When "Close Enough" Fails

In mechanical engineering, tolerance is the allowable variation in a dimension. A drawing that specifies 10.00 ± 0.05 mm means the part can measure anywhere between 9.95 and 10.05 mm. For many applications, this range is perfectly acceptable. But for copper PCB accessories, tight tolerances are often critical.

Consider a press-fit pin that must fit into a plated through-hole. The pin diameter is 1.00 mm, and the hole diameter is 0.98 mm. The interference fit-0.02 mm-provides the mechanical retention and electrical contact. If the pin is 0.02 mm oversize, the insertion force increases dramatically, potentially cracking the solder joint or damaging the PCB. If the pin is 0.02 mm undersize, the interference is lost, and the pin may loosen over time due to thermal cycling.

The same principle applies to other copper accessories. A thermal pad must be flat to within 0.05 mm to ensure full contact with the TIM. A shield can must fit snugly over a connector to maintain EMI performance. A busbar must align with mounting holes to avoid stress on the PCB.

The cost of tolerance failure is high. A single out-of-tolerance part can cause an entire assembly to fail. In high-volume production, a 1% defect rate translates to thousands of failures per million parts. The cost of rework, scrap, and field failures far exceeds the cost of proper tolerance control.

Dimensional Mechanics

To understand tolerance control, we must understand how copper behaves under load and temperature.

• Elastic Deformation.​ Copper follows Hooke's Law up to its yield strength. For annealed copper, the yield strength is about 70 MPa. For half-hard copper, it is about 200 MPa. Within the elastic range, the part returns to its original shape when the load is removed.
• Plastic Deformation.​ Beyond the yield strength, copper deforms plastically. It does not return to its original shape. This is how copper parts are formed-by bending, stamping, and drawing. But plastic deformation also causes work hardening, which changes the material properties.
• Creep.​ Under sustained load, copper slowly deforms over time. This is called creep. At room temperature, creep is negligible for most applications. But at elevated temperatures-above 100°C-creep becomes significant. A press-fit pin under sustained stress may gradually relax, losing its interference fit.
• Thermal Expansion.​ Copper expands at about 17 ppm/°C. A 10 mm part grows 0.17 µm for every 1°C temperature change. Over a 50°C temperature range, the growth is 8.5 µm-enough to affect a tight tolerance.
• Springback.​ When copper is bent, it springs back partially after the bending force is removed. The amount of springback depends on the material temper and the bend radius. For half-hard copper, springback can be 5–10% of the bend angle. The die must be designed with springback compensation.

• Tolerance Classes

• For copper PCB accessories, tolerances are typically classified into three categories:
• Standard Tolerance (±0.05 mm).​ This is suitable for non-critical dimensions, such as the overall length of a bracket or the position of a mounting hole. Standard tolerance is achievable with conventional machining and does not require special processes.
• Precision Tolerance (±0.01 mm).​ This is required for critical dimensions, such as the diameter of a press-fit pin or the flatness of a thermal pad. Precision tolerance requires careful process control, including temperature-controlled machining and measurement.
• Micro-Feature Tolerance (±0.005 mm).​ This is required for the most demanding applications, such as high-frequency RF contacts or precision alignment features. Micro-feature tolerance requires specialized equipment, such as wire EDM or ultra-precision machining, and is significantly more expensive.

Measurement Protocol

Measuring copper parts accurately is as important as machining them accurately. The measurement process must account for thermal effects and measurement uncertainty.

Step 1: Stabilize the part.​ Allow the part to reach thermal equilibrium at the measurement temperature-typically 20°C. This takes at least 24 hours for a large part, less for a small part.

Step 2: Calibrate the equipment.​ The CMM, micrometer, or profilometer must be calibrated with traceable standards before use.

Step 3: Measure at multiple points.​ Take measurements at several locations to assess the variation across the part. This is particularly important for flatness and parallelism.

Step 4: Record the temperature.​ Note the temperature at the time of measurement. If the temperature is not 20°C, apply a correction factor for thermal expansion.

Step 5: Repeat the measurement.​ Measure the same dimension multiple times to assess the measurement uncertainty. The standard deviation of the measurements indicates the repeatability.

Shop-Floor Case

A manufacturer of power connectors required a press-fit copper sleeve with an H7/h6 interference fit. The sleeve had an outer diameter of 8.00 mm with a tolerance of +0.015/−0.010 mm. The initial production used cutting taps to create the internal threads. The threads were rough and the sleeve dimensions varied by ±0.03 mm.

The solution was to switch to roll-form taps. Roll-form taps displace material rather than cutting it, creating smoother threads with tighter dimensional control. The sleeve dimensions stabilized at ±0.008 mm, meeting the H7/h6 specification.

The lesson: the manufacturing process directly affects the achievable tolerance. A process that is not capable of holding the tolerance will produce out-of-spec parts, regardless of the measurement system.

Key Message

Tight tolerance on copper is meaningless without thermal and fixture control. A part that measures correctly at 20°C may be out of tolerance at 30°C. A part that is clamped differently during machining may spring back differently. The entire process-machining, measurement, and handling-must be controlled.

For OEM buyers, the recommendation is to specify only the critical dimensions with tight tolerances. Non-critical dimensions can be specified with standard tolerances, reducing cost. The drawing should clearly identify which dimensions are critical and which are not.

Recommendation

For high-precision copper parts, consider the following best practices:

Use stress-relieved material to minimize internal stress and distortion.

Use multiple finishing passes with light cuts to minimize work hardening.

Use temperature-controlled coolant to minimize thermal expansion during machining.

Measure parts at 20°C in a temperature-controlled room.

Use statistical process control (SPC) to monitor the process capability.