|Ask the Experts|
April 2, 2020
Is Solder Mask Considered an Insulator
If solder from a PTH lead, on the source side, touches solder mask that is over a copper ground plane, but not electrically shorted: is this a defect per IPC standards due to violation of minimum electrical clearance?
Or is solder mask considered an insulating material and not a defect if proven the solder mask has dielectric withstanding voltage > 500volts.
Is solder mask considered a durable insulator over time in harsh environments?
|Expert Panel Responses|
Solder mask primary function is to protect copper of the PCB laminate during manufacturing process and also provide bare copper protection over time.
The dielectric of the solder mask is a secondary benefit and pending the application (Film or SMOBC) should not be relied upon for safety and/or circuit performance control. Soldermask application and standards may permit pin hole or other minor conformity (pending class), which would detrimentally affect dielectric of the Soldermask.
To adequately protect from environments and ensure good dielectric insulation, Conformal Coatings should be applied and managed accordingly to IPC Standard IPC-CC- 830.
Capital Equipment Operations Manager
Specialty Coating Systems
While I cannot speak to the IPC standard requirements, it depends on what reliability specification you are working as to whether you should rely on the solder mask as an electrical barrier.
Also, are you speaking of an actual solder mask, or a conformal coating, as the properties are much different? The type of material, its dielectric strength, its thickness, the voltage through the conductors, etc. These are the variables that must be considered.
In most design facilities, there is an actual Reliability Engineer on staff that can make this analysis. It is not a judgement that can be taken lightly, unless your are manufacturing, for example, children's remote toys, or something on that level of reliability.
We had a similar concern a few years ago. In talking with IPC, it was determined that soldermask is not to be considered an insulator. This would be a defect due to violation of minimum electrical clearances.
Supply Chain Engineer
Solder mask was designed to prevent solder bridging period. Its ability to act as an insulator is an added benefit but should never be considered as a true insulator as the deposit does contain air voids, varies in thickness as a function of surface features and applications, may contain ionic or bi-polar organic species from processing at the fab house etc.
S T and S Testing and Analysis
IPC standards are, for the most part, focused on quality, quality programs and the protection of the environment [http://www.ipc.org/ContentPage.aspx?pageid=Mission-Vision-and-Goals]. There are many other electrical safety standards that may apply, depending on the type of product, a subject too large to discuss here.
If this product is required to meet standards for electrical safety, you must refer to those standards. However, in my experience, most safety standards do NOT recognize solder mask as an effective insulator.
Also, where there is no specific reference to solder mask as an insulator in the standard, acceptability will rest in the hands of the inspector representing the nationally recognized testing lab, AKA: NRTL
Senior Project Engineer
Electronic Controls Design Inc
If solder from a PTH lead, on the source side, touches solder mask that is over a copper ground plane, but not electrically shorted: is this a defect per IPC standards due to violation of minimum electrical clearance? Or is solder mask considered an insulating material and not a defect if proven the solder mask has dielectric withstanding voltage > 500volts.
Depending upon the electrical requirements of the product this may or may not be an issue. With low voltage products like computers, this was never an issue as many leads were over the solder mask and never shorted out. But again, high voltages and dielectric properties of the materials must be considered to verify this is not an issue.
Is solder mask considered a durable insulator over time in harsh environments? By definition solder mask is not an insulator. If it is to be used as an insulator then again its dielectric properties have to be defined and tested to verify it meets the requirements of the product. The function of a solder mask material is to reduce the amount of solder used during the soldering operation and to prevent shorts from occurring between conductive traces.
Once the soldering operation is finish the functionality of the solder is complete. Therefore if one is looking for a durable insulator in harsh environments, the direction would be to use conformal coating and coat the product.
Vice President, Technical Director
Solder mask should never be considered an insulator. It can have porosity and pinholes, and we should therefore not consider any dielectric strength that it may or may not provide.
All comments seems concentrate at Z direction and the consensus here is the thin layer of solder mask (with potential defects like pinholes, etc.) can't be considered as an insulator. This is fine, but what about X and Y axis, that is surface of the PCB? We may have PCB with CTI, say, 400 and then we apply the solder mask.Eugene, MCI
If the CTI of the solder mask is less than the CTI of the laminate, the finished PCB will have the CTI defined by the solder mask, not by the laminate. Yet, UL does not test solder masks for CTI and will certify PCB based just on the laminate CTI.
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