Is Solder Mask Considered an Insulator

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.

Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems

Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

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.

Rick Perkins
President
Chem Logic

Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.

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.

Paul Dickerson
Supply Chain Engineer
Matric Group

Mr. Dickerson is an engineer with 20 years of manufacturing experience. He has worked supporting SMT, THT, cable assembly, and box build processes. He is a Certified SMT Process 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.

Gerard O'Brien
President
S T and S Testing and Analysis

Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.

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

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc

Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

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.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation

At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

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.

Fritz Byle
Process Engineer
Astronautics

Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Reader Comment

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.

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.

Any comments?

Eugene, MCI

Reader Comment

Solder mask is not to be considered an insulator, in any direction. The IPC standards actually allow small skips, opens, and pinholes in the mask. Thus there is no way for us on the team to attempt to accommodate various exceptions to the rule because of factors such as dielectric strength requirements, thickness requirements, types of solder masks used, voltages present during operation, chemical and environmental use, and a myriad of other issues.

Odin Stadheim, Kongsberg Maritime AS

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