The thickness of the copper foil is usually expressed in oz (oz). 1 oz refers to the thickness of 1oz copper evenly covering an area of 1 square foot, which is about 1.4mil, the weight per unit area to express the average thickness of copper foil. For example, 1 ounce = 35 um, 2 ounces = 70 um. According to the big volume practice from PCB mass production, commonly used PCB copper thicknesses are generally divided into 1OZ (35um), 2OZ (70um), 3OZ (105um), etc.
Calculating the current carrying capacity and selecting the thickness of the copper foil is one of the most important criteria in PCB designing work. The current carrying capacity of PCB depends on the following factors: line width, line thickness, and temperature. And there has a positive linear relationship among these factors. Under the same condition of the same temperature and copper foil thickness, the wider the PCB trace width, the greater the current carrying capacity. Under the same condition of the same temperature and same line width, the thicker the copper foil, the greater the current carrying capacity.
From the table below, you can see the line width and current carrying capacity that can be referred to when designing the copper thickness of the PCB.
How to design and select the 1oz thickness, 2oz thickness, or other?
1. The application of the product, especially the current carrying request. From the below pictures, you can see the current-carrying curves of different line widths and copper foil thicknesses at the same temperature.
Regarding applications, 1OZ is enough for common double-layer PCBs, while 2OZ is suitable for most power supply PCBs.
2. Obtain the stacking structure to determine the prepreg pp and core stacking scheme. Usually, the stacking scheme from top to bottom is as follows: pp-core-pp-core-pp-core-pp…-pp.
3. Confirm the line width: the device package and device characteristics used in the design, and then combine the minimum line width, spacing, and other parameters listed in the table above to determine the width of the traces of each layer, the width and gap of the differential line and other parameters.
4. Calculation of impedance. The calculation of impedance requires tools. Usually, the calculation tool provided by allegro is used. However, if pirated copies are used, the calculation parameters may not be accurate. Polar is recommended.
5. Other design factors: The thickness of the copper foil will increase by 0.25~0.5oz after electroplating. The PCB substrate usually used is FR-4 material, and the adhesion strength and working temperature of copper foil are high. Generally, the allowable temperature of PCB is 260 °C. The impact on the power device’s heat dissipation when the PCB temperature is high also needs to be considered.
With the line width, spacing, and dielectric constant, parameters such as core and pp thickness can be comprehensively considered and calculated. By confirming the above main technical parameters and, in parallel, understanding the industry application of the product, the designer is now ready to design the PCB and decide whether the copper foil thickness is 1OZ, 2OZ, or other thickness.
So when it comes to answering the difference between 1oz Vs. 2oz Copper PCB, the answer is not just the difference in the quantity of OZ. 2 oz means higher thickness and cost per unit area. Still, the difference is more about how to design and select copper foil thickness to meet the current carrying requirements of the product. However, in the design process, the designer can still define the line width, consider the characteristics such as operating temperature, and then make an appropriate balance in cost.