FR4 and Voltage and Environment OH MY

Customer Question:

How many volts will the 2 sided boards you sent me handle? The FR4 is good for 50kv.  However, I am concerned with one of my units being used in a high humidity environment, like the south where you can have 98F and 98% humidity.  Add to the mix a little film of whatever is in the air over a course of a few years and you can see where I am a little concerned.

Everyone says, check with your board manufacturer.


ExpressPCB Customer

ExpressPCB Answer:

The dielectric material of our 2 sided PCBs should handle up to around 50kv with respect to dielectric breakdown between layers.

FR-4 material will absorb moisture out of the air but soldermask does a good job of limiting that through the largest surface areas leaving only the boards edges.  This absorption of moisture around the edges typically has very little effect on the dielectric breakdown voltage and may lower the 50kV to 45KV (to be safe).

The main issue where temperature and humidity might come into play is arcing between adjacent current carrying circuits.  

With PCBs with solder mask there are typically no issues with temp or humidity from a voltage standpoint. The dielectric breakdown of our soldermask has been tested to 2800V / mil of thickness.  Our typical thickness in and around the conductors is about 1 – 1.2 mils.  This would allow for about 2500-3000V before there might be any breakdown of this layer.  

For PCBs without solder mask it is theoretically possible for an arc to occur between adjacent copper traces in high temp, high humidity if the conditions and voltage are just right to overcome the breakdown voltage of the surrounding air.

The discussion at gives some of the calculations and constants required to determine what that voltage might be.  

The main concern of bare PCBs in a humid environment would be oxidization of the exposed conductors.  Immersion silver finish has been known to oxidize if stored without environmental protection.  We recommend that bare immersion silver boards be treated similarly to moisture sensitive components, sealing any opened bags, using desiccant and protecting from UV light exposure.  Boards shipped with the Tin Lead finish are not nearly as susceptible to these environmental concerns and can typically be stored for a year without issue. Once a PCB with soldermask is assembled the solder typically coats all of these exposed areas and the oxidation of solder is very low.  


ExpressPCB Staff

2 Responses to “FR4 and Voltage and Environment OH MY”