 µm, coil set-induced variation is unacceptable.
Residual Stress Sources in Copper Strip
Residual stress in copper strip originates from several stages of the manufacturing process. Cold rolling compresses the metal through successive roll passes, creating non-uniform stress distributions across the thickness gradient. The top and bottom surfaces experience different strain levels because of roll gap geometry and friction, leaving the strip with a bending moment even after it leaves the mill.
Slitting adds another stress component. The shear and fracture zones on opposite sides of the cut create asymmetric stress patterns, and the recoiling tension during slitting adds additional bending strain. Finally, leveling and tensioning operations that attempt to correct coil set can introduce new stress patterns if they over-bend the strip in one direction to compensate for an opposite curvature.
Common Residual Stress Measurement Methods
| Method | Principle | Resolution | Best For |
|---|---|---|---|
| X-Ray Diffraction | Strain from lattice spacing shift | ± 5 MPa | Surface stress mapping |
| Layer Removal | Curvature change after etching layers | ± 10 MPa | Through-thickness profile |
| Slitting Method | Strain gauge on slit sections | ± 15 MPa | Longitudinal stress |
| Bending Deflection | Cut strip curvature measurement | Qualitative | Quick production check |
Coil Set Measurement and Quantification
Coil set is quantified by cutting a short sample from the uncoiled strip, placing it on a flat reference surface, and measuring the maximum gap between the sample and the surface. This gap, typically expressed in millimeters over a standard gauge length such as 300 mm, defines the coil set magnitude. Direction is also recorded: positive coil set means the strip curves upward from the coil i
er surface, negative means it curves downward.
For SMT lead frame strip, coil set should be controlled below 1 mm over 300 mm gauge length for standard applications and below 0.5 mm for fine-pitch applications. These limits ensure that the strip feeds smoothly through progressive dies without lifting or camber-induced misalignment. Measurement should be performed at multiple points along the coil length because coil set can vary from the i
er wraps to the outer wraps.
A
ealing Strategies for Stress Relief
A
ealing is the most effective method for reducing residual stress and coil set in copper strip. The process heats the strip above its recrystallization threshold, allowing dislocations to rearrange and locked-in stresses to dissipate. For C11000 pure copper, full a
ealing at 300 to 400°C for 30 to 60 minutes eliminates virtually all residual stress and produces the lowest coil set values.
However, full a
ealing also softens the strip, reducing its yield strength to approximately 30 to 60 MPa. This soft state may not be suitable for stamping operations that require a stiffer material to resist springback after forming. For these applications, partial a
ealing or stress-relief a
ealing at 200 to 250°C provides a compromise: it reduces residual stress and coil set substantially while preserving most of the cold-worked hardness.
A
ealing Parameters and Resulting Properties
eal Type eal (O60) eal after final pass eal after final passA
Temperature YS (MPa) Coil Set (mm/300mm) Full A
350-400°C 30-60 < 0.3 Stress Relief 200-250°C 150-220 < 1.0 Quarter Hard (H01) No a
250-300 2-5 Half Hard (H02) No a
300-350 3-8
Mechanical Leveling Techniques
When a
ealing is not practical because the application requires a harder temper, mechanical leveling can reduce coil set without significantly changing the strip hardness. Roller leveling passes the strip through a series of staggered upper and lower rollers that alternately bend the strip in opposite directions. Each bend cycle progressively reduces the net curvature until the strip lies flat.
Tension leveling combines roller bending with controlled longitudinal tension. The tension stretches the outer fiber of the strip while rollers create localized bending, providing more effective coil set reduction than roller leveling alone. Modern tension leveling lines can reduce coil set to below 0.5 mm over 300 mm on half-hard copper strip without the need for a
ealing.
Process Control Best Practices
Controlling coil set and residual stress requires attention at every stage of strip production and handling. During cold rolling, consistent reduction schedules and uniform roll gap settings minimize asymmetric stress buildup. During slitting, balanced tension on both sides of the cut prevents lateral stress gradients. Recoiling with moderate back tension rather than tight winding reduces the bending strain that causes coil set in the first place.
Incoming quality inspection at the stamping facility should include coil set measurement on every coil before it goes to production. A simple fixture with a flat plate and a dial indicator takes less than one minute per coil and catches problems early. If coil set exceeds the specification, the coil should be returned to the supplier, leveled on-site, or a
ealed before stamping. Skipping this step saves a few minutes of inspection time but risks hours of die adjustment and scrap.
Conclusion
Residual stress and coil set in copper strip are invisible defects that can dominate the dimensional accuracy of SMT lead frames and stamped co
ectors. By understanding the sources of these stresses, measuring them systematically, and applying the right combination of a
ealing, leveling, and tension control, manufacturers can deliver strip that feeds flat, forms predictably, and meets the tight tolerances that modern electronics demand.
