Isolation distance on small packages - Page 1
Forum section: electronics / general

author: Rectifire
author_rank: Newbie
timestamp: December 21, 2023, 11:24:27 pm
content: Hi all,Recently I started designing some higher voltage designs.What I am running agains is that some parts seem to have higher voltage ratings than possible when following IPC rules.As an example, the IGBT STGD25N40https://www.st.com/en/power-transistors/stgd25n40lzag.htmlcan handle voltages of about 400 volt. However, the package (DPAK) has drain and source separated by 1.8mm. According to IPC-2221 I need at least 2.5mm for those kinds of voltageshttps://resources.altium.com/p/using-an-ipc-2221-calculator-for-high-voltage-design. And I've seen this trend across many more high voltage devices, even passives such as capacitors. (0603's of upto 1 kVhttps://content.kemet.com/datasheets/KEM_C1076_X7R_HV_AUTO_SMD.pdf)I am wondering how to design around this. Am I missing something, or misinterpreting the rules? Wat do you think?Kind regards

author: newto
author_rank: Regular Contributor
timestamp: December 22, 2023, 01:41:54 pm
content: You can add routed slots through the PCB between the pads, or add an insulating conformal coating to the surface to prevent.As for the tiny passives...I'm not really sure actually, maybe that includes conformal coating them too? Although that wouldn't help the underside where you probably aren't getting a coating. We have a 1200V board, but opted for 3000V rated 1812 caps in the spots where it goes to ground. I've never had arcs over the 2.7mm gap between pads, even though the IPC rules says I need 6mm for 1200V. We did used to get arc between the 1200V to the ground plane of my predecessor's design though, even through the solder mask, so they needed a conformal coat, and we still coat the new design for good measure

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 22, 2023, 02:23:11 pm
content: IPC is, at best, industry recommendations; and 2221 in particular is very old. Regulations are more up-to-date, and relevant (for obvious reasons). Although IEC 60950 is a bit out of date (deprecated I think?), it's certainly a good starting point for how these things are figured. Calculator:http://www.creepage.com/It's not often (or well?) documented, but semiconductor encapsulant may have a higher CTI, so that basically accounts for that.You do see semis with collared features from time to time, like 1500V TO-220s with plastic extended over the middle pin.Not that it's good to rely on CTI alone, and breakdown means the unit has already failed, more or less, but it's a basis for safety regulations at least.Ceramic capacitors are, well, ceramic, AFAIK the CTI is very high; of course, this is meaningless when the body rests on top of an organic substrate (PCB has low CTI), but this can be routed out, so the component spans a slot in the board. Of course that's even worse for ceramic caps, slot between pads means less stiffness in the board for resisting board flex, making it far more prone to cracking, and all the more reason to get flex-termination or leadframe types. Or to add rout slots beside the part to further reduce deflection, but, that's taking up a lot of footprint area and why don't you just use a THT at that point?There's also the case for potted assemblies. I recall seeing 0603 chip resistors before, rated for 6 or 10kV or something like that. Somewhere in the datasheet, there was the small note, when encapsulated in potting material, or something like that. Later versions of the datasheet actually omitted the high voltage spec (and potting notes) entirely IIRC. Alas I don't remember the mfg and series that I saw this on, so this is just hearsay...Anyway, potting, done properly, amounts to a cemented joint or something like that, so it can do very well indeed. Creepage is only for surface paths in air, and clearance for distance through air; the distances through solid materials (or liquid, in the case of oil immersion) are much shorter.Tim