# What happens if high via current flows in PCB?

If I used a via of 8 mils diameter with 20 um thickness. As per sierra tool calculator it can carry current of 1 A at 10 degree rise in temperature. What happens if the current is more than 1 A? When will the via fail? How the failure will be in PCB?

• Depends on lots of factors such as via length, board material, via's top and bottom surface areas (i.e. if the via is connected to a trace or a copper pour), duration of high current flow, environmental conditions, frequency of the flowing current (i.e. if the current is DC or high frequency AC), adjacent heat sources (if any), etc. Aug 24, 2022 at 7:16
• I have seen like when temperature cross more than Tg of PCB, then board may become instable, Whether it is true to via, if via temperature crosses more than Tg, it will fail? Aug 24, 2022 at 7:27

10 °C rise isn’t much for a PCB trace. Even 20 °C or 30 °C is usually fine. In some cases, voltage drop across the trace, or power dissipated in the trace, are too high for system design reasons when the rise gets high, say 30 °C or 50 °C.

From a strictly thermal standpoint, there are maximum temperatures for the dielectric being used. For common FR4, this is often 110 °C to 130 °C and is determined by the rated Tg (glass transition temperature). Above this temperature, the board becomes soft, and mechanical and electrical properties are degraded.

In the absence of detailed information, I’d keep thermal rise to 10 °C to 30 °C absolute worst case, and if pushing 30 °C, check that voltage drops aren’t an issue.

• Thanks, it means I can allow the via current at the max temp rise of 30C. More than that it is not preferable. Am i right? Aug 24, 2022 at 12:23
• @Selva97 Bear in mind that this is above ambient, so add the maximum expected operating ambient temperature to the temperature rise Aug 24, 2022 at 15:29

Higher temperatures are also undesirable due to thermal stress and cycling. The laminate expands much more than the copper plated onto it, causing the copper to stretch, fatigue, and eventually crack.

Reduce heat by using less current in the first place, using more vias in parallel, wider traces, more layers, etc.

• I think this is at least as important, and possibly more important than the other answer. The Tg will only really matter at high ambient temperatures like 65C+ (unless you're putting like 5A through one trace), but the thermal expansion and contraction will matter at all temperatures Aug 24, 2022 at 23:58