TL;DR
Does recommended creepage distance apply to copper planes under soldermasks?
Yes, it does. Solder mask does not change the required creepage distance on a circuit board.
Is Solder Mask an Insulator?
TI said in this video that "creepage is the shortest distance between two uninsulated conductors along the surface of an insulating material". However, I've also read that soldermask cannot be considered as true insulators.
The statement "soldermask cannot be considered as true insulators" must be considered within its context. Why do we say that it cannot be considered as a true insulator? Because this layer is extremely thin and easily scrapped off the board. So it should not be used as vertical insulation for metal objects above the solder mask, such as metal connectors, shields, solder pins or pads of a chip, another circuit board, etc. This is why the recommended layouts of many connectors specify large keepout zones under their bodies to avoid manufacturing defects.
But for horizontal insulation between two pieces of copper on the same circuit board, adding a layer of solder mask is acceptable. Basically, the actual insulator here is the FR-4 circuit board material, solder mask does not change this fact. The worst-case scenario: the solder mask is scrapped off, who cares? The FR-4 circuit board material continues to function as the insulator.
IPC-2221 vs IEC standards vs Experience
What do industry standards say about solder mask?
IPC-2221
First we have IPC-2221, a voluntary industry standard used by circuit board manufacturers for quality control (not safety). According to this standard, solder mask is beneficial. It classifies solder mask as type B4 (external conductors, with permanent polymer coating). Comparing it to type B2 (external conductors, uncoated, sea level to 3050 m), type B4 significantly reduces the electrical spacing required between the conductors. For example, at 100 V, bare copper (including soldering pads) must maintain a separation distance of 0.6 mm, but traces with solder mask only need a 0.13 mm separation.
However, when we're talking about "creepage" and "clearance", it's inappropriate and wrong to use the IPC-2221 standard as our reference, since IPC-2221 is not a safety standard and does not distinguish or even define "creepage" or "clearance".
The required "creepage" or "clearance" are defined in the context of electrical safety regulations, based on a series of international standards, including IEC 60950-1, IEC 62368-1, IEC 61010-1, and IEC 60664-1. I'll use IEC 60950-1 as a representative example here.
Safety Regulations
Differences
Unlike IPC-2221 - a voluntary quality standard that covers only circuit boards, IEC 60950-1 is (at least was) often a legal standard and is used to design safe electrical equipment under a variety of materials, use conditions, required degree of safety, including protecting humans lives from electrical shock.
The IPC-2221 only defines a single separation distance called "electrical spacing". IEC 60950-1, on the other hand, defines two different separating distances, one is called "clearance" - the distance between two conductors in air, another "creepage" - the distance between two conductors on the surface of a solid insulator. Both clearance and creepage depend on insulation type (functional/basic/reinforced), usage condition (pollution degree). For creepage, it's also a function of material (Material Group / Comparative Tracking Index), because different materials react differently to surface contamination.
IEC 60950-1 realized that, because of surface tracking due to dust contamination, often the separation distance on the surface of a solid must be increased to obtain a reasonable safety margin. And to protect humans from electrical shock, an even higher (2x) safety margin is added to the creepage distance.
Overall, if safety is not critical in your circuit, it's reasonable to use the smaller electrical spacing numbers in IPC-2221 (including the use of the solder mask clause to reduce separation). It's acceptable even in IEC 60950-1, it's called "functional insulation", insulation required only for the correct operating of the circuit with no protection, it's mostly unregulated. But if a short circuit may cause a fire, or if failure of insulation may create an electrical shock hazard, do use IEC 60950-1 Basic Insulation and Reinforced Insulation.
In general, the required separation distances by IEC 60950-1 are much greater than IPC-2221. So if safety is unimportant in a particular case, there's strong motivation to use the smaller number in IPC-2221. But it's also worth noting that, sometimes IEC 60950-1 can give smaller clearance distance than IPC-2221, because the IEC standard allows linear interpolation but the IPC standard does not. Intuitively, going from 100 V to 101 V should not cause a large jump of electrical spacing, yet IPC-2221 requires so. One designer criticized the IPC-2221 requirements as "mostly baseless".
Solder Mask
Then what does IEC 60950-1 say about solder mask and insulation? A lot, but for most people it means very little. Under IEC 60950-1, insulators are classified into four Material Groups based on their Comparative Tracking Index (CTI). Most FR-4 circuit board substrates have a CTI of 175 or greater, and belong to Material Group IIIa. The solder mask material may be unknown, thus, is classified as Metarial Group IIIb by default. However, both IIIa and IIIb are the worst kinds of insulators, and the standard gives the same creepage distance requirements. Hence, the presence of solder mask does not make a difference, end of the story.
To be fair, IEC 60950-1 does not exclude the possibility of using coating (both solder mask and conformal coating) with a better Material Group on a circuit board's surface to improve insulation and reduce creepage requirements as well. But it's only possible with additional quality control requirements. It states that "manufacturing is subjected to a quality control programme that provides at least the same level of assurance" and "The coating process, the coating material and the base material shall be such that uniform quality is assured and the separation distances under consideration are effectively protected." For a mass-produced generic circuit board found in consumer electronics, these requirements are rarely met. Even if one decides to go with this expensive route, one is more likely to use a type of conformal coating specifically designed for this purpose, rather than trying to control the generic solder mask.
Thus, for the purpose of regulation, using solder mask or not does not make a difference (a real conformal coating, on the other hand, does make a difference). You simply proceeds as if it's a bare board.
Experience
From experience, many circuit boards remove the solder mask across an insulation barrier in an attempt to increase its safety. Presumably, the reasoning is the following: the circuit board substrate, FR-4, is a material provided by its manufacturers with controlled electrical properties, on the other hand, a generic type of solder mask's electrical properties are poorly controlled. Thus, it's possible that solder mask has worse insulation than a bare circuit board.
I'm open to correction, but for most designs, so far I didn't find any regulatory requirements to do so. The circuit board substrate belongs to Material Group IIIa. The solder mask, due to its unknown electrical properties, is classified as Material Group IIIb by default. Both are treated as the worst kinds of insulators, and the standard gives identical creepage requirements. Thus, the poor material is already accounted for by the large spacing rules.
The only argument for mandatory solder mask removal comes from a small fine-print in the standard:
Material Group IIIb is not recommended for applications in Pollution Degree 3 with an RMS working voltage above 630 V.
Thus, an argument can be made to remove solder mask for this application, since a circuit board is guaranteed to be Material Group IIIa, but the unknown solder mask can be Material Group IIIb, which the standards warn against. Still, most electronics for home, office, and lab uses are designed for Pollution Degree 2 (non-conductive dust), not Pollution Degree 3, thus, no regulatory requirement exists for most designs at any voltage.
So the conclusion is that, it can be a good design practice, but it only has a small benefit with diminishing return, and it's not required by the regulation in general (which already accounts for the poor materials used and requires a large spacing).
Finally, there are always exceptions. Some circuit boards may be designed with a special material with higher Comparative Tracking Index (CTI), upgrading it from Material Group IIIa/IIIb to Group II or even Group I. In this case, if the board is manufactured with a generic type of solder mask, you definitely want to remove it across a critical insulation barrier. Conversely, special solder mask with high CTI also exists. But in practical, both cases are exceedingly rare.
Summary
In general, solder mask does not change the required creepage distance on a circuit board. According to the industry quality standard IPC-2221, solder mask improves insulation. According to international safety standards, in general, a generic circuit board belongs to Material Group IIIa, a solder mask material with unknown electrical properties belong to Material Group IIIb, both have the same creepage distance requirements. Thus, solder mask usually does not change anything.
When conductive dust is present (Pollution Degree 3), the working voltage is above 630 V, and when the solder mask's electrical property is unknown, removing solder mask across an insulation barrier is justified based on a warning in the safety standards. However, most electronics for home, office, and lab uses are designed for Pollution Degree 2 (non-conductive dust), thus, no regulatory requirement exists to remove solder mask for most designs at any voltage.
For the purpose of regulation, it's also possible to use solder mask with a better Material Group to improve insulation and reduce creepage (not clearance) requirements, but production must be under quality control according to regulatory requirements. This is rarely the case for mass-produced customer-grade circuit boards. Even if one decides to go with this expensive route, one is more likely to use a type of conformal coating specifically designed for this purpose, rather than trying to control the generic solder mask.
