October, 2021
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Page 63
Heavy Copper Design Considerations Part 1: Traces and Temperature
By Stanley L. Bentley, P.E., Technical Director — Americas, ICAPE Group T
he sudden rise of electric everything has had a corre- sponding effect on PCB
technology. If we go back just 10 years, heavy copper was consid- ered anything over 3 oz. Today, that foil weight is common and heavy copper is much thicker. Fabricators are scrambling
to develop technology to turn thick sheets of copper into PCB traces and spaces. It is generally agreed that there are now three categories of heavy copper, though different fabricators will apply slightly different ranges: 1
The failure point of a PCB is reached when the loss of mass exceeds only 5 percent of the original mass due to thermal degradation.
to 3 oz is standard, 4 to 10 oz is heavy, and 11 oz or more is extreme.
Charts are Guidelines At the same time, design
standards (such as IPC 2221) are challenged with providing charts to the designers. The reason that charts are guidelines is that the calculations are complex, and no chart can accommodate every variable of a physical PCB and its operating environment. The chart shown here (Figure 1) has been replicated in numerous publications for 30 years. The problem with charts is that any number of variables can be dis- played. The idea behind this chart
was a clever adaptation of a ther- modynamic chart by using curves of constant temperature. I suspect some of the assumptions and the constraints have been lost to history — as no one really wanted to do the math. For example, why use a con-
stant temperature curve and which one to choose? The “flip- pant” answer is to keep the PCB and circuitry from overheating. So how does this chart know about my application? It doesn’t. If we think of a power resis-
tor on a PCB, we know that we must stand the resistor above the surface to allow the convec- tive process to cause the move- ment of the air and cool the resis- tor. We also know that stagnant air is a very poor conductor of heat (hence the reason a thermal pane window works so well). The same is true of the glass
epoxy substrate of the PCB, except that the fiberglass cannot generate a convective flow and
therefore has little ability to cool the embedded copper traces. The embedded traces will continue to rise in temperature. For this rea- son, we have charts for external traces which can get some con- vective cooling and internal traces which cannot. Again, these are not absolute, they are only guideposts to begin the design process.
How Hot is Too Hot? A constant temperature
curve allows us to consider the effects of the temperature of an individual trace within the PCB substrate itself. The IPC has another document (IPC 4101) that outlines the operating parameters of the raw laminate choice. The designer must con- sider not only the operating tem-
perature of the traces, but the effect these traces have on the PCB laminate. For example, we know that
everything moves with increas- ing temperature. However, the movement is vastly different between the materials of a PCB. The copper traces and (more importantly) the vias expand at
Continued on next page
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