Pad size and soldermask expansion questions for RGB LED footprint - Page 1
Forum section: electronics / general

author: amaningd
author_rank: Contributor
timestamp: January 07, 2025, 06:11:38 pm
content: Hi everyone,I'd like to add thisKingbright 5mm RGB LED (WP154A4SEJ3VBDZGC/CA)to a PCB that I'll have fabricated with JLCPCB. The recommended hole layout consists of four 0.9 mm diameter holes spaced 1.27 mm apart. I've attached what I have so far for the footprint. I usually add a few tenths of a mm to the pad diameter around the hole to give someplace for the solder to flow, so I made the pads 1.1 mm wide.JLCPCBlists a solder mask requirement of 0.038 mm (1.5 mil), so I rounded up the Solder Mask to 0.051 mm (2 mil). But now there's only about 0.051 mm between each pad's solder mask expansions, and JLCPCB's minimum solder mask bridge is 0.1 mm. I've also tried a pad size of 1 mm, which gets me a 0.152 mm gap, but I wasn't sure about the pad size being too small compared to the hole. What's the best way to handle this?Thanks!

author: Kean
author_rank: Unknown
timestamp: January 07, 2025, 07:09:15 pm
content: I recommend keeping the solder mask gap as large as possible. The pins on these will easily bridge during soldering, and can be a bit of a pain to rework. When soldered into a plated through hole, they should still form pretty strong solder joints even if the pad is somewhat smaller. Using oval pads will provide a bit more surface area.I have used similar 5mm RGB LEDs in a client product, except I used the QT Brightek QBL8RGB60D0-2897 which is pretty similar spec and pinout. Pin numbering is reversed vs the Kingbright datasheet but I believe they are actually drop-in equivalents. A little dimmer, but noticeably cheaper.My drills are 0.65mm with 1.0mm wide oval pads, and 1.27mm spacing.The CM I work with actually asked me a few months ago to increase the spacing between the holes as they are seeing too many solder bridges. I think they are using automated lead forming, hand insertion, and selective wave soldering. I have now increased the 1.27mm (50 mil) spacing to 1.52mm (60 mil) as I wasn't tight on space. The new PCBs arrived just before the holidays, so I will be assembling some samples this week but a quick checked showed that they still insert quite easily.For the 0.5mm square leads (per DS) I maybe should have gone with a 0.75mm or 0.8mm drill, but I have not see any issues there. I've hand soldered hundreds of these, and the CM also hasn't mentioned an issue for mass production other than the bridges. Pretty sure the pins on the leadframe are not perfectly square, as a quick measurement shows 0.5mm square but just over 0.6mm on the "diagonal". It is possible that the Kingbright may not fit my holes - not sure if I have any to hand to test...

author: amaningd
author_rank: Contributor
timestamp: January 07, 2025, 10:16:36 pm
content: Kean, thanks for the recommendations! I was so focused on whether it was fab-able that I kinda forgot to think about the part where I hand-solder them... I've increased the lead spacing to 1.52mm / 60 mil. One other question: how critical is the solder mask expansion value?

author: thm_w
author_rank: Super Contributor
timestamp: January 07, 2025, 11:02:54 pm
content: JLC spec is 1.5 mil so you can use 1.5mil.You can use less but the pad might get covered up, which could cause problems. Or they might just modify your design anyway.Oval pads would help as recommended above, then you can expand the pad and guarantee a good connection on at least two sides.

author: tooki
author_rank: Super Contributor
timestamp: January 07, 2025, 11:57:34 pm
content: I also immediately thought that oval/oblong pads would be a wise idea here. You could also stagger the holes slightly, moving e.g. pins 1 and 3 up a little bit, and 2 and 4 down a little bit. The pins still fit in a row, but you gain a bit of distance between pads. Seehttps://www.eevblog.com/forum/eda/zig-zag-pad-spacing-for-self-clamping-connector/for what I mean.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2025, 12:10:13 am
content: I second the suggestion for using oblong shaped pads for mechanical stability and some heatsinking for the LED.I would make the holes smaller. The diagonal size of the pins is 0.71mm. A hole with a final size of 0.8mm is fine. As a general rule you want to leave 0.2mm between pads to avoid bridging so the pad width (assuming oblong pads) is 1.07mm maximum. A pad width of 1.07mm is likely to satisfy both annular ring and clearance requirements for a relatively cheap PCB manufacturing process.I never care about solder mask expansion. I just make the soldermask equal to the pad and let the PCB assembler / PCB manufacturer deal with it as these are parameters which can vary depending on the manufacturing process. For small batches the soldermask is inkjet printed AFAIK so the alignment is excellent.

author: Kean
author_rank: Unknown
timestamp: January 08, 2025, 05:27:11 am
content: I generally don't adjust solder mask expansion except for some special footprints where it is called out in the datasheet.The idea of zig-zagging the pads would make insertion a little less friendly, and it is unlikely to add much benefit with longer pads, but it could provide a little benefit in holding the LED vertical.I guess that also means you need to ensure it is properly aligned before soldering as adjustment becomes harder.In my case, the LED is horizontal so I didn't try that, but I have done something similar before on perfboard.

author: Kean
author_rank: Unknown
timestamp: January 08, 2025, 06:32:10 am
content: I had some stock of Kingbright WP154A4SEJ3VBDZGC/CA and WP154A4SEJ3VBDZGW/CA so I checked fit into my latest PCBs with 0.65mm drill holes. They were very slightly tight during insertion into the holes (not a problem, but definitely tighter than the QT Brightek parts). I believe the leads measured about 0.48mm square and 0.64mm on the diagonal.Based on this, I think JLC correctly adjust drills to match finished hole sizes after plating. I know other PCB vendors can get it wrong, so the pins on some parts can get very tight. That could be due to rounding down to nearest drill and lack of sufficient tolerance allowance in the first place. A 0.9mm hole seems oversized for these LEDs even allowing for manufacturing tolerances.BTW, I prefer the diffused versions of these RGB LEDs so that when looking straight on you don't see the three individual point sources. Depends on your application, but something to consider.

author: amaningd
author_rank: Contributor
timestamp: January 08, 2025, 09:11:17 pm
content: Thanks for the suggestions, everyone. I originally got confused because the datasheet listed a tolerance of +/- 0.25 mm, which is why I tried to use a 0.9 mm hole. Glad to hear it's safe to decrease that.

author: nctnico
author_rank: Super Contributor
timestamp: January 09, 2025, 11:37:00 am
content: It is possible that the datasheet specifies a bigger hole to allow for plating which typically decreases the diameter by 0.1mm. Unfortunately there is still confusion about whether a hole is specified as the final (plated) size or the actual drill size. It used to be worse though.

author: b177
author_rank: Contributor
timestamp: February 01, 2025, 03:33:51 am
content: I was going to post a similar question, optimizing the footprint for a similar 5mm TH RGB. My PN is TJ-L5FYTXHMCYLCRGB-A5The EasyEDA footprint is 1.27mm, 0.7 mm hole 1.00 mm pad diameter.My footprint, 1.27 mm spacing, 0.7mm hole, 0.95mm pad, resulting in 0.32mm gap between pads. My soldermask was only 0.1mm between copper pads, but I do not actual gap (ie vendor probably changed it some).I had some pcbs made and assembled at JLCPCB, no solder bridges in 10 samples, but the pads still look too close and I am amazed there wasn't a single bridge.At least on this LED, the pins are NOT square or even rectangular. The legs do not have sharp corners but radiused, very clear under a microscope. Datasheet indicates 0.5mm width. I measured 0.47mm x 0.53 which would be 0.708mm across. But I measured 0.57mm diagonal due to the radiuses.I am going to use oval pads, keep the 0.7mm hole and use a 0.85 mm (minor) diameter pad.The attached picture shows the new oval layout and new pads from the original layout for reference.

author: Kean
author_rank: Unknown
timestamp: February 01, 2025, 10:05:59 am
content: Quote from: b177 on February 01, 2025, 03:33:51 amThe thing you don't know is whether or not JLC assembly staff had bridges and removed them. The CM I was working with said they were occurring enough that they'd appreciate me adjusting the design.Thanks for that extra info on the LED leads/legs. That specific part seems to be only stocked via LCSC, so probably not one I'll be using on any client BOMs that will go out to a CM, but I may get some to try on my next order.